SERI/TR~217-3036 

DE88001132 

UC Category: 261 

The 

Turbi 

D-2 Wind 

Aeroacoustical Noise 

Sources, EmisSions, and 

Potential ~mpact 

N. D. Kelley 

H. E. McKenna 

E. W. Jacobs 

R. R. Hemphill 

N. J. Birkenheuer 

January 1988 

Prepared under Task No. WE721201 

FTP No. 562 

Solar Energy Research Institute 

A Division of Midwest Research Institute 

1617 Cole Boulevard 

Golden, Colorc;ldo 80401-3393 

Prepared for the 

U.S. Department of Energy 

Contract No. DE-AC02-83CH10093 
NOTICE 

This report was prepared as an account of work sponsored by the United States Government. Neither the 

United States nor the United States Department of Energy, nor any of their employees, nor any of their 

contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any 

legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, 

product or process disclosed, or represents that its use would not infringe privately owned rights. 

Printed in the United States of America 

Available from: 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

Price: Microfiche A01 

Printed Copy A10 

Codes are used for pricing all publications. The code is determined by the number of pages in the publication. 

I nformation pertaining to the pricing codes can be found in the current issue of the following publications, 

which are generally available in most libraries: Energy Research Abstracts. (ERA): Government Reports 

Announcements and Index (GRA and I): Scientific and Technical Abstract Reports (S TA R): and publication. 

NTIS-PR-360 available from NTIS at the above address. 
TR-3036 

PREFACE 

This report summarizes extensive research by the staff of the Solar Energy 

Research Institute into characteristics of acoustic noise emissions of the 

DOE/NASA MOD-2 wind turbine. The results of this study have shown that the 

MOD-2 noise levels are well below annoyance thresholds within residential 

structures a kilometer or more from the turbine rotor. It was also found that 

the inflow turbulent structure has a major influence on the level and characteristics of the low-frequency (2-160 Hz) range acoustic emissions which, in 

turn, have implications for the associated structural response of the rotor 

assembly. The high-frequency range (A-weighted) levels were found to vary 

primarily with the mean hub-height wind speed. In addition, the rotor inflow 

turbulence characteristics at the Goodnoe Hills Site were found to be controlled almost entirely by the diurnal variation in the vertical stability of 

the first 100 m of the atmospheric boundary layer. 

Approved for 

SOLAR ENERGY RESEARCH INSTITUTE 

Robert J. No n, Manager 

Wind Energy Research Branch 

, Solar Electric Research Division 

Neil Do! Kelley I 

Principal Scientist 

Wind Energy Research Branch 

111 
TR-3036 

ACKNOWLEDGMENTS 

The authors wish to acknowledge the excellent support and assistance of the 

following organizations in SERIfs MOD-2 research effort: 

• Boeing Aerospace Corporation 

• Bonneville Power Administration 

• Cornell University, Sibley School of Mechanical and Aerospace Engineering 

• Engineering Dynamics, Inc. 

• Fairchild-Weston, Inc. 

• NASA Langley Research Center 

• NASA Lewis Research Center 

• Oregon State University, Atmospheric Sciences Department 

• Pacific Northwest Laboratories 

• B.C. Willmarth Co. 

Special thanks are extended to Larry Gordon of NASA Lewis and Ron Holeman of 

the Bonneville Power Administration who provided the scheduling and support 

services so necessary to our effort.. The support of Ron Schwemmer and Don 

Fries of Boeing was without reproach. We salute the dedicated efforts of Ben 

Willmarth who expertly operated the tethered balloon system under very adverse 

circumstances. We also extend our deepest thanks to David Long of FairchildWeston who provided us with outstanding data support services. 

~v 
TR-3036 

SUMMARY 

Objective 

This document $ummarizes the results of an extensive investigation by the 

Solar Energy Research Institute (SERI) into the factors relating to acoustic 

emissions associated with the operation of a MOD-2 wind turbine.. The MOD-2 

was the sixth ~ a series of turbine designs developed for the u.s. Department 

of Energy (DOE) by the Lewis Research Center of the National Aeronautics and 

Space Administration (NASA) as part of the Federal Wind Energy Program. The 

MOD-2 turbine has a rotor diameter of 91 m (300 ft) and is capable of generating 2.5 MW of electrical power at its rated wind speed of about 13 mls 

(28 mph), measured at a rotor hub elevation of 61 m (200 ft). A cluster of 

three MOD-2 turbines installed on the Goodnoe Hills near Goldendale, Wash., 

was used for the experiments described in this reporto 

An investigation of the characteristics of the MOD-2's acoustic emissions was 

undertaken as a result of the experience SERI gained with its predecessor, the 

2-MW MOD-l turbine" One of the primary motivations for designing the MOD-2 

turbine with its rotor upwind of the support tower was to avoid the impulsive, 

low-frequency noise associated with the downwind MOD-I.. It was expected that 

placing the MOD-2 rotor upwind would largely eliminate the community annoyance 

problem that was characteristic of the impulsive MOD-1 emissions" It was not 

known, however, whether similar or perhaps greater levels of nonimpulsive, 

low-frequency noise that radiated from the large MOD-2 rotor as a result of 

inflow turbulence interactions would annoy the residents nearby We designed 

our MOD-2 test program to answer these questions, including the following 

specific objectives: 

A general characterization of both low- (under 200 Hz) and high-frequencyrange acoustic emissions 

• The development of a methodology for making acoustic measurements 1n a windy 

environment 

• The development of a methodology relating low-frequency acoustic emissions 

to the turbulent inflow structure 

• The development of a methodology for predicting 

potential of nearby residential structures from 

frequency acoustic loadings 

the interior annoyance 

a wind turbine's low-

• The application of the annoyance potential criteria using MOD-2 emission 

levels measured under a range of operating conditions and a comparison of 

the results with similar ones for the MOD-1 turbine. 

Discussion 

We undertook a series of five experiments from February 1981 to August 1986 to 

characterize the MOD-2' s acoustic emissions 9 The primary experiments, however, were performed during May 1982 and August 1983, using Turbine No.2 at 

the Goodnoe Hills site. The 1982 experiments collected statistical measurements of high-frequency-range emissions as well as low-frequency data. The 

1983 experiment was des igned to be more narrow in scope but included additional parameters not available in 1982, such as rotor surface pressures and 

v 
TR-3036 

high~frequency turbulence measurements made from a fixed tower location and a 

tethered balloon flown in the turbine inflow. Major modifications were made 

to Turbine No. 2 between the 1982 and 1983 experiments as a result of operational instabilities. These included installing vortex generators along the 

rotor's leading edge over 70% of the blade span and establishing a different 

blade pitch sequence in the control system software. These changes required 

us to stratify the low-frequency data collected during these two experiments 

by year. 

In order to make low-frequency noise measurements in a windy environment, we 

developed a technique that employs a pair of ground-mounted microphones spaced 

15 m apart. Cross-correlation signal processing procedures were then used to 

obtain the in-phase acoustic portion of the signal while largely rejecting the 

random, turbulence-induced contribution" Inflow measurements were made from 

fixed meteorological towers located outside the turbine induction zone and 

from a tethered balloon flown approximately laS rotor diameters (1.5D) upwind 

of the rotor plane.. Both standard-response and high-frequency anemometers 

were us~d on both platformso Twelve surface-mounted pressure transducers were 

attached to the upper and lower surfaces of Blade No. 1 on Turbine No. 2 at 

two span locations during the 1983 experiment. 

The data categories--acoustic, atmospheric, and blade surface pressures--each 

necessitated somewhat different processing procedures. Because of the random 

or stochastic nature of the inflow turbulence responsible for exciting the 

acoustic response of the turbine, we developed a statistical sampling approach 

for presenting and quantifying the radiated acoustic spectra. Consistent with 

this approach, we characterized the turbulent inflow using the methods of 

statistical fluid mechanics and calculated a range of "bulk" flow parameters. We employed standard time-series analysis procedures in determining 

~he MOD-2 aeroacoustic and surface pressure response functions. 

Our detailed measurements of the inflow to Turbine No .. 2 revealed a regime 

that is often stably stratified and contains multiple, thin layers of smallscale, anisotropic turbulence. There is strong evidence of the development of 

an internal boundary layer within the rotor disk's vertical envelope, whose 

formation and depth vary diurnally. Further, the vertical layer encompassing 

the rotor disk is influenced by the presence and breakdown of internal wave 

motions, accompanied by intense, small-scale turbulence. For example, under 

stable condi t ions, typical longi tudinal or axial turbulence component length 

scales are in the neighborhood of 200 m, but those of the vertical or upwash 

(in-plane) component are often more than an order of magnitude smaller. 

Measurements of emissions in the high-frequency range (400-8000 Hz) have shown 

that close to the rotor disk the radiation pattern resembles that of a classic 

quadrupole source. This pattern is then distorted by the prevailing wind at 

larger distances, i e e., extended downstream and contracted upstream of the 

rotor disk. Statistical measures of the A-weighted emissions over periods of 

several hours have shown the observed levels to be essentially normally distributed. The decay of these emission levels with distance (at Goodnoe Hills) 

can be described by the following polynomial: 

L (A) = -3.89464 x4 + 4606729 x3 - 191.884 x2 + 287.15 x - 28.4 , eq 

Vl 
TR-3036 

where x is the loglO of the downwind distance, in feet. The departure from an 

r2 dependence is apparently the result of local propagation effects.. The 

average audible range of a single MOD-2 at the Goodnoe Hills site has been 

estimated to be 1220 m (4000 ft) downwind of the turbineo Statistical 

measurements of the acoustic environment downwind of the site with up to three 

turbines operating show that the turbine noise level experienced by an 

observer is dominated by the closest turbine The effects of multiple turbine 

operation, however, are most noticeable when the turbines are located at 

nearly the same upstream distance from the observer. 

The A-weighted, equivalent sound pressure level or L (A) at a distance of 

1.5D (137 m or 450 ft) from the rotor disk was found et~ vary primarily with 

the hub-height wind speed, though some slight dependency was noted on the 

vertical stability (Richardson number) and wind direction. The L (A) variation, at this reference distance, can be expressed to 't'lithin e~0 .. 5 dB by 

1/2 UH + 57 over a hub wind s (UH) range of 6~15 m/s (13 to 34 mph)@ 

An examination of the variation of 1/3-octave band spectra with inflow characteristics revealed that there was essentially a uniform increase in the 

observed average band pressure levels [L (fl /

3)] across the spectrum with 

wind speed and stabilityo A "peaking e~ehavlor" (distinct peaks in the 

exceedence level, 1/3-octave band spectra) was noted, principally in the L10 , 

L5, and L1 levels of the 2500-Hz band This was most noticeable in measurements made in the plane of the rotor, and it appears to be load-related. We 

believe this peaking characteristic may be related to some form of oscillatory 

fluctuations in the rotor's aerodynamic boundary layere 

Comparisons made between on-axis measurements taken during the 1982 and 1983 

experiments revealed a sharp spectral cut-off in the 1983 emissions above 

1600 Hz 0 Whi le some of the "peaking" behavior 1;-7e noted above was present in 

1983, a downshift appears to have occurred in the 1/3-octave band in which it 

was dominant, ioe .. , 2500 Hz in 1982 to 1000 Hz in 1983. We have attributed 

the lower spectral cut-off and lower "peakingU frequency in the 1983 emissions 

directly to the vortex generators, with their inherent ability to limit boundary-layer separation. It is also possible that these changes may be somehow 

related to the modifications in the pitch angle schedule, lee., perhaps 

because they reduced the maximum attack angles encountered 

No significant, steady-tone noise components were found in analyses of representative narrowband (25-Hz resolution) spectra. This indicates that 

mechanical noise sources associated with the drivetrain are well controlled 

and that there are no discrete aeroacoustic sources of consequence. 

We measured the MOD-2 low-frequency (LF)-range acoustic transfer function 

directly by means of balloon-borne instruments flown in the turbine inflow. 

We found that the radiated acoustic spectrum changes characteristics at inflow 

turbulent scales less than the measured longitudinal or axial integral 

scale, i.e., for turbulent eddies less than this scale length. Statistical 

correlations between five characteristic scales of the inflow and the mean and 

the first three moments of the 1/3-octave band spectrum level distributions 

(expressed as the variance, skewness, and relative kurtosis coefficients) were 

derived from the 1983 data set. Using these five inflow scales as predictors 

in a linear, multivariate model of the band spectral levels, we found that a 

V1.1. 
TR-3036 

high degree of convergence of the model could be obtained; 1 .. e., a high 

percentage of the observed variation of the mean and the first three moments 

could be explained.. The most efficient predictors included the following: 

(1 ) 

(2) 

(3) 

(4) 

A reference mean wind speed (U ) measured at a height z within the 

rotor disk layer (vertical laye; occupied by the rotor disk) 

The gradient Richardson number (Ri) stability parameter measured over 

the rotor disk layer 

The Monin-Obukov length scale, L (see Section 2.4.2), or 

The vertical or in-plane turbulence component scale length along the 

vertical z-direction, Iwz , measured at the height noted in (3). 

The statistical distributions of the emitted 1/3-octave band spectra were most 

highly. correlated with the Uz ' Ri, and Iwz predictors or scales. The 

inclusion of (1) and (4) agrees with the generalized theory of Homicz and 

George for subsonic rotor noise generated in homogeneous, isotropic 

turbulence. The need to include the Richardson number reflects the 

inhomogeneous, vertically stratified characteristics of the rotor inflow at 

the Goodnoe Hills site. We found that an increase in the stability above 

cri tical values (Ri > +0 .. 25) led to a decrease in the vertical or in-plane 

turbulence scales. This in turn had the effect of increasing the LF acoustic 

output below a frequency of 10 Hz, with a corresponding decrease above that. 

We attempted to relate the spectral characteristics of the tower-measured 

axial and in-plane (upwash) turbulence components to the shape of the LF 

acoustic mean 1/3-octave band spectra. We suggest using 

as a fixed to rotating space frequency (f') transformation, where Q 

rotor rotation rate and Ro is the effective radius (75% span length). 

this conversion, we found that 

1S the 

Using 

(1) There is a small positive slope change in the mean acoustic pressure 

spectra, in the vicinity of the rotor effective chord length. This 

also seems to coincide with the isotropic turbulence region (indicated by the two turbulence component spectra becoming parallel). 

(2) The acoustic pressure spectral roll-off approximates a -5/3 slope for 

reference wind speeds less than about 10 mise 

Because of the substantive changes made to Turbine No. 2 between the 1982 and 

1983 experiments, we made an effort to compare the acoustic emission characteristics and their relation to the inflow for both years. We were limited to 

comparing the LF-range results, since we did not have sufficient highfrequency (HF)-range acoustic data from the 1983 experiment. We determined 

that the 1983 configuration of the turbine was far more acoustically sensitive 

to inflow stability.. We also determined that the 1982 configuration was 

influenced by flow stability at all frequencies. We found that the 1983 

emissions exhibited less coherent (impulsive) tendencies above 9-10 mls than 

those of the 1982 configuration. It is clear that, because of whatever 

instabilities were present, the upwind 1982 MOD-2 turbine at times performed 

V111 
TR-3036 

acoustically in a manner similar to its predecessor, the downwind MOD-I. 

Thus, a definite improvement was achieved in reducing the degree of coherency 

in the LF-range emissions by adding the vortex generators and making the pitch 

schedule modifications. 

In order to better understand the physical processes responsible for aeroacoustic noise generation, we performed a space-time correlation analysis 

using three parameters measured on the blade itself and the far-field acoustic 

pressure as measured in the 8-Hz octave band. Our results showed, at least at 

the 87% span station, that the processes related to the observed flap and 

chordwise moments, the blade normal surface pressures, and the radiated 

acoustic pressure field are correlated over time periods of 65-75 ms, which 

translate to a movement of the rotor through about 5 m 1n space. 

Our experience with the MOD-l turbine reinforced the desirability of assessing 

the MOD-2' s potential to cause interior annoyance problems in- nearby residential structures by means of low-frequency acoustical loads. Through a 

limited, interior low-frequency noise evaluation experiment, using volunteer 

subjects, we identified what we believe to be an efficient descriptor or 

metric for measuring the degree of annoyance from such stimuli. From data 

available to us, we modified the derivation of this descriptor to include 

internal dynamic pressure effects resulting from the application of external, 

low-frequency acoustical loads. Using this modified descriptor, we then 

developed a procedure for establishing a "figure-of-merit" for a given turbine, which attempted to take into account worst-case conditions of surface 

reflection and atmospheric focusing. By using 1/3-octave band acoustic 

spectra measured at a reference distance from a turbine's rotor plane, we were 

then able to establish a predicted worst-case figure for the MOD-2. We were 

then able to compare that result with the documented community annoyance associated with emissions from the MOD-l operating at both 35 and 23 RPM. 

Conclusion 

We determined from our analysis of both the high- and low-frequency-range 

acoustic data that annoyance to the community from the 1983 configuration of 

the MOD-2 turbine can be considered very unlikely at distances greater than 1 

km (0.6 mile) from the rotor plane. 

1X 
TR-3036 

TABLE OF CONTENTS 

1.0 Introduction ........ e •••••••••••••• 9................................. 1 

1.1 

1.2 

1.3 

Characteristics of the MOD-2 Turbine •••••••••••••••••••••••••••• 

Background ••••••••••••••••••••••••••• 

1.2.1 The MOD-1 Turbine ••••••••••••• 

1.2.2 

SERI's 

Related Studies •••••••••••••••••••••••••••••••••••••••••• 

MOD-2 Acoustic Characterization Program Objectives ••••••• 

1 

1 

2 

4 

4 

2.0 Investigative Procedure.............................................. 6 

2.1 

2 .. 3 

2.4 

MOD-2 

2.1.1 

2.1.2 

2.1.,3 

201.4 

Field Studies."" 

February 1981 ... 

May 1982 0 • Q " • QQ. 0 0 • 0 •• e e 0 EJ e e 0 ••• ., •• 0 6) 0 0 • €I) 0 e e 0 0 • 49 ••• ., ., • e •• 

Augus t 19830. 0 Q • e • G 0 •••• CD " 0 0 • "0 0 0 Ii) ., •• 0 0 ••• 0 •••••• e 0 •••• 1& •• 

Augus t 1986" 0 • eo ... e 0 II ...... cD $I •• " ••• e 0 e •• " • Ill) • " ....... e •••• 4) • 

Ins t rume n tat ion.. " 0 0 " " " e " ....... " " " • • .. .. " " " co .. " • .. " .... " .. co • " 0 0 " " " .. • " • .. .. .. .. 

2.2.1 

2 .. 2.2 

Acoustic Measurement Instruments •• ".o ............. " ••••••• 

Atmospheric Measurement Instruments ................ ""." •• ". 

2 .. 2.2.1 Tower-Mounted Measurements ..... " •• " ....... " •••••••• 

202.2.2 Tethered Balloon Measurements ••••••••• " ......... . 

2.2 .. 3 Turbine Rotor Surface Pressures ••• o •••••••••••••••••••••• 

2.2.4 Turbine Operational Information •••••••••••••••••••••••••• 

2.2.5 Data Recording ••• e ••••••••••••••••••••••••••••••••••••••• 

Experimental Procedures •••••••••••••••••••• e •••••••••••••••••••• 

2.3.1 The 1982 Experiment •••••••••••••••••••••••••••••••••••••• 

2.3.2 The 1983 Experiment •••••••••••••••••••••••••••••••••••••• 

Data Reduction Procedures ••••••••••••••••••••••••••••••••••••••• 

2 G 4 • 1 Ac ou s tic Da t a • • @ .. " " G • " •••• " • " • " ....... " " ...... " ...... " ......... .. 

2.4.1.1 Low-Frequency-Range, Coherent Random Sampling 

Technique." ••••• o •••• 

High-Frequency-Range, Random Sampling 

Tee hn i que. 13 • 0 • til Q Ii) 0 • • e e 0 • e • II • e • Ii) • • " ., • • • ., • • • • • 1& • • e 

2.4.2 Atmospheric Data"" .... "" •••• ".""" •• ,, .......................... .. 

2.4.2.1 Mean Inflow Characteristics ..................... . 

2.4.2.2 Turbulent Inflow Characteristicso ••••••••••••••• 

2.4 .. 3 Rotor Surface Pressures ••• e •• 0 ........................... . 

6 

6 

6 

6 

7 

7 

7 

8 

8 

10 

12 

12 

12 

12 

14 

19 

19 

20 

22 

25 

25 

25 

35 

36 

3.0 Description of Goodnoe Hills Inflow Structure." ........................ 37 

3.2 

3.3 

3.4 

Identification of the Acoustically Important 

Inflow Properties... • .............. " ................................. . 

Determining the Vertical Distributions of UH, I.J, and w'2 •••• 

302.1 Surface Layer Similarity Sca1ing .... Qo ••• : .............. . 

3.2.2 The Vertical Distribution of UH (Vc ) ••••••••••••••••••••• 

3.2.3 Variation of w' Spectra with Height ......... e ................ . 

Inflow Data Statistical Summaries •••••••••• " .................... . 

1983 45-m Inflow Turbulence Spectral Content •••••••••••••••••••• 

x 

37 

41 

41 

42 

42 

43 

45 
TR-3036 

TABLE OF CONTENTS (Continued) 

305 Rotor Disk Inflow Vertical Profilesooooeoooooeoo ••• 0000 ••••••• 0 45 

305.1 PNL Tower Vertical Profiles of Wind Speed and 

Turbulence Intensityo ••. 00 •••••••• 0 ••••• 0 ••••• 00 ••••• 46 

3.502 Representative Tethered Balloon Vertical Profiles 

in Turbine No 2 Inflowooo ••• oo •• oo ••••• ooo •• e •• o ••• o •• 46 

400 Characteristics of MOD-2 High~Frequency-Range Emissions •••• o.e •••• o 60 

4.3 

Observed Directivi Patternoo 

Statistical A-Weighted Emission Distributionsoe. 000 ••• 00.00 .••• 

4 2.1 Single Turbine 

4.2 2 Multiple Turbine 

Influence of Rotor Inflow on HF Noise Generationo" ... 0" 0" 0 0 0 ... 0 

40301 A-Weighted, 

40302 Spectral 

Sound Pressure Level Variationeooo 

High~Frequency~Range Emissions @000 

Typical 

4 3,,2.1 

40302 2 Establishment of 

Measurement Locations00000@ 

Background Spectral 

Reference 0 0 0 000000009 0 

4 3 Rotor Inflow Influence on Spectral 

Distribution~ 0 00 eo 0000000.00000.00 ••• 

Narrowband Spectra@oooaoooo .o@oooo 

60 

60 

62 

64 

65 

65 

67 

67 

69 

71 

85 

5.0 Characteristics of Emissions00o. 90 

5.1 Influence of Rotor Inflow Structure on LF Noise Spectra e •• OO.00 90 

5 1.1 HOD-2 Aeroacoustic Functiono. BOO" 0 • 0 0 ... 0.0 • 90 

5 1el01 Di t Measurement Approach... • •• 000000.0 •• 0 90 

5 1 1 2 Inflow Bulk Scaling Parameter/Multivariate 

Model Approach oooo@ eeao ••••• 0 ••••••••• 0. 93 

501.103 Model Interpretationoo 00 0.000 •••• 0 000.0@0000 114 

501 104 Case Studies of the Role of Inflow Structure 

on Radiated tral Characteristics.eee ••••••• 118 

5 1.1.5 Relationship of Inflow Spectral 

Characteristics to the Mean LF 

Acoustic trum. 000.0 " ••••• 0 .e ••• ooo •• 147 

502 Comparison of 1982 and 1983 Results via Regression Modeling ••••• 151 

5e2.1 Regression Modeling of 1982/83 Data o ••••• e ••••• e.e •• eo 151 

5.3 Comparison of MOD~2 and MOD-2 LF Emissions Characteristicso ••••• 153 

5.3.1 Statistical Measures of Impulsiveness or Coherency ••••••• 157 

5.3.1 1 Band Correlated Spectral Levels.e •• oo •• 157 

503 1 2 Statistical BSL Exceedence Values e ••••• eo •• 159 

50302 Degree of 1982/83 MOD-2 vs. MOD-l Emissions Coherency •••• 159 

5.3.3 Comparison of 1982/83 MOD~2 Exceedence Analysis •.•• 0 ••• 161 

5.,4 Observed Physical Scales of MOD-2 LF Noise Generation. 0 •• e ". 161 

Xl 
TR-3036 

TABLE OF CONTENTS (Concluded) 

Page 

6.0 Measuring the Annoyance Potential of a Single MOD-2 Turbine ••••••••• 168 

6.1 Additional Comparisons of MOD-1 and MOD-2 Emissions 

Characteristics and Their Relationship to Interior 

Annoyance Potential"." 0 .......... " ... " ." ••• " ••• " •• " ...... It It It .... " .. ,," .. It .. 168 

6.2 Use of the PLSL Metric in Assessing Potential Interior 

Annoyance Q •••• tit " • " •• 0: • l1li f) •••• e ... ,. •• e ••• " • ., • ., CIt ., •••••••• cD It 0 •• \9 • 0 • •• 170 

6.2.1 Synopsis of Results of Interior Low-Frequency 

Noise Evaluation Experiment ................................ 170 

6.2.1.1 Identifying an Efficient Estimator of 

Interior LF Annoyance ••••••••••••••••••••••••••• 171 

6.2.1.2 Establishing an LSL Annoyance Scale ................ 173 

6.2.2 A Methodology for Predicting Interior LSL Values ••••••••• 173 

6.2.2.1 Predicting an Interior LSL Level .................. 173 

6.2.2.2 Establishing a Reference External Acoustic 

Load i ng 0 $ • e .... G e • e 0: e •• " •••• lit " • e e • ., EJ e • e •• Ii • lit €I • 4) eel 74 

6.3 Estimating the Community Annoyance Potential of Both an 

Individual MOD-2 Turbine and Clusters of Turbines ................ 176 

6.3 .. 1 Annoyance Potential from High-Frequency-Range 

Emissions ....... " ................................................ " ......................... 176 

6.3.2 Interior Annoyance Potential of Low-Frequency-Range 

Em iss ion See 0 • e e e e e e 0 e e • • II • • $ • • CD • e 8: • e • e e e 0 e ., • e • 4) • • e e • ., • • e. 1 7 7 

7.0 Conclusions ..... ee ••••• eee.e ••••• ee ••• ee •••••••••••••••••••••••••••• 178 

7.2 Low-Frequency-Range Acoustic Characteristics ..................... 178 

7.3 Comparisons of MOD-2 Low-Frequency Emission Characteristics 

with Those of the MOD-I ............................................. 179 

7.4 Community Annoyance Potential of a Single MOD-2 Turbine ••••••••• 179 

8.0 Recommendat ions. " " ... " ............ " " ............. " ... " ......... " ...... " " .... " " " .... " 180 

9 e 0 References 0 0 .... 0 • 0 • " 0 9 I!J 0 0 • e iii 0 e • 009 e 0 lit fit • e e 0 0 •• eo" " 0 ., 0 • 0" ..... e lit ••••• .,. 181 

Xll 
TR-3036 

LIST OF FIGURES 

1-1 The MOD-2 Wind Turbine.ee ••••• Qo ••••• e.eo ••• o ••• e •••••••••••••••••• 2 

1-2 Schematic of the MOD-2 Configuration •••••• e •••••• e ••••••••••••••• 3 

1-3 Cluster Arrangement of MOD-2 Turbines at Goodnoe Hills Site •••••••• 5 

2-1 Microphone Measurement Station.GaGe. 0 •••••••••••••••••••• 0 •••••••• 8 

2-2 Acoustic Instrumentation Configuration for 1982 Experiments •••••••• 9 

2-3 Triad of VLF Microphones Used for 1983 Experiments ••••••••••••••••• 9 

2-4 Site Layout Schematic Showing Locations of Meteorological Towers 

with Respect to the Turbines .0 ••••••••••••••••• e •• 0 ••••• @ •• 0 ••• 10 

2-5 Detail of Inflow Instrumentation Carried by Tethered Balloon.o •• G00 11 

2-6 Typical position of Tethered Balloon in Turbine No 2 Inflow •••••• @ 11 

2-7 Data Flow Path for Inflow Measurements •••• o .000 •• 0.0 13 

2-8 Surface Pressure Tap Installation at Blade No.1, Station 1562, 

15% Chord Position ••• @ •••• 0.0 ••••• 00 0 •••• 0 •••••••• 0 ••••• 14 

2-9 Data Sources and Recording Systems Configurationo ••• e ••• e ••••••••• 15 

2-10 Schematic Representation of an Averaged Radiated Sound 

Pressure Spectrum from a Large Wind Turbinee ••• o •••••••••••••••••• e 21 

2-11 Schematic Sound Spectrum of Figure 2-10 with Residential 

Construction Vibration Modes and Applicative Damping 

2-12 Low-Frequency-Range Coherent Processing Flow o •• o.o.e •••••• 

2-13 Example of Low-Frequency Acoustic Data Reduction Output: 

Observed Frequency Distributions of 8-Hz 1/3-0ctave Band 

Spectrum Level for Turbine Operating (Run #1) and Background 

21 

24 

Conditions (Run #2) •••••••••••• 0 •••••••••••• 00000.0@0.000000 •• 26 

2-14 Example of Low-Frequency Acoustic Data Reduction Output: Mean 

and Peak 1/3-0ctave BSL Spectra for Turbine Operating (Run #1) 

and Background Conditions (Run #2) 000 0 @0$00 GOe. 00.e •••••• 000e 27 

2-15 Example of Low-Frequency Acoustic Data Reduction: Exceedence 

Level (L.) Comparison for Turbine Operating (Run #1) and. 1 Background Conditions (Run #2)0 ••• 0 ••••• 0 ••••••• 00 ••• 0 ••••••••• 0 •• 0 28 

2-16 High-Frequency-Range Acoustic Data Processing .•• ooo •••••••••••••••• 29 

Xll1. 
~-~ 

/( I 

"-~ 

LIST OF FIGURES (continued) 

2-17 Example of High-Frequency Acoustic Data Reduction Output: 

Observed Frequency Distributions of A-Weighted BPL for Turbine 

Operating (Run #1) and Shut-Down Conditions (Run #2) in High 

TR-3036 

Winds., • 0 • e • e It " CD ., • e (} • 0 G .... 0 • $ • 8 CD •• " • lib • (} 0 ID 0 • e • 0 e e • G .... e CIt • CD •••• " \l!l • e • • • 30 

2-18 Example of High-Frequency Acoustic Data Reduction Output: Mean 

and Peak 1/3-0ctave BPL Spectra for Turbine Operating (Run #1) 

and Shut-Down Conditions (Run #2) in High Windse ••••••••••••••••••• 31 

2-19 Example of High-Frequency Acoustic Data Reduction Output: 

Exceedence Level (Li ) Distribution Comparisons for the Case 

of Figures 2-17 and 2-18 •••••••••••••••• 0 •••••••••••••••••••••••••• 32 

3-1 Rotor Geometry Used by Homicz and George.e •••• o •••• e ••••••••••••••• 38 

3-2 Schematic of L x, L x L z Length Scales with Respect 

to Turb1ne RotOr •• '.: •• ' ••• 0 ••• 0 •••••••••••••••••••••••••••••••• 40 

3-3 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983--Run ADS •••••••••••••••••••••••••••••••••••••••••• 47 

3-4 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983-Run A03 •••••••••••••••••••••••• e •••••••••••••••••• 48 

3-5 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983 Run A14-1.e ••••••••••••••••••••••••• ee ••••••• e •• e • 47 

3-6 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983-Run A14-2.e ••••••••• e •••• e •••••••••••••• eee8 ••• e •• 49 

3-7 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983-Run A18 •••••• e.e.e •••••••••• e •••••••• e •••••••••••• 48 

3-8 BPA Tower 45-m Level Longitudinal (u) and Vertical (w) Inflow 

Spectra for 1983-Run All ••• ee ••••••••••••••• ee ••••••••••••••••••••• 49 

3-9 Smoothed Vertical Profile (PNL Tower) of Uhub-Normalized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1 983""""" - Run A 03 (9 fit 0 0 fJ (I) e G 0 lit G • 0 • €I e e 0 lit 0 G 0 0 til e e e e 0 lit I) 0 e • 0 €I 0 0 • 0 •• 0 G • Cit fit • 13 ., ., • e 0 5 0 

3-10 Smoothed Vertical Profile (PNL Tower) of Uhub-Normalized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1983--Run A14-1 e G • 19 0 e " 0 0 G 4) • " • e •• e 0 e 0 0 • e Ii) e e e 0 0 e G fJ e G •• 0. 0 • 0 • 0 ••• 0 e 0 e 0 51 

3-11 Smoothed Vertical Profile (PNL Tower) of Uhub-Normalized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1983--Run A14""""2 €I G a CIt • G a 0 e 19 €I a • €I: CIt €I • 0 e I) 0 • G e G • II/) 1& 0 G fJ ., €I ., •••• 0 " ••• G " ••• 0 • 0 • 52 

3-12 Smoothed Vertical Profile (PNL Tower) of Uhub-Normalized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1983-....... Run Al8 .. 0 e €I e 0 0 e €I: 0 e 0 Q e 0 0 it III e 0 0 m e 0 0.0 a a. 0 It 0 0. e e e €I e e ••• 0.0 •• e.. 53 

X1V 
TR-3036 

LIST OF FIGURES (continued) 

3-13 Smoothed Vertical Profile (PNL Tower) of Uhub-Norma1ized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1983--Run AlleeGooeooo 00000 0 

3-14 Smoothed Vertical Profile (PNL Tower) of Uhub-Normalized 

Horizontal Wind Speed (a) and Turbulence Intensity (b) for 

1983--Run A05 .. ". eo. e 0 00. • 0 

3-15 Smoothed and High-Resolution Vertical Profiles of Wind Speed 

54 

55 

for Run AOSOGeee00G 6000$000 0 e0eee r:g')e 130000000.006 •••• 000 •• 00 56 

3-16 Smoothed and High-Resolution Vertical Profiles for Wind Direction 

for Run A05 ••••••• 00 •••• 0 57 

3-17 Smoothed and High-Resolution Vertical Profiles of Sensible 

Temperature for Run AOSao 00 ••• e. 000 0 0 ••••• 0 ••• 0.00 ...... 0 ••• 58 

3-18 Narrowband Longitudinal Turbulence Spectrum Measured at 

78-m (±6 m) Height for Run AOS. 00 000 .0 ••• 0 ••• 00 ••••• 0 •• 0 •••••••• 59 

4-1 Microphone Locations Used in High-frequency-Range Measurements.e ••• 65 

4-2 A-Weighted Sound Pressure Level Directivity Pattern for a Single 

MOD-2 Turbine. 0.000 ••••• 000 •••••••• 62 

4-3 Cumulative Distributions of A-Weighted SPL at Three Downwind 

Distances from a MOD-2 Turbineoe00@@ ••..• 000 ••••••••••• 90. 63 

4-4 Average Tu:bine Leq(A) Emissions Decay Downwind of a Single 

MOD-2 Turb1neeoGoGGG QOtloCi 0 00 006 00000 @ GO<OO'lllloeoaGoG&eOeeOfi!l00 •• 63 

4-5 Cumulative Distributions of A-Weighted 8PL at Site No.2 (see 

Figure 4-1) for Two and Three Operating MOD-2 Turbines ••••••••••••• 64 

4-6 Leq(A) Levels as a Function of Hub-Height Wind Speed ••••••••• o ••••• 66 

4-7 Observed Frequency Distributions of A-Weighted SPL for Highest 

and Lowest Mean Hub-Height Wind Speedse •• 0.0 ••••••• 0 ••••••••••••• 68 

4-8 Comparison of Background Mean BPL Spectra for Highest and Lowest 

Mean Hub-Height Wind-Speed Condi t ions o. 0 • • •• .. •• " ............. 69 

4-9 Comparisons of Observed Mean SPL Spectra for Operating and 

Background Condition~ in High Ca) and Low (b) Windso •••• o ••• o •••••• 70 

4-10 Comparison of Background HF Acoustic Spectra at On-Axis Microphone 

Location for the Same Mean Hub-Height Wind Speed during 1982 and 

1983 Experimentseeo GOOGe 000GGQlQ00Ge 0 GG0000"'000GOooooeeooO.e.OG 71 

xv 
TR-3036 

LIST OF FIGURES (continued) 

4-11 1982 HF-Range Acoustic Spectral Distribution for On-Axis (solid) 

and In-Plane (dashed) Locations as a Function of Wind Speed and 

4-12 

4-13 

4-14 

4-15 

4-16 

4-17 

4-18 

4-19 

4-20 

4-21 

4-22 

4-23 

Stability. e • .,. o. e e G Il.t. fit 4) 0 e eo" e •• ., .... 0" 0 0 e Ii) (I) I/) 0 fiJ CD. 0 e 0 e. e e e 0 e •• " t!J CD ... e.. 72 

Comparison of the On-Axis Component of the HF Spectral 

Distribution between 1982 and 1983 under High Loading Conditions ••• 

Comparison of On-Axis (solid) versus In-Plane (dashed) HF Spectral 

Emissions for Neutral to Slightly Stable Inflow Conditions ••••••••• 

Comparison of L1 , LS' LI0 , L20 , and LSO 1/3-0ctave Level Spectra 

at 1.5D at the On-Axis and In-Plane Measurement Locations for 1982 

Runs: (a) 17-2; (b) 19-1; and (c) 26-1 ••••• 0 •••••••••••••••••••••• 

Comparison of Mean 1/3-0ctave BPL Spectra for Mean Wind Speeds 

of 9.3 and 1003 mps under Stable Conditions •••••••••••••••• oo •••••• 

Comparison of L1 , LS' L10 , L20 , and LSO 1/3-0ctave Level Spectra 

at 1.SD at the On-Axis (a) and In-Plane (b) Measurement Stations ••• 

Comparison of Mean 1/3-0ctave BPL Spectra for High and Moderate 

Wind Speeds under Stable Inflow Conditions ••••••••••••••••••••••••• 

Comparison of Ll , LS' LlO ' L20 , and L50 1/3-0ctave Level Spectra 

for High and Moderate W1nd Speeds under Stable Inflow Conditions ••• 

Comparison of Mean 1/3-0ctave BPL Spectra for a Mean Wind Speed 

of 13.3 mps and Stable Conditions (Run 27) and a Mean Wind Speed 

of 14.5 mps and Unstable Conditions (Run 25-1) ••••••••••••••••••••• 

Comparison of L1 , L5 , L10 , L20 , and L50 1/3-0ctave Level Spectra 

for High-Wind, Stable vs. High-Wind, Unstable Inflow Conditions •••• 

Mean and L20 BPL Spectra for 1982 Run R27 and 1983 Run A-II •••••••• 

Mean and L20 BPL Spectra for 1982 Run 26-1 and 1983 Run A18 ..... """". 

Comparison of Acoustic Environment at 1.5D, On-Axis Measurement 

Location with and without the Turbine Operating under High Wind 

74 

75 

76 

77 

78 

79 

80 

81 

82 

83 

84 

Conditions 4) 0 g. 0" 0 It e 0 <it. 0., .... 000. e €t." 0 e e 11) • 0 0 •• ., 0.0.0" tit" 8 0. eo •• 8,. • .,... 85 

4-24 Comparison of L1 , L5 , LI0 , LZO ' and L50 On-Axis 1/3-0ctave Level 

Spectra for Identical Mean W1nd Condit10ns during 1982 and 1983 

Experiment See ... 0 e 0 0 e fill elite •••••• 0 0 13 0 0 II e 0 0 " e • 0 ..... 19 II) G • Q 4) 0 <11 Q • 0 ••• e <I) •• e • 86 

4-25 Comparison of Ll , L5 , LI0 , L20 , and L50 On-Axis 1/3-0ctave Level 

Spectra for Sim1lar Wind Speed Regimes in 1982 and 1983 ••••••••• e •• 86 

4-26 Representative 1982 HF-Range Narrowband (Be = 1e25 Hz) Spectra 

for Very Low Wind Conditions.e.e •••• ee ••••••••••••••••••••••••••••• 87 

XV1 
TR-3036 

LIST OF FIGURES (continued) 

4-27 Representative 1982 HF-Range Narrowband Spectra for Moderated 

but Below-Rated Wind Conditionso0G000000o •••••••••••••••••••••••••• 87 

4-28 Representative 1982 HF-Range Narrowband Spectra for Above-Rated, 

Unstable Inflow Conditions .0 ••••• &00 •••• 0 •••• 000 •••••• 0 ••••••••••• 88 

4-29 1983 On-Axis HF-Range Narrowband Spectra for Above-Rated, Very 

Stable Inflow Conditions.o ••• 0000 ••• 0 •• 88 

4-30 1983 On-Axis HF-Range Narrowband Spectra for Near-Rated, Stable 

Inflow Conditions000 0000000 0000 Q00eO$Og$Oeeae.Cl00&o.oeeOGOoo •• 89 

5-1 Relationship of Location of Measurements on Blade No.1, Rotor 

Disk Geometry, and Height of Elevated Turbulent Layer ••••••• oe •••• e 92 

5-2 Measured Low-Frequency Acoustic Response Function.e •• e 0 •••• 0 ••••• 92 

5-3 Measured Low-Frequency Acoustic Response as a Function of 

Turbulent Space Scale •••• oe •• 0000 ••••••••• 0 •••••••••• 0 •• 0 ••••••• 93 

5-4 Comparisons of the Observed Run-to-Run Variations of the BSL Mean 

and First Three Statistical Moments and the Variations Expected 

from a Purely Random Process ••••••• 0 •• 00 ••••••••• 0 •••••••••••••• 94 

(a) mean BSL comparlSOTIe 

(b) varlance coefficient comparisono 

(c) skewness coefficient comparison. 

(d) kurtosis coefficient comparison. 

5-5 Spectral Run-to-Run Variation ANOVA Results for the 45-m 

Longitudinal (u) and Vertical or In-Plane (w) Turbulence 

Componen t S 8 e 0 e $ 0 I) G @" 0 $ €.I a a 0 @ <) e E:) 19 0 " 0 0 0 a 1& (If 0 e EI 0 I[) iii) e 0 e 0 • t!J •• e 0 G & 0 e tD • 97 

(a) longitudinal component. 

(b) vertical or in-plane componento 

5-6 Observed Variation of the Disk Gradient Richardson Number as a 

Function of the Time-of-Day for the Combined 1982/83 Experimental 

Periods" " •••• 0 0 ••• " 

5-7 Observed Variation of the Disk Gradient Richardson Number as a 

98 

Function of the Time-of-Day for the 1983 Experimental Period ••••••• 98 

5-8 Observed Variation of the Mean Hub-Height Wind Speed as a Function 

of the Disk Gradient Richardson Number for the Combined 1982/83 

Experimental Periods 0 00 eo.., e e Q e 9 0 e e Ii) 0 0 Q e I;} 0 Q • e e .. a ., e (I) €.I 43 e e e 0 0 00 e e e e e 0 99 

XVll 
1.;-~ 

II II 

~-~ 

LIST OF FIGURES (continued) 

TR-3036 

5-9 Observed Variation of the Mean Hub-Height Wind Speed as a Function 

of the Time-of-Day for the Combined 1982/83 Experimental Periods ••• 100 

5-10 Observed Variation of the Height of the Mean Wind Speed Maximum as 

a Function of the Gradient Richardson Number for the 1983 

Experimental Period ••••••••••••••••••••• a •••••••••••••••••••••••••• 101 

5-11 Observed Rotor Disk Maximum Mean Wind Speed as a Function of the 

Richardson Number for the 1983 Experimental Period ••••••••••••••••• 101 

5-12 Observed Rotor Disk Maximum Mean Wind Speed as a Function of the 

Time-of-Day for the 1983 Experimental Period ••••••••••••••••••••••• 102 

5-13 Observed Variation in the Mean Wind Speed at the Five Levels of 

the PNL Tower as a Function of the Gradient Richardson Number for 

the 1983 Experimental Perioda •••••••••••••••••••••••••••••••••••••• 102 

5-14 Observed Variation of the Mean 45-m Horizontal Wind Speed (U) and 

the Horizontal (luX) and Vertical (lwZ ) Length Scales as a 

Function of the Richardson Number •••••••• o ••••••••••••••••••••••••• 103 

5-15 Observed Variation of the Mean 45-m Horizontal Wind Speed (U) and 

Horizontal (fmu ) and Vertical (fmw ) Spectral Peak Reduced 

Frequencies as a Function of the Richardson Number ••••••••••••••••• 103 

5-16 Observed Variations of the 45-m Horizontal and Vertical Length 

Scales as Functions of the Spectral Frequency Reduced 

Frequencies. e II 0 g e Et e flJ (j) 0 0 0 e & 0 • e e 0 6) (} 0 e f!I (I) eo. CIt •• e • G 0 fl) eo. II e 11'1 Ilt 0 tit 0 0 .. eo. e • fit 0 104 

(a) variation of luX versus f mu • 

(b) variation of lwz versus f mw • 

5-17 Ratio of Hourly-to-Diurnal Mean Hub-Height Wind Speed as a Function 

of Time-of-Day for Summer and Winter of 1985/86 Wind Season •••• o ••• 105 

5-18 Observed Diurnal Variation of Actual Hub-Height Mean Wind Speed for 

Summer and Winter of the 1985/86 Wind Season ••••••••••••••••••••••• 105 

5-19 Observed Variat~on .. o~ the 45-m Mean Wind Speed (UH) 

and the Brunt-Vaisala Frequency (N) as a Function of the Disk 

Richardson Number •••••••••••••••••••••••••••••••••••••••••••••••••• 106 

5-20 Observed Variation of the Brunt-Vaisala Frequency (N) and the 

Horizontal (luX) and Vertical (lwZ ) Length Scales as a Function 

5-21 

of the Disk Richardson Number •••••••••••••••••••••••••••••••••••••• 106 

Same as Fig. 5-20 except for Spectral Peak Reduced Frequencies fm 

(Horizontal Component) and f mw (Vertical Component) •••••••••••••• ~. 107 

XVlll 
TR-3036 

LIST OF FIGURES (cont nued) 

5-22 ANOVA Analysis of the Observed Run-to-Run Variation in the 

Longitudinal or Axial (u) Turbulence ComponentOGOe ••• a ••••••••••• eo 108 

5-23 Same as Figo 5-22 but for the Vertical or In-Plane (w) Turbulence 

Component.ooo •• o. 

5-24 ANOVA Analysis of the Efficiency of Multivariate Models Using a 

Series of Predictors for the Observed Run-to-Run Variation of the 

108 

Acoustic MBSL Spectra ••••• 0 ••••• 0 ••• 0 ••••••••••• 0 ••••••• 0 ••••••••• 109 

5-25 ANOVA Analysis of the Observed Run-to-Run Variation of the 

Acoustic MBSL Spectra as a Function of Four Inflow Predictor 

5-26 Same as Figo 5-24 but for the Observed Variation ln the Acoustic 

BSL Variance Coefficient <b2> S pe c t r a Q G 0 !it 0 e 0 0 • () @ 0 e 0 Q G 6 ... 0 ,. • e 0 €I 0 0 I) e & 

5-27 Same as Fig. 5-25 but for the Observed Variation ln the Acoustic 

BSL Variance Coefficient <b2> Spectra •• 00eo 0000." 0000$000 ••• eGO •• CD 

5-28 Same as Fig. 5-24 but for the Observed Variation ln the Acoustic 

BSL Skewness Coefficient <b3> Spectra .. 0 00000 €JGGS €I 00 •••• 000000000 Itt 

5-29 Same as Fig. 5-25 but for the Observed Variation ln the Acoustic 

BSL Skewness Coefficient <b3> Spectra e 0 19 Il'I 0 0 e G e /Ii) 0 9 fit " 0 • 0 0 0 G G 0 0 0 0 0 GO. 0 

5-30 Same as Figo 5-24 but for the Observed Variation ln the Acoustic 

BSL Kurtosis Coefficient <b4> Spectra . 000'11000000&.0.008000000000 • 

5-31 Same as Fig" 5-25 but for the Observed Variation in the Acoustic 

BSL Kurtosis Coefficient <b4> Spectra" ...... o •• &00000 00GOe00.00.8. 

5-32 Characteristic Scaling Sensitivities for Mean Band Spectrum 

Leve 1 s ...... G G • 0 • 0 036$000 $O¢OElCllOOae0eOO00eeOOeQ0000eoeo •• oOOOO.e •• 

5-33 Characteristic Scaling Sensitivities for BSL Variance 

5-34 Characteristic Scaling Sensitivities for BSL Skewness 

5-35 Characteristic Scaling Sensitivities for BSL Kurtosis 

CoefficientSoa0 0QGOalllteao@.ao 00 001900000 000000000009009 

110 

110 

111 

112 

112 

113 

113 

115 

115 

116 

117 

5-36 Normalized 45-m Turbulence Spectra for Runs AlB and A05 •••••••••••• 119 

5-37 Ensemble Statistical Moments of 1/3-0ctave Acoustic Band 

Spectral Levels for Runs Al8 and A050oeo ••••• 0 .•••• 6 •••••••• 0 ••••• 120 

X1X 
TR-3036 

LIST OF FIGURES (continued) 

5-38a Smoothed (a) and High-Resolution (b) Vertical Profiles of Wind 

5-38b Smoothed (a) and High-Resolution (b) Vertical Profiles of Wind 

Direction for Run A05 •••••••••••• o •••••••• e ••••••••••••• 9 •••••••••• 124 

5-38c Smoothed (a) and High-Resolution (b) Vertical Profiles of Sensible 

Temperature for Run A05 •••••••••••••••••••••••••••••••••••••••••••• 125 

5-39 Normalized 45-m Turbulence Spectra for Runs A18 and A14-l •••••••••• 126 

5-40 Ensemble Statistical Moments of 1/3-0ctave Acoustical Band 

Spectrum Levels for Runs A18 and A14-l ••••••••••••••••••••••••••••• 127 

5-41 Normalized 45-m Turbulence Spectra for Runs A14-2 and All •••••••••• 129 

5-42 Ensemble Statistical Moments of 1/3-0ctave Acoustical Band 

Spectrum Levels for Runs A14-2 and All •••• a •••••••••••••••••••••••• 130 

5-43 Normalized 45-m Turbulence Spectra for Runs A03 and A14-l •••••••••• 132 

5-44 Ensemble Statistical Moments of 1/3-0ctave Acoustic Band 

Spectrum Levels for Runs A03 and A14-l ••••••••••••••••••••••••••••• 133 

5-45 Normalized 45-m Turbulence Spectra for Runs A03 and A05 •••••••••••• 136 

5-46 Ensemble Statistical Moments of Acoustic 1/3-0ctave Band 

Spectral Levels for Runs A03 and A05 ••••••••••••••••••••••••••••••• 137 

5-47a Detailed Vertical Profiles (3-m Resolution) of Wind Direction 

for Runs A03 and A05 ••••••••••••••••••••••••••••••••••••••••••••••• 139 

5-47b Detailed Vertical Profiles of Wind Speed for Runs A03 and A05 •••••• 139 

5-47c Detailed Vertical Profile of Thermal Structure (Potential 

Temperature) for Runs A03 and A05 •••••••••••••••••••••••••••••••••• 140 

5-48 Normalized 45-m Turbulence Spectra for Runs All and A18 •••••••••••• 141 

5-49 Ensemble Statistical Moments of Acoustic 1/3-0ctave Band 

Spectrum Levels for Runs All and AlB ••••••••••••••••••••••••••••••• 142 

5-50 Normalized 45-m Turbulence Spectra for Runs A14-l and A14-2 •••••••• 144 

5-51 Ensemble Statistical Moments of Acoustic 1/3-0ctave Band 

Spectrum Levels for Runs A14-1 and A14-2.ee ••••••••• e •••••••••••••• 145 

5-52 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su(f') and Upwash Sw(fl) Turbulence 

Components for Run A05 ••••••••••••••••••••••••••••••••••••••••••••• 148 

xx 
TR-3036 

LIST OF FIGURES (continued) 

5-53 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su(f') and Upwash Sw(f t ) Turbulence 

Components for Run A03.oGaG.a •••••••• o ••••••••••••••••••••••••••••• 148 

5-54 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su{fl) and Upwash Sw(f') Turbulence 

Components for Run A14-1 ••••••• oo •••••••••••••••••••••••••••••••••• 149 

5-55 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su(f') and Upwash Sw(f t ) Turbulence 

Components for Run A14-2 ••••••••••••••••• e ••••••••••••••••••••••••• 149 

5-56 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su(f') and Upwash Sw(f') Turbulence 

Components for Run A18 ••• eo ••• 0 ••••••••••••••• 00 •• 0.0 •••• 0 •••••••• 150 

5-57 Relationship of Mean Acoustic Pressure Spectral Distribution 

Sap(f) to 45-m Axial Su{f') and Upwash Sw(f t ) Turbulence 

Components for Run Al1 ......... ,," ...... e .............................. 150 

5-58 Difference in Mean Band Spectrum Levels for 1982 and 1983 •••••••••• 153 

5-59 Results of ANOVA Analysis for Mean Band Spectrum Level Inflow 

Parameter Sensitivities ••• e.e •••••••••••••••••••••••••••••••••••••• 154 

(a) 1982 turbine configuration 

(b) 1983 turbine configuration 

5-60 Normalized Inflow Parameter Predictor Slopes for Observed Mean 

B and S p e c t rum Level s • • •• ••• 0 • • • " €I • • • • • • • • • • • • • • • • • • 0 • " • • " eo. • • 0 • •• 15 5 

(a) 1982 

(b) 1983 

5-61 Relative MBSL Spectral Sensitivity to Inflow Stability 

(Richardson Number) for 1982 and 1983 Configurations ••••••••••••••• 156 

5-62 Relative MBSL Spectral Sensitivity to Inflow Stability for Mean 

Hub-Height (UH) Wind Speeds •••••••••••••••••••••••••••••••••••••••• 156 

5-63 Relative MBSL Spectral Sensitivity to Inflow Stability for 

Hub-Height Wind Speed Variance (Uf)oeoeoe 000.00 ••• 0 •• 0.0.0.0 •••••• 157 

5-64 Comparison of 8/l6-Hz CBSLs vs. Hub-Height Wind Speed for 1982 

and 1983 MOD-2 Configurations and the MOD-1 Turbineo ••••••••••••••• 159 

5-65 Comparison of 16/31.5-Hz CBSLs vs. Hub-Height Wind Speed for 1982 

and 1983 MOD-2 Configurations and MOD-1 Turbine •••••••••••••••••••• 160 

XXI. 
TR-3036 

LIST OF FIGURES (concluded) 

5-66 Comparison of 31.5/63-Hz CBSLs vs. Hub-Height Wind Speed for 1982 

and 1983 MOD-2 Configurations and MOD-l Turbine •••••••••••••••••••• 160 

5-67 Comparison of Inflow Predictor Sensitivities on Adjacent CBSLs 

and Ccoefs for 1982 nd 1983 Turbine Configurations ••••••••••••••••• 162 

5-68 Sensitivity of Inflow Predictors on Adjacent CBSLs and 

Correlation Coefficients for 1983 MOD-2 Configurations ••••••••••••• 164 

6-1 MOD-1 Mean 1/3-0ctave Band Acoustic Spectra Associated with 

Numerous Complaints of Community Annoyance at 1 km from the 

Turbine <it .. 0 e • G • " tit • e II CD " Q • 0 • 0 ... f) e (i) e •• ., • fit •••• " lit •• ., ••• ., " •• " ., ,. • ., •• flJ • cD .. .. •• 169 

6-2 Comparison of MOD-l Acoustic Spectra of Figure 6-1 with Spectra 

from the 1983 MOD-2 under Lowest (Run AOS) and Highest (Runs All 

and A18) Wind Conditions ••••••••••••••••••••••••••••••••••••••••••• 169 

6-3 Comparison of MOD-l Acoustic Spectra of Figure 6-1 with Spectra 

from the 1982 MOD-2 Operating in a Severe Off-Design Condition 

( Run 25 -1 ) •. 0 e • <I •• CD ...... 4) ,. • '" •• ,. ... " .......... 4) •• CD eo ••••• " ••• ., • II II • II • •• 1 70 

6-4 Low-Frequency Noise Metrics Spectral Weightings •••••••••••••••••••• 172 

6-5 Typical Indoor/Outdoor Transmissibility Functions for Impulsive 

and Nonimpulsive Acoustic Loadings ••••••••••••••••••••••••••••••••• 175 

6-6 Modification of Original LSL Frequency Weighting for Impulsive 

and Nonimpulsive PLSL Measurements ••••••••••••••••••••••••••••••••• 175 

xx!.!. 
TR-3036 

LIST OF TABLES 

1-1 MOD-2 Configuration Characteristics and Features................... 4 

2-1 Original Test Matrix for 1982/1983 MOD-2 Experiment •••••••••••••••• 17 

2-2 Excerpt of Run Configurations for 1982 Single Turbine Experiments •• 18 

2-3 ISO Octave and 1/3-0ctave Low-Frequency Analysis Bands ••••••••••••• 22 

2-4 ISO/l/3-0ctave Bands Used for High Frequency Analysis •••••••••••••• 23 

2-5 Magnetic Recorder Bandwidths and Dynamic Ranges for HF Acoustic 

Recordings during 1982 and 1983 Experiments •••••••••••••••••••••••• 23 

2-6 Bulk Surface Layer Parameters Measured during 1983 Goodnoe Hills 

Experiments from Towers and Tethered Balloon ••••••••••••••••••••••• 34 

2-7 Pressure Tap Locations on Turbine No.2, Blade No.1 ••••••••••••••• 36" 

3-1 Comparison of 1982 and 1983 Inflow Statistics from the BPA 

Meteorological Tower ••••••••••••••••••••••••••••••••••••••••••••••• 44 

3-2 Summary BPA Tower Statistics of Reduced 1982/1983 Data Sets •••••••• 44 

3-3a 1983 Mean Inflow Characteristics Summary from BPA Tower: 

Wind Speeds eo ••• e • ,. 4) e • II 0 ,. Q\ €I 0 •• e ... e €I • 0 .... II ••• e ••• a .. " ......... ., .... 0: ,. .. • • 45 

3-3b 1983 Mean Inflow Characteristics: Characteristic Scaling 

Par arne t e r S It 0 4) ••• 0 • 0 • ., a 0 III 0 • tlI •• " e fit ..... 0 ••••• ., • 8 '" .. lit .... " ......... 8 • CD •• " • • 45 

3-3c 1983 Mean Inflow Characteristics: Spectral Scaling and Rotor 

Disk Layer Parameters •••••••••••••••••••••••••••••••••••••••••••••• 46 

4-1 Inflow Conditions and Resulting Leq(A) Levels for Representative 

1982 and 1983 Data Runs at 1.5De.e ••••••••••••••••••••••••••••••••• 65 

4-2 Multivariate Regression Correlations of MOD-2 <Leq(A» Levels 

with Hub-Height Wind Vector and Vertical Stability ••••••••••••••••• 66 

5-1 Summary of Important Turbulence Excitation and Turbine Operating 

Flow Angles for the Analyzed Segment of Run ADS •••••••••••••••••••• 91 

5-2 Bivariate Correlation Coefficients for Inflow Turbulence Scales 

and Intensities versus Stability, Velocity, and Roughness •••••••••• 96 

5-3 Cross-Correlation Matrix for Stability, Velocity, and Roughnesse ••• 96 

5-4 Inflow Structure Comparisons Data •••••••••••••••••••••••••••••••••• 118 

5-5 Overlap Ranges of Reduced 1982/83 Inflow Data Sets ••••••••••••••••• 152 

XXIII 
TR-3036 

LIST OF TABLES (concluded) 

5-6 Comparison of Mean 1/3-0ctave Band Spectral Levels for the 

Conditions in Tables 3-2 and 5-9 for 1982 and 1983 Runs •••••••••••• 152 

5-7 Statistical Summary of Correlated or Coherent Octave Band 

Spectrum Levels Measured On-Axis at 1.5D for 1982/83 MOD-2 

Data Runs Compared with the MOD-l Turbine at 35 and 23 RPM ••••••••• 158 

5-8 Comparison Summary of 1982/83 Adjacent-Band Correlated 

Spectrum Levels for Reduced Meteorological Data Set of 

5-9 

5-10 

5-11 

Table 3-2 and the 23-RPM Operation of the MOD-I •••••••••••••••••••• 163 

Comparison 

Levels for 

the 23-RPM 

Exceedence 

MOD-2 Rotor 

Aeroelastic 

Summary of 1982/83 LF-Range Octave Band Exceedence 

Reduced Meteorological Data Set of Table 3-2 and 

Operation of the MOD-l •••••••••••••••••••••••••••••••••• 165 

Level Summary for 1983 LF-Range Octave BSL Values ••••••• 166 

Space-Time Correlation Scales for Four Aeroacoustic/ 

Par arne t e r s ... " .......... " • " .. " •• " " " ..... to ....... " .. • • • • • • • • • .. .. .. .... 167 

6-1 Low-Frequency Noise Environments Subjective Ranking Criteria ••••••• 172 

6-2 Correlation Coefficients of Volunteer Ratings of Low-Frequency 

Noise Stimuli versus Six Noise Metrics ••• " ............................ 173 

6-3 SERI Interior Low-Frequency Annoyance Criteria Employing the 

LSL Metrics .••••••••••••••••••••••••••••••••••••••••••••••.•••••••• 174 

6-4 Predicted Interior LSL (PLSL) Values at 1 km from the MOD-2 

Turbine (1983 Configuration) ........................................ 176 

XX1V 
A 

ANSI 

a o 

B 

b 

b2 

b3 

b4 

BAC 

BPA 

BPF 

BPL 

BSL 

BUREC 

C 

c 

CBSL 

Ccoef 

CNA 

c p 

D 

dB 

DOE 

DOI 

Dr 

EDS 

f 

FM 

f . ml 

NOMEECLATURE 

ISO "A" spectral weighting 

American National Standards Institute 

local acoustic velocity 

Bowen ratio; number of blades ln rotor 

blade span length 

variance coefficient 

skewness coefficient 

kurtosis coefficient 

Boeing Aerospace Corporation 

Bonneville Power Administration 

blade passage frequency 

band pressure level 

band spectrum level 

Bureau of Reclamation 

ISO "c" spectral weighting 

rotor chord dimension 

coherent octave band spectrum level 

cross-correlation coefficient 

community noise analyzer 

rotor chord length at effective radius Ro 

specific heat at constant pressure for air 

turbine rotor diameter 

decibel a 

u.S. Department of Energy 

u.S. Department of the Interior 

rotor-surface force fluctuation spectral density 

Engineering Data System 

reduced frequency parameter, f=nz/U 

frequency modulation 

i-th component Si (f) peak reduced frequency 

local gravity acceleration 

proposed ISO spectral weightings 

vertical heat flux 

xxv 

TR-3036 
Hz 

ISO 

I x 

u 

I x 

w 

I z 

w 

K 

L1/ 3 

Leq 

LF 

Li 

LSL 

LSPL 

Mc 

Mo 

MOD-l 

MOD-2 

MSL 

N 

n 

NASA 

Pa 

PG&E 

Phub 

Pi 

PLSL 

PNL 

NOMENCLATURE (continued) 

surface vertical heat flux 

high frequency 

SI unit of cyclic frequency (cycles per second) 

International Organization for Standardization 

longiLudinal length scale along x-axis 

vertical length scale along x-aX1S 

vertical length scale along vertical (z) axis 

degrees Kelvin 

von Karman constant 

Monin-Obukov length 

one-third octave band level 

time-averaged sound pressure level 

low frequency 

sound level exceeded i-th percent of time 

low-frequency sound level 

low-frequency sound pressure level 

axial convection Mach number 

rotor speed Mach number 

DOE/NASA 2-MW wind turbine design 

DOE/NASA 2.S-MW wind turbine design 

mean spectrum level 

Brunt-Vaisala frequency 

cyclic frequency (Hz); BPF harmonic number 

National Aeronautics and Space Administration 

rotational-broadband transition frequency 

inflow/rotor scaling frequency 

2:1 frequency ratio 

SI unit of pressure 

Pacific Gas and Electric Company 

hub elevation barometric pressure 

i-th component peak turbulence energy 

predicted interior LSL value 

Pacific Northwest Laboratories 

XXVl 

TR-3036 
Psfc 

Qo 

R,r 

Ri 

R· . 1J 

Ro 

Sap 

SERI 

SI 

S· 1 

T 

T,;'( 

TBV 

T u 

Tw 

u l , w' 

U45 

UH 

Umax 

Uz 

Vc 

VLF 

W4S 

WH 

x 

z 

Greek 

NOMENCLATUEE (continued) 

surface barometric pressure 

surface kinematic heat flux 

slant-range distance to observer 

Richardson number 

autocorrelation function 

effective rotor radius 

far-field acoustic pressure spectral density 

Solar Energy Research Institute 

International System of Units 

i-th component logarithmic spectral density 

absolute air temperature 

surface layer temperature scale 

Brunt-V~is~l~ period 

longitudinal time scale 

vertical time scale 

1-10 Hz band, mean square turbulence values 

friction velocity 

mean along-wind component at 45 m height 

mean horizontal windspeed at hub height 

rotor-disk peak mean horizontal wind speed 

mean horizontal wind speed at height z 

axial convection velocity 

very low frequency 

mean vertical wind component at 4S m height 

mean vertical wind component at hub height 

along-the-ground downwind distance 

geometric height above the ground 

height of peak mean wind speed in rotortdisk 

surface roughness length 

local a1r density 

rotor plane-to-observer angle 

XXVll 

TR-3036 
NOMENCLATURE (concluded) 

potential temperature 

rotor disk mean potential temperature 

surface Reynolds stress 

rotor rotational frequency 

non-dimensional turbulence wavenumber 

ensemble statistical quantity 

areferenced to 20 ~Pa unless usage indicates otherwise. 

XXVlll 

TR~3036 
TR-3036 

1.0 INTRODUCTION 

This document summarlzes the results of an extensive investigation into the 

factors relating to the acoustic emissions associated with the operation of a 

MOD-2 wind turbine.. Three of the turbines used for the work reported here 

were located at the Goodnoe Hills Wind Turbine Site near Goldendale, Wash., 

which was operated by the Bonneville Power Administration (BPA) for the u.S. 

Department of Energy (DOE). The data for this study were taken primarily from 

the Unit No. 2 turbine at Goodnoe Hills. However, some measurements of highfrequency-range acoustic emissions from all three of the turbines were made 

and are reported on.. The investigations herein were conducted by the Solar 

Energy Research Institute (SERr) on the MOD-2 turbine since 1981 .. 

1 .. 1 of the MOD-2 

The sub ject of thi s study, referred to as the MOD-2 wind turbine, was the 

sixth in a series of turbine designs developed for DOE by the National Aeronautics and Space Administration (NASA) Lewis Research Center as part of the 

Federal Wind Energy Program. Five turbines of this design were constructed by 

the Boeing Aerospace Corporation (BAC).. Three were located in a cluster at 

the Goodnoe Hills Wind Site near Goldendale, Wash.. One was installed near 

Medicine Bow, Wyoo for the u.s. Department of the Interior (DOl), Bureau of 

Reclamation (BUREC). A fifth and privately financed unit was installed in the 

Solano Hills near San Francisco, Calif for the Pacific Gas and Electric 

Company (PG&E). Only the PG&E unit remains in operation. 

The MOD-2 turbine has a rotor diameter of 91 m (300 ft) and is capable of generatil1g 2 .. 5 MW of power at its rated wind speed of about 13 mps (28 mph) The 

rotor is located upwind of the supporting tower and turns on a hub 61 m 

(200 ft) above the ground 0 Figures 1-1 and 1-2 show a photograph and a 

schematic of the turbine, respectively. Table 1-1 summarizes the turbine's 

design and mechanical specifications. Figure 1-3 illustrates the turbine 

cluster arrangement at the BPA Goodnoe Hills wind turbine site. 

1 .. 2 

An extensive investigation of the characteristics of the MOD-2 acoustic emissions was undertaken as a result of the experience we gained with the MOD-I, a 

large downwind turbine. One of the primary reasons for moving the MOD-2 rotor 

upstream of the support tower was to avoid the impulsive, low-frequency noise 

generated by its predecessor.. An extensive study of the MOD-l was made by 

SERI and others and is reported in Ref" [1] We expected that the MOD-2 

upwind configuration would largely eliminate the low-frequency, impulsive 

characteristic of the MOD-l emissions.. We did not know, however, whether 

similar or perhaps greater levels of nonimpulsive, low-frequency noise 

(radiated from the larger MOD-2 rotor) would excite the interiors of nearby 

residential structures enough to reach or exceed the detection threshold of 

the occupantso SERI designed its MOD-2 test program to answer these 

q,ues tions" 

1 
TR-3036 

1-1.. The MOD-2 

1.2.1 The MOD-l Turbine 

From the extensive investigation of the MOD-1 n01se situation detailed 1n 

Ref. [1], we arrived at the following conclusions: 

• The annoyance to the affected residents near the turbine was caused by a 

low-frequency noise phenomenon 

• The source of the phenomenon was a more or less steady stream of acoustic 

impulses, caused by the unsteady aerodynamic loading of the MOD-1 rotor 

blades as they passed through the wake of the support tower. 

• The severity of the situation was enhanced by the focusing of this lowfrequency, coherent acoustic energy on the homes of complaining residents by 

a combination of terrain reflection and atmospheric refraction. 

2 
• The impul s i ve 

resonances 1n 

both direct 

structurese 

Wind 

300 ft 

Foundation 

45 ft 

Teetered 

rotor 

200 ft 

..... ---Controllable tip 

Nacelle 

Tower 

10ftOD 

20 ft OD 

of HOD-2 

lD 

(') 

;;; 

o 

o 

TR-3036 

acoustic loading of the homes produced strong, short-lived 

the structures, which 'lrlere transmitted to the occupants by 

and secondary acoustic emissions within the vibrating 

As a part of the MOD-l investigation, several analysis techniques were developed along with criteria for measuring the degree of coherency in the radiated 

acoustic fielde On at least three occasions we had detailed responses from at 

least two of the affected homes when measurements at the turbine were available We also were able to make simultaneous acoustic and vibration measurements indoors and outside on two of the homes, and correlate these along with 

residents' comments, during the varying acoustic loads of the turbine. Thus, 

we had a very good idea of what was responsible for the annoyance from the 

MOD-l turbine and were in a position to employ this knowledge in evaluating 

the annoyance potential of a , upwind turbine such as the MOD-2. 
Table I-Ie MOD-2 Configuration Characteristics and Features 

Parameter 

Rated power 

Rotor diameter 

Rotor type 

Rotor airfoil shape 

Rotor orientation 

Rated wi~d at hub 

Cut-off wind speed at hub 

Rotor tip speed 

Rotor rpm 

Generator rpm 

Generator type 

Gear box 

Hub height 

Tower 

Pitch control 

Yaw control 

Electronic control 

System power coefficient 

Design Feature 

2,500 KW 

91 m (300 ft) 

Teetered, tip controlled 

NACA 230XX 

Upwind, 2.5 0 tilt 

13 mps (28 mph) 

20 mps (45 mph)a 

84 mps (275 fps) 

17.5 

1,800 

Synchronous 

Compact planetary gear 

61 m (200 ft) 

Soft-shell type 

Hydraulic 

Hydraulic 

Microprocessor 

0" 382 (max) 

TR-3036 

aThe Medicine Bow and Solano turbines have cut-offs of 27 mps 

(60 mph)" 

1.2.2 Related Studies 

The NASA Langley Research Center has been involved in a companion effort in 

studying acoustic emissions and impacts of large wind turbines. The Center's 

primary focus has been on field measurements, propagation studies, and turbine 

characterization as well as studies of psychoacoustical response to wind turbine noise. A number of reports that summarize their efforts are available, 

including Refs. [2], [3], and [4] .. 

1.3 SERIes MOD-2 Acoustic Characterization Program Objectives 

SERI's program for MOD-2 noise characterization had the following objectives: 

A general characterizing of both low- (under 200 Hz) and high-frequencyrange acoustic emissions. 

• The development of a methodology for making acoustic measurements in a windy 

environment. 

• The development of a methodology for relating low-frequency acoustic emissions to the turbulent inflow structure. 

• The development of a methodology for predicting the interior annoyance 

potential of nearby residential structures from acoustic loading from wind 

turbine emissions. 

• The application of the annoyance potential criteria to measured MOD-2 emlSSlons under a range of operating conditions and a comparison of the predictions with similar ones for the downwind MOD-1 turbine. 

4 
TR-3036 

Figure 1-3. Cluster Arrangement of MOD-2 Turbines at Goodnoe Hills Site 

5 
TR-3036 

2.0 INVESTIGATIVE PROCEDURE 

2.1 MOD-2 Field Studies 

Five field experiments were undertaken by SERI from February 1981 to 

August 1986 to support the characterization of MOD-2 acoustic emissions. 

Three of the five were major activities requiring up to four weeks in the 

field. These major experiments were performed in May 1982 and August 1983 at 

the Goodnoe Hills site and in October and November 1983 at the Medicine Bow 

site. Although wake measurements were the primary objective of the Medicine 

Bow experiment, acoustic emission characterization in that environment was 

also programrrled Unfortunately, the acoustic data were affected by the presence of another large, downwind turbine which operated during the bulk of the 

MOD-2 data-taking periods, corrupting the recordings. The sole source of the 

MOD-2 acoustic data utilized in this report was Turbine No. 2 at the Goodnoe 

Hills sitee 

2 .. 1..1 

A brief survey was performed early, during the acceptance tests of Turbine 

No. 1 in February 1981 The purpose of these measurements was to see if any 

impulsiveness could be detected in the MOD-2 emissions, similar to those of 

the large downwind turbines An analysis of the resul ting tape records did 

not reveal a tendency for impulsive characteristics. 

2 .. 1 .. 2. 

A major field measurement was planned for May 1982 at the Goodnoe Hills site 

using all three turbines While continuous statistical distributions of highfrequency-range noise levels for one, two, and three operating turbines were 

collected, low-frequency-range noise data were collected from Turbine No.2 

onlye SERI's tethered balloon system was used to obtain detailed descriptions 

of Turbine No.2 inflow on runs that were within its wind-speed operating 

range. Unfortunately, at that stage in the program's development the turbines 

were operating erratically, particularly in high winds. As a result, major 

reV1Slons were made in the control algorithms and vortex generators were 

installed on the rotors after our measurements were completed. 

2.1.3 August 1983 

A second major experiment was performed using Turbine No. 2 during the last 

two weeks of August 19830 While the experiment's scope was more limited than 

that of the previous year, considerably more information was collected. 

Twelve surface pressure taps were installed on Blade No 1 of the turbine and 

an improved hot-film anemometer was flown on the SERI tethered balloon system. 

A two-axis, hot-film anemometer was also installed at the 45-m level of the 

BPA meteorological tower, about 400 m from Turbine Noo 2. 

A very limited amount of high-frequency-range acoustic data was collected 

since the primary objective of this test series was to ascertain low-frequency 

emission characteristics and their relationship to the turbulent inflow. As 

mentioned above, the turbine had undergone a number of major revisions, 

including a different pitch angle schedule and vortex generators installed on 

70% of the rotor surface. These changes have made it very difficult to 

6 
TR-30J6., 

compare the 1982 and 1983 data sets since the operating characteristics of the 

turbine were substantially altered. Thus, the 1983 data set is considered a 

reference for the MOD-2; four of the five turbines were converted to 

essentially the same configuration. 

2.1 August 1986 

Analysis of the 1982 and 1983 inflow data revealed the existence of wave 

structures that would successively form and then break down into highfrequency turbulence over a period of 30 minutes to an hour., A monostatic 

acoustic sounder and minimal wind recording equipment were installed in 

August 1986 to aid in verifying that (1) the wave structure and breakdown were 

actually occurring, and (2) the phenomena were characteristic of the Goodnoe 

Hills site. 

, 202 Instrumentation 

To understand the complex noi ion processes, several kinds of instruments were necessary.. To characterize the turbine inflow structure both 

tower- and balloon-supported measurements were needed Surface pressure taps 

on one of the turbine blades had to be installed, to understand the unsteady 

blade loads responsible for low-frequency noise emissions. Multiple measuring 

microphone systems used to (1) determine the ,spatial. characteristics of 

the radiated acoustic pressure field and (2) to reduce the effects of the wind 

on individual microphones" Multiple data recording systems accommodated the 

number of data channels required to fully document the noise generation 

processes on the MOD-20 

2 .. 2 .. t 

TJo;·pai~sof ground-level, very-low-frequency (VLF) microphone systems (Biuel 

& KjaerMbdel 2631 preamplifiers with Type 4144 back-sealed microphones) were employed during the 1982 measurements. The microphorie ~air~ were 

spaced 15 m (50 ft) apart, or about a 1/4 acoustic wavelength at 5 Hz. During 

the 1982 program, one pair was placed on the rotor axis 1.,5 roto~ diameters 

(1.5D, 137 m or 450 ft) upwind, and one pair was placed in the rotor plane a 

similar distance from the turbine hub axis" A single, precision sound level 

rnet~r' (ANSI Type-I, Bruel & Kjaer Model 2209 with a Type 4165 microphone) was 

l6cated halfway b~tween the two low-frequency microphones. Figure 2-1 shows 

the ~ placement of one of the microphone stations" In addition to the fixed' 

rni~roph~fie stations, an averaging Type-l sound level meter was used to ·establishthe directi~ity of the MOD-2 A-weighted acoustic emissions making measurement~ over a predetermined grid pattern around the turbine. Two community 

noise analyzers (CNA, GenRad Model 1945) were used to obtain stat'istical 

distribu'tions' of "the A-weighted emission levels at several locations" 

Figure 2-2 summarlzes the acoustic measurements used during the 1982 

experiment" 

A slightly different arrangement was employed during the 1983 experiment. A 

triad' of surface-mounted VLF microphones was located on the rotor axis as 

s,howrl. in Figure 2-3.. This arrangement allowed us to use acoustic ranging rnea- ' 

surement techniques, such as those discussed by Hemphill [5], for locating 

emission source regions within the rotor disk" While not part of the original 

test plan, a Type-l sound level meter was placed between the VLF ~icrophones 

7 
IM 

00 

o 

o 

Figure 2-1. Microphone Measurement Station 

TR-3036 

at the 137-m distance for a few data runs. The on-axis VLF microphone placement was the same as for the 1982 tests, to allow for direct comparisons 

between the two years.. All acoustic measurements taken in 1981, 1982, and 

1983 were referenced to a Bruel &: Kjaer Type 4220 pistonphone corrected for 

the local barometric pressure. 

2.2.2 Atmospheric Measurement Instruments 

Information about the details of the turbine inflow during these experiments 

was very important if any quantitative relationship between the inflow structure and the spectral emission levels was to be achieved. Historically, this 

information has been gathered from multiple, fixed measurement heights 

installed on a meteorological tower near the turbine.. Some attempts have been 

made to use remote probing devices such as bi -static acoustic sounders or 

laser velocimeters, but often with limited success because of operational 

limitations as well as the long averaging times required.. We chose to use 

both tower measurements and a package carried by a tethered balloon flown 

directly upwind of the turbine. 

2.2.2 .. 1 Tower-Mounted Measurements 

The Goodnoe Hills site has two meteorological towers. One, operated by the 

Pacific Northwest Laboratories (PNL), is 107 m tall with five levels instrumented with cup and vane anemometry, sensible air temperature at 15 m, a 15-

to 107-m temperature difference, and local barometric pressure at the 61 m 

(200 ft) hub-height level. This tower is located near Turbine No.1 on the 

8 
'4-", 

[I II 

~-~ 

1.50 (450') ........ ---..., 60 (1800') 

\ ' 

100 (3000') 

\ 

TR-3036 

Turbine No.2 0.7500 (225') " " \ \ 

o 

\ 

\ 

\ 

2-2 

2-3 .. 

'0" 

/ .... -~-.... ' \ " \ \ ? 9 I 

Q-cy.0 ? \ I 

o I 

,.... /o~ 

..... O+A.--" CNA 

site #1' 

.............. / 

--- -0- ....-

400' 

3D (900') 

\ 

o 

J 

/ 

/ 

/ 

/0 

1350 // 

"" ,/ 

/' 

/' 

,/ 

P 

MOD-2 Unit #2 

ttl t t 

Wind 

I 

~---------------------~.#1 

I 

I 

/ 

\ 

I , 1800

? --fOIoWind direction 

O~ 

: CNA, site #2 

I 

I 

I 

I 

I 

, 

I 

I 

I 

I 

/ 

I 

I 

I 

I 

+ Low-freqency microphone pair 

High-frequency microphone (SLM) 

Airborne hot-film probe 

o Roving microphone position 

"f' Tower-mounted microphone 

~ Community noise 

analyzer (CNA) locations 

Legend: 

A solid symbol 

indicates fixed 

measurements. 

for 1982 

Microphone systems 

~--------------~.-----.#2 

Tria.d of VLF Used for 1983 (Plan view) 

9 
TR-3036 

east side of the site. The second tower, operated by BPA, is located on the 

west side of the site between Turbine Nos. 2 and 3. It has a maximum elevation of 61 m with propeller-type anemometry at the 15- and S9-m levels, sensible temperature at 15 m, and surface barometric pressure.. A three-axis 

propeller anemometer, wi th a response of about 1 Hz, was al so temporari ly 

installed at the 61-m level.. SERI mounted a two-axis, hot-film anemometer 

with a response of 10 Hz at the 4S-m level. Figure 2-4 shows the location of 

the two towers with respect to Turbine No.2. 

2.2.2.2 Tethered Balloon Measurements 

In order to make detailed measurements of both the hub-height wind speed and 

direction, as well as the vertical distribution of the wind vector and temperature, SERI employed a commercial tethered balloon system (AIR, Inc.) which 

was modified to carry a O.IS-mm diameter, hot-film anemometer. The standard 

instrument package transmits wind speed and direction, dry- and wet-bulb 

temperatures, and barometric pressure (height) once every 10 seconds over a 

radio link to a ground receiving station where it is displayed and recorded. 

The hot-film anemometer transmits a nonlinear velocity signal with a 12S-Hz 

bandwidth over a special, digital FM-telemetry link.. This link provides a 

signal dynamic range (maximum Signal-to-noi se ratio or SNR) of 70 dB.. The 

received signal is converted to an analog voltage, linearized, and recorded. 

Figures 2-5 and 2-6, respectively, are photographs of the measuring system 

. Prevailing wind direction 

for experimental periods 

~ <00) / 

~/ 

~ BPA 61 m-tower 

_ "9~9 '(('-

N 

t 

o 

... 

ro 

o 

o 

Turbine No.1 

L:::. PNL 107-m tower 

Figure 2-4. Site Layout Schematic Showing Locations of Meteorological 

Towers with to the Turbines 

10 
TR-3036 

Figure 2-5 .. Tethered Balloon 

Figure 2-6 .. ition :in No .. 2 Inflow 

11 
TR-3036 

carried by the tethered balloon and its posltlon in the turbine inflow. Figure 2-7 schematically shows the data flow paths associated with the hot-film 

anemometer and meteorological parameter measurements. 

2.2.3 Turbine Rotor Surface Pressures 

During the 1983 experiment, 12 surface-mounted, pressure transducers were 

attached to the upper and lower surfaces of Blade No. 1 of Turbine No.2. The 

transducers were subminiature, 35 kPa (5 psi) capacity semiconductor, strain 

gage type, each with a frequency response of at least 5 kHz. The site engineering data system (EDS) unfortunately limited the data bandwidth from these 

transducers to 37@5 Hz (-3 dB point).. Six transducers were installed at 

locations equalling 65% and 87% of the blade span at 15%, 40%, and 85% chord 

positions on the upper and lower surfaces. Figure 2-8 shows a typical 

installation at the outer span station. 

2 .. 2.4 Information 

A number of standard engineering measurements were recorded to support these 

tests.. The parameters included the rotor position angle, Blade No.1 pitch 

angle, generator power output, chordwise and flapwise moments at the 21%, 65%, 

and 87% span locations, and flapwise accelerations at the 21% and 67% span 

locations. 

2,,2 .. 5 Data 

Multiple recording systems were required to handle the volume and diversity of 

data sources for these experiments 0 Figure 2-9 summarizes the data sources 

and recording system configurations for the 1983 experiment.. Five separate 

systems were used. Acoustic data were recorded on SERI's l4-channel FM/direct 

tape together with two of the outer span station pressure taps, the corresponding chord and flapwise moments, and the linearized hot-film anemometer 

signal from the tethered balloon. The PNL minicomputer-based data logging 

system was used for the standard meteorological data from both towers. The 

data were later transferred to digital cassettes for transport to SERI. 

The bulk of the pressure tap, aeroelastic information, and operational data 

from the turbine were recorded using the NASA Data Van in FM multiplex format. 

The standard 10-second data from the tethered balloon system were logged on 

its own recorder in serial-digital format. A 7-channel FM recorder was used 

to acquire the two-axis hot-film and the standard 59-m level wind speed and 

direction from the BPA tower All recording systems included time channels 

that were synchronized at the beginning of each experimental period" The 

maximum time uncertainty between recording systems is estimated at less than 

5 seconds for a given run and less than 1 second for the NASA and SERI 

recording systems in particular. 

2 .. 3 Procedures 

A consistent objective of both the 1982 and 1983 experiments was to document 

the acoustic noise characteristics of an isolated MOD-2 rotor under a range of 

atmospheric inflow conditions.. The primary difference between the two years 

was that in 1983 the low-frequency-range emissions were of particular interest, rather than both high and low frequencies, which was the case in 1982. 

12 
Constant 

temperature 

hot-film 

anemometer 

Time-multiplexed 

meteorolog ical 

data via 

UHFFM 

radio link 

Digitally 

encoded 

UHF FM radio 

transmitter 

/ 

10-sec 

duty 

cycle 

Wind speed 

Magnetic heading / 

/ 

Balloon-borne 

SERI data van 

Dry bulb temperature 

Wet bulb temperature*/ 

Static pressure / 

---

Calculator/ 

pri nter 

UHF receiver 

ground 

station/ 

decoder 

• Height 

• Wind speed 

• Wind direction 

• Temperature 

*Not used. 

I 

I 

-- --

-- UHF FM receiver 

Digital-toanalog 

converter 

v 

1-20 Hz 

bandpass 

filter 

Mean 

square 

converter 

Cassette 

recorder 

", 

4th-order 

polynomial 

linearizer 

100-Hz BW 

14-track 

FM analog 

recorder 

Time-of-day 

Acoustic data 

Blade dynamics data 

Operational data 

Voice annotation 

Figure 2.1 Data Flow Path for Inflow Measurements 
TR-3036 

Figure 2-8. Surface Pressure Tap Installation at Blade No.1, Station 1562, 

15% Chord position 

Experience gained from acoustic surveys of other large wind turbines showed 

that the inflow turbulence structure was a major factor in determining not 

only the level and spectral content of radiated emissions, but the degree of 

coherence or impulsiveness as well" Furthermore, SERI v s wind-tunnel and 

atmospheric tests [1,6] showed that inflow turbulence length scales, the order 

of the rotor's mean chord length, were the most important in terms of lowfrequency emissions. Therefore, the experimental procedures used were 

designed to document these key elements: the degree of rotor aerodynamic 

loading, and the turbulent inflow structure and its relationship to other 

atmospheric surface layer parameters. 

2.3.1 The 1982 Experiment 

The 1982 experiment was originally planned to document not only the acoustic 

emission characteristics of a single MOD-2, but the aggregate effects of the 

cluster triad as well. We recognized that these objectives would require at 

least two experimental periods. The original plan, therefore, was to complete 

the individual MOD-2 characterization in 1982 and the cluster effects in 

1983.. The 1982 objective was basically met. Subsequent material changes in 

the turbine physical and operational configuration, however (an alteration of 

the pitch angle schedule and the installation of vortex generators on the 

rotor), necessitated a reassessment of the modified machine. Either or both 

of these modifications had the potential to substantially alter the 

characteristics of the radiated acoustic field. 

14 
Data Sources 

Acoustics 

Low-frequency noise (0.1-200 Hz) 

High-frequency noise (400-8000 Hz) 

Continuous unweighted 

A-weighted SPL 

Statistically-sampled 

Spatially-sampled 

Blade surface pressures/dynamics 

d.c. - 37.5 Hz data bandwidth 

Turbine operational data 

d.c. - 37.5 Hz data bandwidth 

----......,.p.. SERI single FM channel/track 

-----i.p.. SERI single direct channel/track 

----......,.p.. Community noise analyzer 

--------l.1II>_ Type-1 sound level meter 

: 

SER! single FM channel/track 

-:-_-_-_-_---:-;: NASA data van 16 FM channels/track 

- Site EDS 16 FM channels/track 

: 

SERI single FM channei/track 

-:-_-=--=--=--=-~: NASA data van 16 FM channels/track - ~ Site EDS 16 FM channels/track 

BPA meteorological tower 

Hot-film anemometry (d.c. 

Wind direction/speed (d.c. 

10 Hz) -----;.l1li>- SERI single FM channel/track 

PNL meteorological tower 

1 • SERI single FM channel/track 

"-----:.~ PNL site data system (2-s averages) 

Wind direction/speed (2-s averages) 

Temperature/temp. difference 

------r~)rlll>- PNL site data system 

Tethered balloon 

Available 

SNR* 

55 

40 

95 

50 

55 

35 

30-35 

55 

35 

30-35 

45 

45 

46+ 

46+ 

Wind direction/speed, ---.,.-==JfIJ>- Stand-alone digital recording (10-s sampling) 60 

Temperature, height (d.c.- 0.1 Hz) 

Hot-film anemometer (d.c. - 100 Hz) 

*SNR = Signal-to-noise ratio 

----...,-.111>- SERI single FM channel/track 55 

Figure 2-9. Data Sources and Recording 

.n 

.,. 

co 

0 

0 

H 

:::0 

I 

W 

o 

W 

Q"\ 
TR-3036 

The experimental test matrix had to be based not only on field-measurable 

parameters having a direct influence on the inflow turbulent structure and the 

degree of rotor aerodynamic loading, but also on the turbine's relative alignments to assess the acoustic impacts of upwind turbine wakes on downwind 

machines. The turbulent structure reaching the turbine rotor is a function of 

the wind speed regime, the hydrodynamic stability of the vertical atmospheric 

layer occupied by the rotor disk, and the upwind fetch or surface conditions 

(landforms, vegetation, man-made structures, etc.). These parameters are not 

independent and may be a strong function of the local wind azimuth and timeof-day. The diurnal dependence was automat ically determined by the avai lability of sufficient winds for the turbine's operation and not included 

directly in the test conditions. The most commonly used measure of the 

vertical stability, which can be readily determined under field conditions, is 

the gradient Richardson number (Ri) defined in Eq. (2-1), where g is the local 

gravity acceleration, ~8 and 80 are the difference and mean potential 

temperatures for the layer defined by ~z, and ~u is the difference in wind 

speed between those two levels. 

(2-1) 

The potential temperature 8 is given by 

8(z) = T(z)[100/p(z)] 0.286 (2-2) 

where T(z) and p(z) are the absolute temperature (K) and barometric pressure 

(kPa) at height z" Three turbulence regimes have often been assigned to 

ranges of the Richardson number and include the unstable (0 < Ri), the stableturbulent (0 < Ri < 0.25), and the stable-laminar (Ri > 0.25). The turbulent 

structure has been observed to undergo distinct changes in each of these three 

regimes. 

The test matrix elements chosen for these experiments then were based on 

10-minute averages of three locally determined criteria: the hub-height 

(59 m) wind speed and direction from the BPA tower and the gradient Richardson 

number as measured from the 15- and 107-m levels on the PNL tower. Table 2-1 

summarizes the test condition matrix originally planned to be used for the 

1982/83 experiment. The original matrix in Table 2-1 employed three wind 

direction ranges (SW, W, and NW; see Figure 1-4), two wind-speed ranges (7-11 

and 12-20 mps), and the three vertical stability regimes discussed above. The 

two wind-speed regimes were chosen to separate conditions below and above the 

turbine's rated operation (12 mps or 28 mph). The upper operational limit of 

SERI's tethered balloon system at that time was also about 11 mps, so runs in 

the high-speed regime of Table 2-1 were not planned to be supported with balloon inflow measurements (that limitation was removed by redesigning the balloon rigging system before the October 1983 experiment). Table 2-1 also lists 

the desired matrix conditions for the multiple turbine portions of the experiment by alignment (see Figure 2-4); i.e .. , 5 rotor diameters (D) or approximately 450 m (1500 ft) with Turbine 2 and 3 within ±10° of on-axis alignment; 

7D (640 m or 2100 ft) with Turbine Nos. 1 and 2 aligned; and 10 D (915 m or 

3000 ft) with Turbine Nos. 1 and 3. 

Table 2-2 presents an excerpt from the run configuration for the single turbine portion of the experiment. The probability of success was based on what 

was known about the climatology of the site. For example, while we wanted to 

16 
Table 2-1 

Average Wind 

Direction 

SW direct 

(205°-235°) 

or 

W direction 

(255°-285°) 

or 

7D spac 

(270°-290° 

NltJ direction 

(300°-330°) 

or 

laD spac 

(245°-260°) 

a, 1 b' slng e tur lne 

for 

Average Wind 

range 

(7~~ 11 mps) 

range 

(12-20 mps) 

L01;,j'-S range 

range 

turbine 

TR-3036 

Ri Average, 

15- to 107-m Layer 

Unstable 

Stable-turbulent 

Stable-laminar 

Unstable 

Stable-turbulent 

Stable-laminar 

Unstable 

Stable-turbulent 

Stable-laminar 

Uns'table 

Stable-turbulent 

Unstable 

Stable-turbulent 

Stable-laminar 

\ 

Unstable 

. Stable-turbulent 

Stable-laminar 

evaluate the turbine under flow conditions flowing up out 

v.7inds are below 

ratinge Each run 

background runs with the 

of the Col umbia ) 9 a of the 

cut-in speed from this direction; thus, the 1 

with the turbine was 30 minut long while 

turbine shut dOvin were 10 minutes 

17 
Table 2-2. Excerpt of Run Configurations for 1982 Single Turbine Experiments 

;,;.;;;:::~ 

I 

Operating ~ - ~ 

Site Turbines 

Run Code Success WD/ ws Stability Airborne Acoustic Data 

Number Priority Probe Spacing Range Range Config. - Config. Config. 1 2 3 Remarks 

ST27 2 G W II SL AC1 SD1 -'-

ST28 3 F NW L US AD ACI SDI ..,,< 

ST29 1 G NW L US AC ACI SDI ~': background 

ST30 1 G NW L US AC ACI SD1 * 

ST31 3 E NW L ST AD AC1 SDI 

ST32 1 E NW L ST AC ACI SDI ,,;t~ 

ST33 1 E NW L ST AC ACI SDI * ~ 

00 ST34 3 E NW L SL AD ACI SDI background 

ST35 1 E NW L SL AC AC1 SDI * 

ST36 1 E NW L SL AC ACI SDI background 

ST37 2 G NW II US ACI SDI -'-

ST38 2 E NW II ST ACI SDI -'-

ST39 2 E NW II SL ACI SDI 

ST40 1 E W L US AD AC2 SD1 -'-

ST41 1 E W L ST AD AC2 SD1 -'-

ST42 1 G W L SL AD AC2 SDI -'-

ST43 1 E W L ST AC AC2 SDI -,-

Success Probability 

E Excellent 1-3 G Good ~ 

I F Fair w 

P 0 Poor w 

~ 
TR-3036 

2 .. 3 .. 2 The 1983 

The test matrix was slightly in 1983 in response to the reduced 

scope of the objectives and also to reflect the shorter testing period availableo Because of structural cracks developing within the low-speed shaft of 

Turbine No Q 2, the actual time was very 1 imi ted. Enough data were 

collected during the 1982 experiment to that inflow turbulence played a 

major role in determining the ensi ,coherency, etc.) of 

the MOD-2 radiated emi ssions, for acoustic energy contained in 

frequencies below 200 Hz. A is indicated that many of these 

characteristics could be cted with some accuracy from a knowledge of 

(1) the mean hub-height wind , (2) the turbulence intensity or level, and 

(3) the vertical stabili of the occupied by the rotor 

disk. 

The major objective of 

acoustic performance after 

a direct measurement of the 

respons The rotor 

The 1982 

255°-285°, which 

opera t ing hours, 

previous year's 

parameter range 

bulent charac 

foll 

o Unstable ) : 

o Stable-turbulent ST) 

o Stabl ( 

Thus, the modified 1983 test 

Ri, 

used s 

in addi tion to re-evaluating 

modifications, was to attempt 

t turbulence spectral 

this objective., 

wind~direction range, ioeo, 

the desired 

reali ic, in view of the 

the Richardson number stability 

of ion in the turredefined ranges included the 

e wind-direction range, two 

hub-height ranges (same as the 1982 ) and the redefined 

vertical s cla ses US 

consisted of a 30-minute 

a IS-minute period of 

(used when the hub-level ii7inds 

vertical profile of the inflow 

conclusion with the instrument 

nominally 1 S-2 0 rotor 

2 

The acoustic, atma c 

each required somewhat different 

of the stochast of the 

characterized us 

in terms of 

MOD-2 aeroacousti 

continuous time-series 

defined aboveQ Each data run 

was ional concluded by 

tions The tethered balloon system 

were less than 11 mps or 25 mph) produced a 

before the operational run and again at its 

upstream 

19 

near the hub elevation and 

turbine operation. 

hot-film inflow data 

Furthermore, because 

acoustic response of 

been developed for presenting 

This is explained 

the turbulent inflow was 

fluid mechanics and described 

termine direct estimations of the 

response functions, standard 

oyed 
TR-3036 

It was convenient and reasonable to separate the radiated acoustic spectra 

presentations into low- and high-frequency ranges.. The low-frequency range 

« 200 Hz) is essentially controlled by the unsteady airload driven by the 

inflow turbulence encountered in the rotor disk (rotational noise sources). 

The high-frequency range, whose emissions dominate the contributions to the 

A-weighted sound pressure level" is often influenced by so-called broadband 

"self-noise" mechanisms; i .. e., trailing edge vortex shedding, as an example, 

though rotor/turbulence interaction may still playa significant role. The 

two-level frequency range classification was further supported by our previous 

experience with the MOD-l turbine and an Oregon gas turbine peaking generator [25]. Acoustic emissions in the 5 to 100-Hz range were found to produce 

resonant interactions with residential structures [1] causing annoyance to the 

occupants .. 

Figure 2-10 illustrates schematically the typical characteristics of a large 

wind turbine's acoustic spectra, and Figure 2-11 schematically delineates the 

relationship of these emissions to acoustically sens1t1ve resonances in 

residential constructions. The 5- to 100-Hz, resonance-controlled modal range 

shown in the Figure 2-10 damping range can be reasonably described by the 8-, 

16-, 31 .. 5-, and 63-Hz ISO (International Organization for Standardization) 

octave bands or, in more detail, by the corresponding ISO 1/3-octave bands. 

Table 2-2 lists these bands and their associated frequency ranges, with the 

frequency range covered by the 8-, 16-, 31.5-, and 63-Hz ISO octave bands 

highlighted. These four bands encompass 87% of the desired range of 95 Hz 

from 5 to 100 HZe Using the 1/3-octave bands as the minimum frequency resolution is justified over this frequency range because (1) the frequency resolution is small, ranging from 1.2 Hz for the 5-Hz band to 22 Hz for the 100-Hz 

band; and (2) even including the remainder of the bands shown in Table 2-3, 

the numerical and statistical processing demands remain tractable in comparison with performing the same operations with a narrowband resolution of 0.5 

or even 1 Hz over the desired frequency range of 95 Hz. The bookkeeping may 

be compared by noting the volume of data required for summarizing the multidimensional statistics of 20 maximum (ISO 3 to 22) or 12 minimum (ISO 8 to 19) 

1/3-octave bands versus 95 bands with I-Hz resolution and the 190 bands at 

Oo5-Hz resolution for the desired 5- to 100-Hz range. 

The 1/3-octave band resolution was also employed for the high-frequency range 

emission spectra as well. Table 2-4 lists the ISO 1/3-0ctave Bands used for 

the high range as well as the corresponding A-scale weightings.. The highfrequency ranges available in terms of recorder channel bandwidths and dynamic 

ranges for the 1982 and 1983 experiments were limited, as shown in 

Table 2-5. The A-weighted values displayed in later sections are based on a 

truncated frequency range and should be considered estimates rather than 

strictly defined by ISO and American National Standards Institute (ANSI) 

standards. We do not believe the values used are seriously deficient, 

however, since the range of most sensi t i ve human hearing and nominal atmospheric absorption are covered by the bands of Table 2-4 for both experimental 

years. 

20 
r0-

Il. 

::::J.. 

0 

C\I 

(!) 

1.. 

OJ 

:> 

(!) 

..J 

(!) 

!.. 

:::I 

Iff) 

Iff) 

(!) 

1.. 

Il. 

"C 

s:: 

:::I 

0 

U) 

TR-3036 

<D 

'T 

100 Rotational Noise s=-o_u_rc_e_s __ "t"'<---T=-rans.itio-n_,t,<------B roadband Noise ___ .... 

Dominant RegIOn Sources Dominant 

;;; 

o 

o 

80 

70 

60 

50 

2-10 

/ 

/ 

/ 

2-11 

I 

/ 

I 

I 

I 

/ 

/ 

/ 

I 

I 

/ 

Steady and Long Period Unsteady Loading Noise 

r Short Period, Impulsive Loading Noise 

/riTTf"/ /1 I I III "- ;1 I IIIII 

I I I II I 

I I II I 

I :: I 

I I I 

II I I I 

I 

I 

10 100 

Non-Periodic, 

Incoherent 

Loading Noise 

1000 

Schematic of 

21 

Sound 
TR-3036 

Table 2-3 .. ISO Octave and 1/3-0ctave Low-Frequency Analysis Bands 

Nominal Third-Octave Octave Center Band No .. Frequency Passband Passband 

(Hz) (Hz) (Hz) 

3 2 1 .. 78- 2 .. 24 1 .. 41- 2 .. 82 

4 2 .. 5 2 .. 24- 2 .. 82 

5 3.15 2.82- 3 .. 55 

6 4 3.55- 4 .. 47 

a - ....... - ....... - - -- - ..... ........ ..... ....... ....... - ....... - - -- - - - ..... - 7 5 4.47- 5 .. 62 

8 6 .. 3 5.62- 7 .. 08 

9 8 7.08- 8 .. 91 5 .. 62- 11.2 

10 10 8 .. 91- 11.2 

11 12 .. 5 11 .. 2- 14.1 

12 16 14,,1~ 17 .. 8 11 .. 2- 22 .. 4 

13 20 17 .. 8- 22 .. 4 

14 25 22.4- 28 .. 2 

15 31 .. 5 28.2- 35 .. 5 22.4- 44.7 

16 40 35.5- 44 .. 7 

17 50 44 .. 7- 56 .. 2 

18 63 56 .. 2- 70 .. 8 44 .. 7- 89.1 

19 80 70.8- 89 .. 1 

....... -- - -=- ....... - ....- ...... - -- ...... - ................ - 20 100 89.1- 112 

21 125 112- 141 89 .. 1- 178 

22 160 141- 178 

aRegion within dashed lines represents the portion of the 5- to 

100-Hz structural resonance range included by the 8-, 16-, 31.5-, 

and 63-Hz ISO octave bands used in the analysis .. 

2 .. 4.1.1 Low-Frequency-Range, Coherent Random Sampling Technique 

Figure 2-12 summarizes schematically the random, coherent processing technique 

used for the reduction of MOD-2 low-frequency-range data.. The purpose of this 

technique, which is explained more fully in Ref. [7], is to minimize the 

effects of wind-induced or "pseudo" noise on the results.. The approach is 

based on the assumption that (1) the wind-induced-noise components will 

exhibit a random phase characteristic at 5 Hz and above between two microphones spaced a minimum of a quarter wavelength apart at 5 Hz, as compared 

with the more coherent radiation from the wind turbine rotor; and (2) the use 

of random sampling will produce statistically significant differences between 

the turbine's operating acoustic pressure spectra and the local background. 

The actual processing consisted of feeding the recorded signal from each of 

the two microphones into a dual-channel, narrowband (400-line, effective bandwidth of 0.5 Hz) spectrum analyzer and calculating the in-phase or cospectrum 

for the desired frequency range or time period, or both. The resulting elemental cospectrum frequency-band amplitudes were then summed into the appropriate ISO 1/3-octave and octave bands listed in Table 2-3. Each 30-minute 

data collection run with the turbine operating was subdivided into three 

22 
TR-3036 

Table 2-4 .. ISO Bands Used for High Analysis 

Nominal Third-Octave A-Scale Band Center Passband Weighting No .. Frequency 

(Hz) (Hz) (db) 

23 200 178- 224 -11 

24 250 224- 282 -9 

25 315 282- 355 -7 

a '""""'" '""""" """"" """"" """"" <='» """'" c=:I> ""'""" .....,. """"'" 

26 400 355- 447 -5 

27 500 447- 562 -3 

28 630 562- 708 -2 

29 800 708- 891 -1 

30 1000 1- 1120 0 

31 1250 1120- 1410 +1 

32 1600 1410- 1780 +1 

33 2000 1780- 2240 +1 

34 2500 2240- 2820 1 

35 3150 2820- 3550 1 

36 4000 3550- 4470 +1 

37 5000 4470- 5620 +1 

38 6300 5620- 7080 0 

39 8000 7080- 8910 1 

aAvailable high-frequency-data range for the 1983 experiment 

(dashed area)., 

Table 2-5 

Recording Bandwidth Signal Dynamic Range 

(kHz) (dB) 

1982 

Racal 7DS o 1 to 19,,0 38 

1983 

0.4 to 62e5 a Honeywell 5600E 33 

aSignal bandwidth limited to 20 kHz before recording .. 

10-minute segments, and individual 1/3-octave and octave co spectra were 

calculated from 100 2-second, randomly sampled (wi th replacement) ensemble 

recordS0 The same processing procedure was applied to the corresponding background recordings" 

The statistical distributions of the band spectrum levels (BSL) up through the 

fourth moment were then calculated for each 1/3-octave and octave band listed 

in Table 2-3 In addition, both band-to-band cross and conditional correlations as well as frequency distributions were calculated for the four octave 

23 
Collection Configuration 

VLF 

microphone 

no.3 

VLF 

microphone 

no.2 

FM recording 

Data Reduction Process 

Microphone 2 

Microphone 3 

400-line 

dual-channel 

narrowband 

spectrum analyzer 

·0.5-200 Hz 

Co-spectrum 

estimate 

Re {S23 (f)} 

100 randomly sampled 

2-second records 

from 10-minute 

recording period 

Obtain co-spectrum 

of S2 (f) & S3 (f) 

Time-ofday 

30-minute periods with turbine operating 

is-minute periods with turbine shut down 

Desktop 

computer 

SERf 

specialized 

software 

Sample statistics of 

1/3 & i-octave band 

Mean BSL <S1/3 (f» 

BSL variance coefficent <b2 (f» 

BSL skewness coefficient <b3 (f» 

BSL kurtosis coefficient <b4 (f» 

8,16,31.5,& 63 Hz octave 

cross-correlation analysis 

1/3-octave band exceedence levels, 

Graphics 

& 

printed 

output, 

, 

Disk 

storage 

Figure 2-12. Low-Frequency-Range Coherent Processing Flow 

co 

"<:t 

00 

o 

o 

1-3 

lA:l 

I 

w 

o 

W 

0'1 
TR-3036 

band amplitudes" The summary statistics as well as the raw band spectrum 

level distributions for each 10-minute record were then stored in disk 

files. The aggregate statistics for each 30-minute run were later arrived at 

from the ari thmetic average of these shorter segments.. Figures 2-13 through 

2-15 present representative examples of the distributions and displays that 

are available from typical la-minute analysis segments. 

A somewhat similar random sampling technique was used for the la-minute 

records of the measurements, and it is summarized in Figure 2-16 For acoustic emissions in this range, only a single microphone was 

used" Band pressure levels (BPL) of the 1/3-octave bands listed in Table 2-4 

were assembled from an 800-line, narrowband autospectrum with an effective 

bandwidth (B ) of 2S Hz Rather than the 100 random samples of a 2-second 

record, whicfi were used for 10'1:'1 frequencies, the high-frequency 1/3-octave 

spectra were calculated from 31 s of the average of three consecutive 

autospectra with a total record length of 0,,06 seconds.. The observed 

frequency distributions and carre 1/3-octave band statistical distributions were calculated (inc band level estimate discussed in Section 2" 1) and stored in disk filea No band-to-band cross or 

conditional correlations were derived As before the statistics 

for a particular 3D-minute run were formed from the arithmetic mean of the 

individual statistics" 2-17, 2-18, and 2-19 are 

representative the result high-frequency-range statistical 

summaries" 

2" ,,2 

Two to relat the MOD-2 acoustic SSlons to the turbine inflow 

\Vere used in our evaluation. One stochastic is methods in 

which the statistical di ibutions of the 1/3-octave emission spectra were 

correlated using bivariate and multivariate linear regression techniques with 

characteristic bulk measures of the turbulent inflow.. The other approach 

relied on more-or-less conventional t techniques using continuous 

records as a basis of comparison With the exception of continuous analog 

recordings of the two-axis, hot-film anemometer and the hub-height wind vector 

on the BPA tower, the majori of the measurements from the two meteorological 

towers were used to determine the mean inflow characteristics for each 30-

minute data collect SERI' tethered balloon system was used during 

many of the low- runs of Table 1 and its 1983 counterpart to 

estimate a mean vertical ile of the Turbine No 2 inflow, with a level of 

detail unavailable from the tower measurements The details of the tethered 

balloon operation and associated data processing may be found in Ref .. [8]. 

2.4 .. 2.1 Mean Inflow Characteristics 

Table 2-6 lists the surface-layer bulk characteristic parameters for Goodnoe 

Hills measured from the PNL and BPA cal towers and SERI's tethered 

balloon system for the 1983 s The BPA tower measurements, except 

for the vertical stability (Richardson number), were used to correlate the 

test turbine acoustic emissions because of the tower's close proximity.. The 

PNL tower measurement were al so used in determining. the .. value and height of 

the rotor disk mean wind and the Brunt-Vaisala frequency (period) 

25 
1/3-octave BSL Frequency Distribution 

Run #1: A05-C 

Run #2: A05-BG ----

1/3 oct. band = 8 Hz 

Upper freq. limit = 8.915 Hz 

Lower freq. limit = 7.085 Hz 

25------~----~----~----~----, 

.......... 

~ 20 (J) 

() 

c 

(J) 

"-

~ 15 () 

() 

o 

'+-

o 

1/3-octave BSL (dB/Hz) 

~tOO-Hz band distribution statistics 

Run #1 

Mean BSL (dB/Hz) 61.9 

Peak BSL (dB/Hz) 72.7 

Min. BSL (dB/Hz) 50.8 

Coef. variance 0.50 

Skewness coef. = 1.80 

ReI. kurtosis coef. = 5.59 

Mode BSL (dB/Hz) = 61.0 

Median BSL (dB/Hz) = 61.0 

Run #2 

48.4 

69.1 

25.7 

1.60 

3.88 

17.70 

37.0 

42.0 

0) 

'<t 

00 

o 

o 

TR-3036 

Figure 2-13. Example of Low-Frequency Acoustic Data Reduction Output: 

Observed Frequency Distributions of 8-Hz 1/3-0ctave Band 

Spectrum Level for Turbine Operating (Run 11) and Background Conditions (Run ) 

26 
(l) 

> 

(l) 

1/3-0ctave BSL Frequency Spectrum 

Run #1: A05-C 

Run #2: A05-BG ---~ 

Mean BSL comparison 100~~~~~~=-~~--r=~rM 

85 

70 

55 

~ 40 ctl 

!Il 

u 

25L-~~~~~~~~~~~~~ 

105 

90 

75 

60 

o 

1/3-octave center frequency {log Hz) 

1/3-0ctave BSL 

#1: A05-C 

Run #2: A05-BG ~ 

Peak BSL comparison 

c 45 ctl 

!Il 

30 ,~~~-b~~ __ ~~~~~~w 

o 2 3 

1/3-octave center frequency (log Hz) 

27 

o 

LD 

;;; 

o 

o 

TR-3036 
Run A05-C 

Run #1: 1/3-octave· BSL exceedence level summary (dB/Hz) 

Center frequency 

(Hz) 1% 5% 10% 20% 50% 

2.00 82 79 78 75 72 

2.50 79 77 76 74 71 

3.15 78 76 74 72 69 

4.00 78 75 73 71 67 

5.00 74 73 71 69 66 

6.30 75 70 68 66 64 

8.00 73 67 66 64 61 

10.00 67 65 63 62 60 

12.50 65 65 63 62 59 

16.00 64 62 61 60 56 

20.00 64 60 58 57 54 

25.00 60 58 57 55 53 

31.50 62 59 58 56 54 

40.00 56 54 54 53 51 

50.00 54 52 51 50 49 

63.00 54 50 49 48 47 

80.00 51 48 47 47 45 

100.00 52 46 45 44 43 

125.00 48 44 43 42 41 

160.00 46 41 41 40 40 

Run A05-BG 

Run #2: 1/3-octave BSL exceedence level summary (dB/Hz) 

Center frequency 

(Hz) 1% 5% 10% 20% 50% 

2.00 83 77 74 71 63 

2.50 83 74 70 66 59 

3.15 81 71 69 64 57 

4.00 77 68 66 61 51 

5.00 72 67 65 59 50 

6.30 67 62 60 55 46 

8.00 69 60 56 50 42 

10.00 62 55 52 47 39 

12.50 55 52 46 42 34 

16.00 53 46 42 39 32 

20.00 48 42 39 35 30 

25.00 42 39 35. 33 29 

31.50 39 34 32 29 26 

40.00 47 34 31 30 27 

50.00 34 33 32 31 29 

63.00 33 29 28 27 24 

80.00 26 25 23 22 21 

100.00 27 25 25 24 22 

125.00 28 27 27 27 23 

160.00 26 25 24 23 22 

Figure 2-15. Example of Low-Frequency Acoustic Data Reduction: 

Exceedence Level (L.) Comparison for Turbine 

1 Operating (Run ) and Background Conditions (Run ) 

28 

TR-3036 

l[) 

;x; 

0 

0 
Collection configlJration 

SLM Direct recording 

Time-ofday 

Data reduction process 

800-line 

narrowband 

spectrum 

analyzer 

25-20,000 Hz 

31 random samples 

of averaged spectra 

of three consecutive 

0.02-s measurement 

periods 

computer 

SERI 

specialized 

software 

Sample statistics of 

400-800 Hz 

1/3-octave bands 

-Mean BPL <S1/3(f» 

N 

l!) 

;;; 

0 

0 

-SPL standard deviation <S21/3>1/2 

-SPL skewness coefficient <b3 (f» 

-BPL kurtosis coefficient <b4 (f» 

distributions 

Figure 2-16. High-Frequency-Range Acoustic Data Processing 

I"l:j 

r-" 

()Q 

c 

Ii 

(1) 

N 

I I 

~ 

0\ 

~=:;; "-

~ ~ 

-

H 

~ 

I 

W 

o 

W 

0\ 

I 
CI) 

(.) 

c 

CI) 

I... 

I... 

:J 

(.) 

(.) 

o 

'+-

o 

~ 

(.) 

c 

CI) 

:J 

0-

CI) 

l... 

IJ.. 

A-weighted Level Distribution 

Run #1: S251 AX 

Run #2: 251 BGX ----

50 , ..----~----,r----__ r-----...__-___, 

40 

30 

'I 

'I 

20 / I 

, I {l 

I I, , 

10 I \1 \ 

I II I 

I r \ 

I 

0 50 I 60 70 80 100 

A-weighted level (dB) 

.A-\I\U~IClnl[eCl level distribution statistics 

Mean BPL (dB) 

Peak BPL (dB) = 

Min. BPL (dB) 

Std. dev. (dB) = 

Skewness coef. 

Kurtosis coef. 

Mode (dB) 

Median (dB) 

Run #1 

63.7 

66.0 

61.1 

1.26 

-0.50 

-0.34 

64 

64 

Run #2 

57.0 

62.2 

53.6 

2.05 

0.54 

-0.27 

56 

56 

(') 

l() 

;;; 

o 

o 

TR-3036 

Figure 2-11. Example of High-Frequency Acoustic Data Reduction Output: 

Observed Frequency Distributions of A-Weighted BPL for 

Turbine Operating (Run ) and Shut-Down Conditions 

(Run ) in High Winds (>14 mps) 

30 
(i) 

> 

(j) 

Q) 

b, 

:J 

CI) 

CI) 

Q) 

b, 

Q. 

1/3-octave 

Run #1: S251 AX 

Run #2: 251 BGX 

60F---r---r-'-1I---'---'-'-" 

Mean BPL comparison 

55 

50 

45 

1/3-octave center frequency (log Hz) 

Run #1: S251 AX 

60 Run #2: 251 BGX 

55 

50 

\ 

\ 

\ 

\ 

\ 

\""\ 

\ 

"-'..---

~ 

l!"J 

co 

o 

o 

TR-3036 

"0 45 c 

(~ 

..a 

350~---+---~~~d----L--·~~~~ 

t:.. 

1/3-octave center frequency (log Hz) 

Figure 2-18e of Acoustic Data Reduction Output: 

Mean. and Peak BPL for Turbine Operating 

) and Shut~Down ) in High Winds 

31 
TR-3036 

Run S251AX 

Run #1: 1/3-octave BPL exceedence level summary (dB) 

Center 

frequency (Hz) 1% 5% 10% 20% 50% 

400 59 58 58 57 55 

500 57 57 57 56 55 

630 56 56 55 54 53 

800 55 55 54 53 52 

1000 54 54 54 53 51 

1250 54 54 52 52 50 

1600 53 53 51 50 49 

2000 52 52 52 50 49 

2500 51 51 50 48 47 

3150 48 47 47 46 45 

4000 45 45 45 44 42 

5000 43 43 42 42 42 

6300 44 43 43 43 42 

8000 44 44 44 43 43 

A-weighted level 66 66 65 65 64 

Run 251 

Run #2: 1/3-octave BPL exceedence level summary (dB) 

Center 

frequency (Hz) 1% 5% 10% 20% 50% 

400 56 52 52 50 48 

500 53 53 51 49 47 

630 49 47 46 46 43 

800 50 45 45 44 42 

1000 48 46 44 43 41 

1250 47 45 44 43 41 

1600 46 43 43 42 40 

2000 47 44 43 43 41 

2500 47 43 42 42 40 

3150 46 44 43 42 40 

4000 45 43 42 42 40 

5000 46 43 42 42 41 

6300 46 44 43 43 43 

8000 46 44 43 43 43 

A-weighted level 62 61 59 59 56 

Figure 2-19. Example of High-Frequency Acoustic Data Reduction Output: 

Exceedence Level (L.) Distribution Comparisons for the 

Case of Figures 2-1~ and 2-18 

32 
( 

TR-3036 

of the rotor disk Unless se specified, the parameters 1n 

Table 2-5 were calculated for the 3D-minute data collection period@ 

A number of parameters li ted in Table 2-6 and calculated for this experiment 

have not generally been used in research field programs. These 

parameters brief 10n of what they 

represent 

ent 

which air is 

upstream fetch 

of 0.1 to 0 m 

Nos 2 and 3 

Friction veloci 

where i 

the IC VI 

and a measure of 

portion of the 

where 

ture; 

Karman constant; 

to latent heat flux 

an 

a knowledge 

Eqs ( ) 

z/L Ri/( 

L :: ( 

33 

o 

o 

er s the s of the 

measure of the roughness over 

the local wind 10n or 

to 10+ me Roughness lengths 

imilar to that upwind of Turbine 

veloci i defined 

(2-3) 

1 9 and \) is 

units of velocity 

in the lower 

number defined by 

to mechanical (shear) 

(unstable) correspond 

thermal convecwhich energy is 

buoyancy-damped, 

of zero repreconditions for only 

IS defined 

07 )] (2-4) 

(2-5) 

(2-6) 
TR-3036 

Table 2-6. Bulk Surface Layer Parameters Measured during 1983 

Coodnoe Hills Experiments from Towers and Tethered 

Balloon 

Rotor Disk Mean Flow Quantities 

Mean l5-m horiz. wind vector, U15 

Mean 38-m horiz. wind vector, U38 

Mean 45-m u-(long.) component, U45 

Mean 45-m w-(vert.) component, W45 

Mean hub-ht. horizo wind vector, UH 

Mean hub-ht. vert. component, WH 

Mean 84-m horiz. wind vector, U84 

Mean 107-m horiza wind vector, U107 

Surface roughness length, Zo 

Friction velocity, u* 

Gradient Richardson number, Ri 

Monin-Obukov length, L 

Rotor disk peak horizo wind speed, Umax 

Rotor disk peak velocity height, zmax 

Rotor disk Brunt-V~is~l~ period, TBV 

Surface barometric pressure, Psfc 

Hub-height barometric pressure, Phub 

BPA 45 m Bulk Inflow Turbulence Quantities 

BPA Tower 

o 

o 

o 

o 

o 

o 

o 

o 

o 

Longitudinal component (u), x-direction length scale, 

Vertical r-omponent (w), z-direction length scale, I z 

w 

Vertical component (w), x-direction length scale, I x w 

Longitudinal (u) time scale, Tu 

Vertical (w) time scale, Tw 

Longitudinal (u) frequency spectral estimate, S (f) u 

Vertical (w) frequency spectral estimate, Sw(f) 

Longitudinal (u) spectra reduced frequency peak, fu 

Vertical (w) spectra reduced frequency peak, fw 

Longitudinal (u) peak turbulence energy, P u 

Vertical (w) peak turbulence energy, Pw 

34 

I x 

u 

PNL Tower 

o 

o 

o 

o 

o 

o 

o 

o 

o 

o 

o 

o 
TR-3036 

Table 2-6 .. Measured 1983 

and Tethered 

Tethered Balloon ities 

wind 

( )1/2 turbulence 

inflow turbulence 

ra, Suw(n) 

scales, I 

It IS 

heignt 

is defined 

upward heat 

or downliilard 

an indicator 

near zero 

positive 

generation [9] 

i~ a measure of 

stratified f10vla 

existence of such 

calculated for the rotor 

and local values from SERl i 

2 4 2 2 Turbulent Inflow 

~Nhere T· i the 

uw 

resolution velocity 1es, U(z) 

ical 

ia1 

co 

profiles, dU/dz and du/dz 

les, Rily(z) 

iles 8(z) 

les, 8s (z) 

and Ri1o(z) 

measure of the 

10n is dominant L 

are (negative or 

stratification exists (positive 

io z/L can also be thought of as 

going on Large 

dominant; 

and a 

shear 

(2-7) 

of of densitystable boundary 

an measure of the 

in Table 2-6, it has been 

tower measurements and layer 

les 

The turbulent or 

for i=1,2,3 , (2-8) 

I . cross-correlatIon 

time~scale for the ith-component and R· .(T) is the 

f . b h' d' h lJ unctl0n _ etween . t. elan J s T ere are n1ne 

possible terms in 

the 1983 

the . (T) matrix 0 Three of these have been computed for 

fromJthe hot~film anemometer measurements and listed in 
TR-3036 

Table 2-6" Iu xis a charac teri s tic scale length of the longi tudinal turbulence component, u, along the x-direction (positive east).. Similarly, I x 

is a scale length of the vertical component, w, along the x-directio~" 

Finally, I z is a scale length of the w-component in the vertical or 

z-direction~ The time and length scales were calculated from component velocity signals over a frequency range of 0.05 to 10 Hz for the 30-minute datacollection periods. 

Normalized turbulence spectra descriptors. Frequency spectra ampli tudes of 

the turbulent wind components (u, v, w) are often normalized by the cyclic 

frequency or nS{n). The corresponding cyclic frequency, n, is normalized by 

the ratio z/U{z) or nz/U{z) and referred to as the reduced frequency f, where 

U{z) is the mean wind speed at height z.. The quantities f and f in 

Table 2-6 represent the value of the reduced frequency where f'§{f) hasw its 

maximum. Further, P and P parameters in Table 2-6 refer to the maximum 

values of nS (f) and ~S (f), ;espectivelye u w 

2.4.3 Rotor Surface Pressures 

Table 2-7 lists the locations of the 12 semiconductor pressure transducers 

used on Blade No. 1 of Turbine No.2 in the 1983 experiment. In this volume, 

only the measurements near the leading edge of the rotor have been analyzed to 

any great extent.. This is because coherent, low-frequency acoustic radiation 

is associated with turbulence-induced, unsteady blade loads, whose greatest 

effects tend to be concentrated near the leading edge. 

Table 2-7. Pressure Tap Locations on Turbine No.2, Blade No.1 

Tap Blade Percent Percent 

No .. Station Span Chord Surface 

1 1164a 65 15 

2 1164 65 15 Lowerc 

3 1164 65 40 Upper 

4 1164 65 40 Lower 

5 1164 65 85 Upper 

6 1164 65 85 Lower 

7 1562d 87 15 Upper 

8 1562 87 15 Lower 

9 1562 87 40 Upper 

10 1562 87 40 Lower 

11 1562 87 85 Upper 

12 1562 87 85 Lower 

~Fixed pitch portion of blade. 

Low pressure surface. 

~High-pressure surface" 

Movable tip portion" 

36 
TR-3036 

3@O DESCRIPTION OF GooDNOE HILLS INFLOW STRUCTURE 

A knowledge of the details of the turbulent inflow structure is important if 

any statistical correlation between the inflow properties and the resulting 

acoustic emission characteristics is to be made. We must first establish what 

properties are most important In terms of acoustic noise generation by the 

MOD-2 in a turbulent atmo Summaries are then given of the available 

inflow properties for the 1982 and 1 experimental periodse 

3 .. 1 

Homicz and George [10,11] and 

theory using generalized functions 

blades in a turbulent 

determining the cri tical inflow 

and Kim [17] have developed a unified 

of aeroacoustic noise generated by rotating 

We will use their results as a basis for 

to be used as statistical predictors for correlations with the MOD-2 emlS 10ns 

Homicz and have shown that 

radiated acoustic s (f) 

is givec 

f) 

ized function for the far-field 

localized disturbance source region 

(n (3-1) 

where f IS the acoustic IS the sound Po IS the alI" 

density, y is the distance between the observation and the rotor, n is 

the harmonic number, n is the position vector of the disturbance on the rotor, 

PL. L. is the cross~spec ral densi of the lift, and ~o i the unit vector 

albng Figure shows of the and associated angles 

and symbols 0 shown that the characteristics of the radiated 

acoustic spectrum on the quasi-static load due to mean loading 

conditions plus the fluctuat loads due to rotor-turbulence interaction. 

The far-field radiated spectrum due to a turbulent upwash spectrum is of the 

form 

co n2 CD 

2 

<s (f) OJ. f d C;; [ I G (c;;,n,l1,)JnB_

l (fet) (3-2) ap Q 

C;; 

M 

J2 a x I l1, rZ 

w 

where <S (f», the acoustic pressure , is expressed in the form of an 

integral ap over C;;, the turbulent velocl .. spectrum dlmensionless wavenumber, and 

where the two series involving Bessel functions arise because of the resolution of the turbulence into polar coordinates and because of the Doppler shift 

due to the rotation0 The function G(c;;,n, ~) includes the ingested turbulence 

spectrum and unsteady aerodynamic effect and the finite span effect of the 

rotor bladeso r Z is the vertical or upwash turbulence integral scale; V is 

the axial convec;ion velocity of the turbulence (usually taken as O.8UH); ~ is 

the number of blades in the rotor and Mc is the axial convection Mach number, 

Hc = Vc/ao · 
Figure 3-1 

Turbulent eddy 

Microphone 

~ 

ro 

o 

o 

Rotor Geometry Used by Homicz and George [10,11] 

TR-3036 

The dynamic characteristics of the low-frequency portion of the radiated spectrum is determined by the degree of blade-to-blade correlation with turbulent 

eddies passing through the rotor plane [10]. Larger eddies may be cut by more 

than one blade, producing peaked, essentially discrete tones in the lowfrequency spectrum. Smaller eddies produce much higher frequency blade loads, 

but generally there is less likelihood of repeated encounters with the same 

eddy, and random, broadband noise is radiated. Homicz and George have shown 

that the frequency at which this transition to broadband takes place is 

Bn (1 + Mo/Mc) f ~ (3-3) o -2 (1 - Mo cosy) • 

They also were able to show that the values of ~ that provide significant contributions to the acoustic spectrum at frequency fare 

(fin) (1 - Mo cosy) (fin) (1 + Mo cosy) 

< ~ < (3-4) (Vc/lun) (1 + Mo/Mc) Vc Iun 

When (3-3) and (3-4) are combined, the lower limit of ~, which makes significant contributions for f>f

o 

1S 

~f o 

> B I n/2 V • 

u c (3-5) 

For the MOD-2 turbine with ~ dimensionalized by the chord Co at R

o

' the corresponding turbulent eddy wavelength would be -10 m or 5 chord lengths for a 

longitudinal integral scale of 125 m. 

Homicz and George assumed that the inflow turbulence could be considered homogeneous and isotropic. This enabled them to use only one length scale. As we 

see in Sections 3.2.2 and 3.3, even treating the MOD-2 inflow twodimensionally requires additional length-scale specification, because of the 

38 
TR-3036 

degree of anisotropy that may be present in the (inflow) acoustically significant eddy wavelengths.. In anticipation, we define three integral scales: 

lux, the longi tudinal or axial scale; I x, the scale length of the vertical 

turbulence or upwash component'J w, alon; the axial direction; and I z, the 

scale of the vertical component in the rotor planeo Figure 3-2 sketc~es the 

relationship of these length scales to the rotor plane 

Homicz and George arrived at the following expression for the nondimensionalized, one-sided acoustic from a turbulent envi ronment 

moving through a rotor: 

(3-6) 

co 

x f E. E E E E -lnt ~ turb aero span ' 

where 

E. 1nt 

E ::::: J2 

ll- 1 

E ::::: 

turb 

[COS11 

H 

0 I 

= {l 

J2[ 

0 

::::: 

s 

/ ) 

1 

2 2 

J nB- l 

f cosy) Q 

2 2 4Tf r; ) 

kc and ks are the reduced aerodynamic ~ ies along _thr/

2

hord (c) and span 

(s) averaged over one blade revolution, and kT :::::( k~) 

E2, the relative of the ll- loading modes for a given nond1mensional wavenumber describes the acoustic of these 

upwash modes with the nB harmonic modes Et b establishes the . . ur. weighting ied to thecontrlbut1on from any turbulent wavenumber; b 1S the 

blade span; 11 i the between the 1 force vector and the vertical; n 

is the harmonic number of the blade passage ; Mt is the tip Mach 

number; and i the tip radius" The E and E terms reflect the 

.aero span . influence of rotor's unsteady aerodynam1c response and a spanWlse correction to account for the loss of acoustic compactness at high frequencies .. 

Homicz and have developed expres ions for these terms under what we 

believe to be highly idealized conditions For now, we have chosen to lump 

these effects together with those fixed by design by experimentally 

determining the MOD-2 aeroacoustic transfer functions 

'10 
TR-3036 

W(z) 

U(x) 

V(y) 

Figure 3-2 .. 

W 

U 

Schematic of I x, I x u w' to Turbine Rotor 

I~ 

I Z Length Scales with Respect w 

The acoustically significant inflow properties, which need to be determined 

within a stochastic framework, can now be summarized and include these: 

• Axial convection veloci ty, V , . c making UH an 1nflow parameter 

which we will approximate by 0 .. 8UH, thus 

'2 • Upwash velocity turbulence intensity, w 

• Turbulence integral scale, which we have expanded into three components: 

I x, I x, and I z, by treating the problem two-dimensionally u w w 

Local atmospheric state (barometric pressure, temperature), which 1S used to 

calculate the air density, Po' and acoustic velocity, a o • 

The remainder of the scaling parameters in Eq. (3-6) are fixed for the MOD-2 

design and include the 

• Rotor rotational frequency, Q 

• Number of blades, B 

• Rotor tip radius, Rt or D/2 

• Blade span, b 

Blade chord, co' at effective radius Ro (typically taken as 0.7SRt ) 

• Airfoil shape of the rotor blades. 

Furthermore, a reference distance, r, and its relationship to the rotor disk 

must be chosen. We have decided to use the upwind, on-axis station located 

1.SD ahead of the rotor disk or at an angle of 24° with respect to the turbine 

hub height as this reference point. It also allows this measuring station to 

40 

to 

CX) 

o 

o 
TR-3036 

"see" thrust and torque components of the acoustic radiation.. This location 

is also useful in verifying the predictive methods of Homicz and George by 

avoiding the complications associated with modeling the conditions in the 

vicinity of the rotor plane. 

3 .. 2 

In the lower portion of the earth's atmo~pheric boundary layer (usually 

referred to as the surface layer), UH, Ii J, and Wi 2 are all functions of 

height, z. The rotor blades of large, horizontal-axis wind turbines, such as 

the MOD-2, traverse deep vertical layers perhaps exceeding the height of the 

surface or constant flux (see Section 302.1) a percentage of the time. 

The existence of strong vertical shearing and layering associated with this 

layer, particularly under stable flow conditions, makes the specification of 

acoustically significant essentially impossible except for some 

well-defined reference atmo What is needed are characteristic or 

scaling parameters, which by physical processes are efficient predictors of 

the statistical distribution of the vertical or upwash velocity , w', spectrum 

(or, more generally, the three-dimensional and the horizontal wind 

velocity UH across the vertica.l depth of the turbine I s rotor disk. Such 

scaling is discussed below 

3 .. 2 .. 1 Surface 

It is known that, within a locally homogeneous and quasi-steady surface layer, 

the vertical fluxes of heat, momentum, a.nd moisture are essentially 

constant. Ideally, under such conditions, the turbulent structure should be 

related to vnly a few Monin and Obukov [13,14] proposed that turbulent motions in the surface layer scaled with the height z above 

the surface, the local buoyancy, giS, the kinematic surface stress Ls/po 

(where LS is the surface stress), and the surface kinematic heat flux 

Q = H Ip co By combining these parameters, they defined a series of surfaceo 0 0 

layer scaling factors (knovm as similarity or M-O scaling), including a velocity scale, u)" (the friction velocity; see Section 2.4 2.1); a temperature 

scale, T* = -Qo/u*; the M-O length, L (defined in Section 2.4 .. 2 .. 1); and the 

geometric height, z 

When surface layer flow properties are appropriately nondimensionalized 

(velocities by U~I(, temperatures by T~I.., etc.), M-O theory has shown that they 

become universal functions of z/L. It is generally accepted that the "universal function" aspect of M-O similarity applies in surface layers over homogeneous terrain [15]. In the real world, and that includes most wind plant 

sites, there is often a departure from strict M-O theory as a result of such 

influences as nonsteady boundary conditions, complex terrain features, local 

radiative effects, or inhomogeneous flow conditions due to upstream (fetch) 

effectse These "departures" usually have the effect of changing "universal 

functions" into site-specific ones. 

41 
TR-3036 

3.2.2 The Vertical Distribution of UM (Vc ) 

Because of the effects of surface friction, the horizontal wind vector 

increases with height at least part of the way in the surface layer. Under 

neutral stability conditions (Ri = z/L = 0),. and over homogeneous terrain, 

this vertical profile of UH can be represented by the familiar log profile, or 

(3-7) 

where z is the roughness length defined in Section 2.402.1. Equation (3-7) 

is modi~ied for non-neutral surface layers by the diabatic wind profile [8], 

or 

where 

and 

UH(z) = (u*/ka ) . [In(z/zo) - Wm(z/L)] 

z/L 

f 

zo/L 

[1 - Ym (r,;)] dr,; 

l;; (3-8) 

The important point here is the influence of the M-O scaling parameters u*, L, 

and z on the value of UH' or equivalently V , as a function of height within 

the turbine's rotor disk. If the condition~ of true M-O scaling are met, u* 

and L are not functions of z 0 Thus,. we may think of these parameters as 

potential candidates for predictors of turbine acoustic emissions 

characteristics .. 

3.2.3 Variation of with Height 

If M-O scalin~ is applicable, Panofsky and Dutton [9] show the vertical varlation of Wi spectra, S (f,z), is of the form w 

(3-9) 

where n is the cyclic frequency and f is the reduced or normalized frequency 

given by f(z) = nz/UH(z). The relationship defined in Eq .. (3-9) has a different form for unstable/neutral and stable environments. For stable conditions, Panofsky and Dutton recommend the equation developed by Kaimal [16]: 

(3-10) 

where P /u,,~2 is the maximum of S (f ,z) at f = f and is a function of z/L .. w " .. w m • For unstable and neutral condltlons, they recommend the HOJstrup [18] model, 

or 

nSw(n,z) 

2 

u ... }~ 

32f (~)2/3 + 2f 

(1+17f)5/3 -L (1 + 5 .. 3f)5/3 (3-11) 

Assuming M-O similarity holds, Eqs. (3-9), (3-10), and (3-11) demonstrate the 

statistical dependency of the upwash velocity spectra on the UH(z), U,.o and 

z/L scale lengths characteristic of the flow.. Since the M-O length, L, can be 

estimated from a knowledge of the Richardson number (see Section 2.4.2.1), the 

Ri could be substituted for L. 

42 
TR-3036 

In addition to the M-O similarity theory scale lengths discussed above, it is 

also possible to empirically determine characteristic length scales of the 

inflow itself. These scale lengths, called integral scales, are defined by 

CD 

Ii j = ui fo RUij(T)dT , 

where U· = [u,v,w] and j = [x,y,z), but Slnce 1 

CD 

(3-12) 

(3-13) 

IiJ contains information .about Sij(f) and should also be related to the M-O 

scales. The measured I.J scales contain information about the spectral content of the flow, even il it does not strictly follow M-O similarity. 

From the above, it is clear that one \vould expect S (f,z}) to have some rela-

( ) ' • 1.'1 ( tionship with S f,z2 ' vnth zl = z2' Slnce both S f and UH 0:;.7. 

scale with 'I'l • height in the surface layer whlle u* and L do not. Thus, lt seems plausible 

that detailed spectral measurements made at a given height within the rotor 

disk layer should be at least statistically related to those at other heights 

if the scaling laws apply. Since we do not have the ability to gather direct 

measurements of the upwash or WI spectra throughout the entire disk, we have 

chosen to establish a reference to measure the actual flow scale lengths 

related to it as well as the normal M-O scaling parameters. These parameters 

include the following: 

o Mean reference or hub-height wind speed, UH 

o Rotor disk gradient Richardson number, Ri 

o Longitudinal (axial) velocity component length scale, lux 

o Vertical (upwash) velocity component length scale along the axial (x) direc-

. I x t lon, w 

o Vertical component length scale parallel to the rotor plane Iwz • 

Thus, the parameters above provide scales for Vc (1:25~H)' the surface layer 

vertical stability (Ri and/or z/L), and characterlstlc flm.; scale lengths 

related to Sw(f). 

3.3 Inflow Data i tical Summaries 

As previously stated, no detailed, high-frequency turbulence measurements ,.;ere 

taken during the 1982 experiment. This has limited our ability to establish 

adequate correlations with the MOD-2 acoustic emissions spectra for that 

experimental period. Hm.;ever, since the current MOD-2 configuration is the 

one in use during the 1983 series, we believe it is the more important. We 

have summarized be.low what information was available for the 1982 data runs. 

A reduced 1982 data set was selected based on available inflow properties 

whose values fell within the observed ranges of the same parameters in 1983. 

This '.vas done to try to establish a common inflow basis for year-to-year 

comparisons. 

43 
TR-3036 

A comparison of the 1982 and 1983 overall inflow statistics are given in 

Table 3-1. While lower wind speeds were, on the average, encountered during 

1983, Table 3-1 shows that these data runs were made in conditions that were, 

on the average, more stable and more turbulent than the 12 runs made in AprilMay 1982.. Table 3-2 lists the reduced data set summary statistics for 1982 

and 1983, with its closely overlapping parameter rangese Missing from 

Table 3-2, of course, are the measured characteri s t ic scale lengths of the 

inflow .. 

Tables 3-3a, b, and c summarize the detailed inflow data for the six 1983 data 

runs used for the low-frequency-range analysis in Section 5 .. 0. Table 3-3a 

lists the air motions data from three measuring heights on the BPA tower. The 

two-axis, hot-film anemometer was installed at the 45-m level, from which the 

high-frequency turbulence statistics were gathered. Eight runs were made in 

1983, but one was lost to a failure in the recorder collecting the hot-film 

datae The other run was lost when the wind direction moved out of the desired 

40° cone for the operation of the two-axis, hot-film anemometer. The inflow 

characteristic scaling parameters are listed in Table 3-3b; the air motions 

data were deri ved largely from the BPA tower but the stabili ty information 

came from the PNL tower. Table 3-3c summarizes the longitudinal and vertical 

component spectral scaling parameters P /u~2 and P /u~2 discussed in .... ··u .. w " 

Section 3.2 .. 3 as well as th~ Brunt-Vaisala period TBV " The peak mean wind 

speeds and heights listed in Table 3-3c were determined by vertically 

smoothing the five speeds available from the PNL tower. 

Table )-1. Comparison of 1982 and 1983 

from the BPA Meteorological 

Parameter 

Total number of data runs 

59-m mean wind speed, mps 

59-m mean wind speed variance, (mps)2 

59-m mean turbulence intensity, % 

Mean Richardson number (PNL tower) 

Peak 59-m 10-min mean wind speed, mps 

Minimum 59-m 10-min mean wind speed, mps 

Inflow 

Tower 

Statistics 

1982 1983 

12 8 

11 .. 4 9.5 

1.51 1 .. 75 

10 .. 8 13 .. 9 

0 .. 30 2.36 

14 .. 5 13 .. 6 

7.5 6 .. 9 

Table 3-2 .. BPA Tower Statistics of Reduced 1982/1983 

Data Sets 

Parameter 1982 1983 

Number of data runs 6 4 

59-m mean wind speed, mps 9.85 9 .. 58 

59-m mean wind speed variance, (mps)2 1.15 0.99 

59-m mean turbulence intensity, % 10 .. 9 10 .. 4 

Mean Richardson number 0.25 0 .. 34 

Mean M-O z/L parameter 2.86 3.02 

44 
~-~ 

II I 

~-:-' 

Table 3-3a .. 1983 Mean Inflow 

59-m 59-m 59-m Data Wind UH (U ,)1/2 Run Direction 

(deg) (mps) ~mps) 

A05 268 7 .. 3 1 .. 02 

A03 261 8 .. 1 1 .. 03 

A14-1 261 9.2 1 .. 31 

A14-2 256 9 .. 9 1 10 

Al8 254 12 .. 4 0 .. 89 

All 257 13 .. 6 0 .. 31 

Means 260 10 .. 1 0 .. 94 

TR-3036 

from BPA Tower: 

45-m 45-m 45-m 60-m 

(u,)1/2 (w,)l/2 u w 

(mps) (mps) (mps) (mps) 

8.7 1 .. 13 0,,16 0 .. 52 

9 .. 0 1 .. 32 0 .. 19 0 .. 76 

9 .. 6 1 .. 30 0,,19 0 .. 95 

10 .. 5 1 37 0 .. 15 0 .. 78 

13 .. 9 0 .. 90 0 .. 12 0 .. 99 

14 .. 3 1 .. 10 0 .. 17 0 .. 52 

11 .. 0 1 .. 19 0016 0 .. 75 

Table 3-3b .. 1983 Mean Inflow Characteristics: Scaling 

Parameters 

Data Ri z/L u,,;'( Zo I x I x I z Tu Tw 

Run (mps) (em) (~) (~) (~) (sec) (sec) 

ADS 11 .. 7 5 .. 62 0635 56@7 136 125 7.5 15 .. 6 14 .. 4 

A03 -0.12 -0 .. 12 .461 5.6 210 157 13 .. 3 23 .. 4 17.5 

A14-1 0 .. 13 .. 334 1 01 11 .. 9 157 166 16 .. 5 16 .. 4 17 .. 3 

A14-2 0 .. 26 5 .. 62 0,,736 25 .. 6 8605 8107 6.1 8.22 7.76 

A18 1 .. 25 5 .. 62 0 .. 943 27,,4 125 123 8.8 9 .. 02 8 .. 84 

All 6 .. 68 5 .. 62 1,,09 4603 336 86 .. 8 6 .. 8 23.4 6 .. 06 

Means 3 .. 32 3 .. 78 .. 813 2809 175 123 9 .. 8 16 .. 0 12 .. 0 

3 .. 4 Inflow Turbulence Content 

The frequency spectra for six analysis runs of the longitudinal and vertical 

turbulence components measured at the 45-m level on the BPA tower are presented in Figures 3-3 through 3-89 The corresponding Richardson number 

(Ri), mean hub-height wind speed (Uhub )' and longitudinal (IX) and vertical 

(I z ) integral scale lengths are listed on each plot.. u w . 

3 .. 5 Rotor Disk Inflow Vertical Profiles 

It is important to know the vertical distribution of wind speed and turbulence 

across the MOD-2 rotor disk.. Since the height of the BPA tower extended only 

to hub height, we were left with two other sources of vertical profiles of 

wind distribution information: the five levels of the PNL tower and SERI' s 

tethered balloon sounding system. The latter was limited, in 1982/83, to wind 

speeds under about 11 mps.. We have used both resources to assemble a picture 

of the distribution of the horizontal wind with height. 

45 
TR-3036 

3.5.1 PNL Tower Vertical Profiles of Wind Speed and Turbulence Intensity 

The vertical resolution (22.9 m or 75 ft) of the PNL tower is limited, but by 

regression smoothing we can achieve a reasonable estimate of a mean vertical 

profile. Also, it is clear that these profiles may not exactly agree with the 

vertical inflow structure into Turbine No. 2 because of the degree of spatial 

separation. However, they are useful because they track the vertical structure (shape) of the profiles under the range of stability and speeds encountered during the six 1983 data runs" Figures 3-9 through 3-: 14 plot the 

smoothed wind speed normalized by the hub-height value and the un-normalized 

turbulent intensity for each of the six data runs. We should point out that 

we consider the turbulent intensity plotted here to be the "long wave" contribution due to the lack of response of the cup anemometers at small wavelengths 

(high frequencies)" 

3.5.2 Representative Tethered Balloon Profiles in Turbine NOG 2 Inflow 

A number of 1982 runs were supported by the tethered balloon system but only 

three were available in 1983 because of equipment problems, high winds, or 

both. In 1983 the runs that were supported included A03, A03-1, and A05. The 

45-m level turbulence data were unavailable from the A03-1 run because of an 

out-of-tolerance hub wind direction. Figures 3-15 through 3-17 present 

smoothed, high-resolution vertical profiles of wind direction and speed, 

sensible temperature, ana atmospheric stability taken just before the 

beginning of Run A05" It is interesting to compare the vertical wind-speed 

profile measured by the five discrete levels on the PNL tower in Figure 3-14 

and the tethered balloon profiles taken in the inflow of Turbine No" 2 in Figure 3-15.. A representative sample of a high-resolution inflow turbulence 

spectrum measured by the hot-film anemometer is plotted in Figure 3-18 for the 

same run. 

Data 

Run 

A05 

A03 

A14-1 

A14-2 

A18 

All 

Means 

Table 3-3ce 1983 Mean Inflow Characteristics: Spectral 

Scaling and Rotor Disk Layer Parameters 

fm fm 2 TBV Mean pu/U.,.", pw/u-;,,,, 

2 

u w (sec) Speed 

(mps) 

.081 .081 0 .. 836 0 .. 038 247 6 .. 8 

.. 078 ,,078 0.729 0 .. 162 0 9.0 

.. 147 .147 0 .. 489 0 .. 061 319 11.5 

.. 134 .. 067 0 .. 941 0 .. 050 343 11 .. 3 

,,101 .. 051 0 .. 324 0 .. 031 234 13.6 

.049 .167 2 .. 43 0 .. 043 177 13.2 

.098 .099 0 .. 958 0 .. 064 264 10 .. 9 

aDetermined from the PNL tower. 

46 

Peak Winda 

Height 

(m) 

44.8 

107 

107 

107 

97 .. 9 

68 .. 4 

8807 
N 

I 

~ 

en 

a. 

E 

'-' 

c 

~ :i 

(f) 

.~ 10-1 

c 

(\) 

'"0 

~ "-

..... 

() 

(\) 

a. 10-2 en 

E 

I 

L() 

'<:j" 

1983 Run AOS BPA 4S-m inflow spectra 

Sw(n) 

Ri = 

Uhub = 7.3 mps 

lux = 136 m 

Iwz = 7.S m 

10-3 ,L-. ____ -.L _____ "'-=-____ --L _____ _ 

10-2 10-1 10 102 

Cyclic frequency, n (Hz) 

Figure 3-3. BPA Tower 45~m Level and Vertical 

N 

I 

........ 

N 

en 

a. 

E 

c 

~ :i 

(f) 

>. 

."t:: 

en 

c 

(\) 

'"0 

~ )... 

..... 

l) 

(\) 

a. 

en 

E 

I 

L() 

'<:j" 

Inflow for 1983-~Run A05 

10 

10-1 

10-2 

Ri = -0.12 

Uhub =8.1 mps 

lux=210m 

Iwz = 13 m 

Cyclic frequency, n (Hz) 

Figure 3-4. BPA Tower 45-m Level 

Inflow 

47 

-5/3 slope 

and 

1983--Run A03 

TR-3036 

co 

l{) 

co 

o 

o 

fii 

00 

o 

o 
10 

N 

I 

........ 

C\J 

en 

0.. 

E 

......... 

...-... 

c 

-; 

ct 

>-

:!:: 

(/) 

C 

<D 

'"0' 

ro 

h... 

.... 

() 

<D 

0.. 

(/) 

E 

I 

~, 

10-3 

10-2 

Figure 3-5 .. 

N 10 r 

I 

........ 

C\J 

en 

0.. 

E 

1 E .-

c 

---3_ 

en 

>-

:!:: 10-1 

(/) 

C 

<D 

'"0 

ro 

h... 

.... 

() 

<D 10- 0.. 2 

(/) 

E 

I 

L() 

"'" 

10-3 

1 0-2 

Figure 3-6 .. 

Ri=+0.13 

Uhub = 9.2 mps 

lux = 157 m 

Iwz = 17 m 

Cyclic frequency, n (Hz) 

-5/3 slope 

BPA Tower 45-m Level Longitudinal (u) and 

Vertical (w) Inflow Spectra for 1983--Run 

Su(n) 

-5/3 slope 

Sw(n) 

Ri = +0.26 

Uhub = 9.9 mps 

lux = 87 m 

Iwz = 6.1 m 

Cyclic frequency, n (Hz) 

BPA Tower 45-m Level Longitudinal (u) and 

Vertical (w) Inflow Spectra for 1983--Run 

48 

A14-1 

A14-2 

TR-3036 

<0 

00 

0 

0 

o 

<0 

00 

o 

o 
N 

I 

.......... 

C\J 

(j) 

Q. 

E 

,..-... 

c 

~ 

10 

-5/3 slope 

of Sw(n) 

.£ 10-1 

en 

c 

(l) 

"'0 

«l 

l.-

+-' 

U 

(l) 

Q. 

en 

E 

I 

L() 

-<::t 

10-2 

Figure 3~1 

N 

I 

.......... 

C\I 

(j) 

Q. 

E 

......... 

c 

---~ 

::i Cf) 

10 

~ 

:=: 

~ 10-1 

Q) 

"'0 

cu 

l.-

+-' 

U 

Q) 

Q. 

en 

E 

I 

L() 

-<::t 

10-2 

Figure 3-8 

Ri = +1.25 

Uhub 12.4 mps 

lux := 125 m 

Iwz = 8.8 m 

Cyclic frequency, n (Hz) 

BPA Tower 45-m Level 

Ri = +6.68 

UhUb = 13.6 mps 

lux 336 m 

Iwz = 6.8 m 

Inflow 

Cyclic frequency, n (Hz) 

BPA Tower 45-m 

49 

and 

1983--Run Al8 

-5/3 slope 

and 

1983--Run All 

TR-3036 

C\I 

<0 

00 

o 

o 

C') 

<0 

00 

o 

o 
.......... 

E 

........ 

Q) 

en 

ct1 

..a 

Q) 

> 

o 

..a 

ct1 

...... 

..c 

0) 

Q) 

TR-3036 

'<;f" 

<D 

Profile for Run A03 ro 

o 

o 

125 __ ----__ ----~----__ ----__ ----~ 

100 

75 

50 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 7.75 m/s 

Hub wind dir. = 273.4 deg. 

Grad. Rich. no. = -0.12 

I 25 

O~ ____ d_--__ ~ __ --~------~ __ ~ 

0.7 0.8 0.9 1.0 1.1 1.2 

(a) UH normalized horizontal wind speed 

If) 

to 

ro 

o 

o 

Run A03 PNl Turbulence Intensity Profile 

125-------~------r----~.----~r-----, 

E 100 

Q) 

en 

ct1 

..a 

Q) 

> 

o 

..a 

ct1 

...... 

..c 

0) 

75 

50 

Q) 25 

I 

3 6 9 12 15 

Turbulence intensity (%) 

(b) turbulence intensity 

30-min. parameter means: 

Hub windspd. = 7.75 m/s 

Hub wind dir. = 273.4 deg. 

Grad. Rich. no. = -0.12 

Figure 3-9 .. Smoothed Vertical Profile (PNL Tower) of Uh b-Normalized 

Horizontal Wind Speed (a) and Turbulence In~ensity (b) for 

1983--Run A03 

so 
,.-.. 

E 

Q) 

en 

al 

.0 

1.-

Q) 

~ 

0 

+=' 

(j) 

> 

0 

.0 

al 

Q) 

> 

o 

.0 

al 

)-10 

100 

75 

50 -

25 

0 

0.7 

100 

75 

50 

25 

TR-3036 

Norm.PNl Wind Profile for Run A14-1 

<D 

<D 

00 

o 

o 

1.2 

U/UH 

3 6 9 12 15 

Turbulence intensity (%) 

51 

Tower) of 

and Turbulence 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 8.98 m/s 

Hub wind dir. = 271.8 deg. 

Grad. Rich. no. = +0.13 

r-

<D 

00 

o 

o 

30-min. parameter means: 

Hub windspd. = 8.98 m/s 

Hub wind dir. = 271.8 deg. 

Grad. Rich. no. = +0.13 
Norm.PNL Wind ·<W'IIJ.I;;'I;U Profile for Run A14-2 TR-3036 125r-----r-----r-----r---~r_--__ 

......... 100 

E 

Q) 

en 

ro 

.0 

Q) 

> 

o 

.0 

ro 

+-' 

..c 

0) 

Q) 

I 

75 

50 

25 

o 

0.7 0.8 0.9 1.0 1.1 1.2 

U/U H 

(a) UH normalized horizontal wind speed 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 9.78 m/s 

Hub wind dir. = 274 deg. 

Grad. Rich. no. = +0.26 

Run A14-2 PNl Turbulence Intensity Profile 

125~------------r----~r_--~r_--~ 

E 100 

CD 

en 

ro 

.0 

l... 

CD 

3 

o 

+-' 

CD 

> 

o 

.0 

ro 

+-' 

..c 

0) 

'Q) 

I 

Figure 3-11 .. 

75 

50 

25 

0'-----'-----'-----11..---11'-----

o 3 6 9 12 15 

Turbulence intensity (%) 

(b) turbulence intensity 

30-min. parameter means: 

Hub windspd. = 9.78 m/s 

Hub wind dir. = 274 deg. 

Grad. Rich. no. = +0.26 

Smoothed Vertical Profile (PNL Tower) of Uh b-Normalized 

Horizontal Wind Speed (a) and Turbulence In~ensity (b) for 

1983--Run A14-2 

52 

ro 

<0 

ro 

o 

o 

en 

<0 

ro 

o 

o 
Q) 

(f) 

ctI 

.Cl 

10-

Q) 

3: 

0 

+-' 

Q) 

> 

0 

J:l 

ctI 

+-' 

..c 

0) 

Q) 

I 

TR-3036 

Norm.PNL Wind Profile for Run A 18 :2 

co 

o 

o 

100 

75 

50 

25 

o 

0.7 0.8 0.9 1.0 1.1 

) normalized horizontal 

1.2 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 12.8 m/s 

Hub wind dir. = 265.8 deg. 

Grad. Rich. no. = +0.72 

;::. 

Run A18 PNl Turbulence Init,An~~ihl Profile co 

o 

o 125 ~--"iF""""--~--....----...,.....----... 

100 

75 

50 

15 

Turbulence intensity (%) 

turbulence intensity 

30.;.min. parameter means: 

Hub windspd. = 12.8 m/s 

Hub wind dir. = 265.8 deg. 

Grad. Rich. no. = +0.72 

Smoothed Vertical 

Horizontal Wind 

1983~-Run AlB 

Tower) of Uh b-Normalized 

Turbulence In~ensity (b) for 

53 
Norm.PNL Wind Profile for Run A 11 

TR-3036 

C\J 

r--

;;; 

o 

o 

125~----~----~----~----~--~ 

Q) 

> 

o 

.c 

m 

+-' 

.c 

0> 

100 

75 

50· 

Q) 25 I 

o 

0.7 0.8 0.9 1.0 1.1 1.2 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 13.1 m/s 

Hub wind dir. = 270.3 deg. 

Grad. Rich. no. = +6.68 

(a) Us normalized horizontal wind speed 

Run A 11 PNL Turbulence Intensity Profile 

125 _--.,-----r---..,..---~-____, 

-

E 100 

Q) 

CJ) 

m 

.c 

Q) 

> 

o 

.c 

m 

+-' 

.c 

0> 

75 

50 

Q) 25 

I 

o 

o 3 6 9 12 15 

Turbulence intensity (%) 

(b) turbulence intensity 

30-min. parameter means: 

Hub windspd. = 13.1 m/s 

Hub wind dir. = 270.3 deg. 

Grad. Rich. no. = +6.68 

Figure 3-13. Smoothed Vertical Profile (PNL Tower) of Uhub-N?rmalized 

Horizontal Wind Speed (a) and Turbulence Intens1ty (b) for 

1983--Run All 

54 

C'"J 

r--

;;; 

o 

o 
125 

E 100 

Q) 

C/) 

ct:l 

..a 

I.-

Q) 75 

3: 

0 

...... 

Q) 

> 

0 50 ..a 

ct:l 

25 

o 

0.7 

100 

Q) 

C/) 

ct:l 

..0 

I.-

Q) 75 

3: 

0 

...... 

Q) 

> 

0 50 ..0 

ct:l 

...... 

..c 

Ol 

Q) 25 I 

o 

o 

Figure 3-14 .. 

TR-3036 

Norm.PNL Wind Profile for Run A05 ;:: 

;;; 

o 

o 

Profile ref. to WS at hub ht. 

30-min. parameter means: 

Hub windspd. = 6.47 m/s 

Hub wind dir. = 290.7 deg. 

Grad. Rich. no. = +11.7 

0.8 0.9 1.0 1.1 1.2 

normalized horizontal wind 

Run A05 PNL Turbulence Profile 

30-min. parameter means: 

Hub windspd. = 6.47 m/s 

Hub wind dir. = 290.7 deg. 

Grad. Rich. no. = +11.7 

3 6 9 12 15 

Turbulence intensity (%) 

turbulence intensity 

Smoothed Vertical Profile (PNL Tower) of Uh b-Normalized 

Horizontal Wind (a) and Turbulence In~ensity (b) for 

1983--Run AOS 

55 

l{) 

I'--

OJ 

o 

o 
100 100 CD 

roo 

0 

0 

• • • • 80 • • • 80 • • • • • • 

Hub height Hub height 

- 60 60 

E E VI ...... ---- 0'\ ..c • ...... 0> ..c 

(J) 0> 

I (J) 

40 I 40 

20 20 

o 

4 5 6 7 8 9 10 11 12 o 4 5 6 7 8 9 10 11 12 

Wind speed (m/s) Wind speed (m/s) 

Figure 3-15. Smoothed and High-Resolution Vertical Profiles of Speed for Run A05 
E 

I..n 

-....J -c 

--

0) 

<D 

I 

100r-----------------------------------~ 

80 ' 

Hub height 

60 

40 

20 

o 

180 

-----

• 

• 

Wind direction (deg) 

) Smoothed profile 

Figure 3-16. Smoothed and 

100 p---------------------------------------~ ~ 

;;; 

o 

o 

80 

Hub 

60 

E 

-c 

--

0) 

<D 

I 40 

20 • 

o ~--~--~----~--~--~----~--~------~ 180 220 260 300 340 

Wind direction (deg) 

High-resolution vertical profile 

Vertical Profiles for Wind Direction for Run AOS 
V1 

00 E 

..... 

..c. 

0) 

Q) 

I 

100 ~--------------------------------~ 

80 

Hub height 

60 

40 

20 

0

16 17 18 19 20 21 22 

Temperature (0 C) 

E 

..... 

..c. 

0) 

Q) 

I 

100~--------------------------------------~ 

80 

Hub height 

60 

40 

20 

17 18 20 21 22 

Temperature (0 C) 

a:J 

r--

a:J 

o 

o 

Figure 3-17. Smoothed and High-Resolution Vertical Profiles of Sensible Temperature for Run AOS 
TR-3036 

101 

........... g: c: 00 '--' 0 (f) Mean sensor height = 78 m 0 

c: 

----

>. Mean windspeed = 7.4 ms-1 

+-' 

(f) Integral scale = 2.9 m c: 

Q) 100 "0 

cti 

lt> 

Q) 

a. 

(j) 

IQ) 

$: 

0 10-1 

a. 

"0 

Q) 

.~ 

cti 

E 

b0 

z 10-2 

10-1 100 101 102 

Frequency, n (Hz) 

3-18" l"arrowband Turbulence Measured at 

18-m 6 Run A05 

59 
TR-3036 

4.0 CHARACTERISTICS OF MOD-2 HIGH-FREQUENCY-RANGE EMISSIONS 

This section summarizes the characteristics of the high-frequency-range (HF) 

MOD-2 emissions found during both the 1982 and 1983 experiments. The bulk of 

the data presented reflects measurements made during the 1982 period. As 

previously stated, the objectives of the 1983 experiment were much more 

limited and focused on low-frequency-range (LF) emissionse Since an analysis 

of the 1982 data indicated a relatively small run-to-run variation in emission 

levels, no additional HF data were initially planned for the 1983 period .. 

During the end of this experimental period, an on-axis HF measurement was 

added, however.. The characteristics of the MOD-2 HF emissions include an 

estimate of the rotor (source) directivity, temporal statistics of the Aweighted emissions at various downwind distances for both single and multiple 

turbine operation, and a limited correlation of HF levels with various inflow 

regimes .. 

4.1 Observed Directivity Pattern 

A survey to estimate the rotor HF radiation pattern was accomplished during 

the 1982 experimental period.. A grid of markers was surveyed from the east to 

the southwest of Turbine No.2, as shown in Figure 4-1. Stakes were placed at 

a minimum range of 0.75D (69 m or 225 ft) to a maximum of 10D (915 m or 

3000 ft). On the evening of May 13, 1982, between the hours of 1900 and 2100 

(local time), a hand-held precision sound level meter was carried to each 

marker position as rapidly as practical and a 2-minute average value of the Aweighted sound pressure levels (SPL) was obtained. This period of the day was 

chosen because it was observed that local propagation effects (e.g., extensive 

refractive focusing) were at a relative minimum during these hours. The resulting A-weighted SPL directivity pattern is plotted in Figure 4-2 with the 

readings taken east and south of the turbine reflected to the north and west 

to form the symmetrical pattern shown. The shape of the contours in 

Figure 4-2 appears to resemble a classic quadrapole radiation pattern distorted by the prevai ling wind at larger di stances" i" e .. , extended downwind and 

contracted upwind of the rotor plane. 

4.2 Statistical A-Weighted Emission Distributions 

Important considerations in the future siting of turbines the size of the 

MOD-2 are the fall-off of the noise with distance and the temporal characteristics of the A-weighted SPL (a measure of loudness). It has been recognized 

that the A-weighted noise level received is a random variable because of the 

random nature of the turbulence driving many aeroacoustic mechanisms [10] and 

the modulation by atmospheric refraction between source and receivere In 

order to achieve a measure of both of these MOD-2 HF noise qualities at the 

Goodnoe Hills site, two community noise analyzers (CNA) were used at various 

positions downwind of the turbine rotor plane(s)e The CNA consists of an integral measurement-quality microphone, a preamplifier, and a specialized computer that continuously calculates the statistical cumulative distribution of 

the A-weighted SPL over three contiguous preset collection periods ranging 

from 30 minutes to several hours. Data were collected from the CNAs for both 

single and multiple turbine operation. 

60 
.....-

00 

o 

, 

, 

, 

Turbine No.2 

\ 

" "'- ~ 

I 

.~ 

\ 

......... 

1.50 (450') 

r 

,,;'" 

---. --- --0---

3D 

\ d 0 --. 

J I 1800 

I 

/ 

60 (1 

\ 

\ 

~ I 

, 

I 

I 

I 

o 

• 

100 (3000') 

\ 

\ 

, ~ 

Wind direction 

Legend: 

High-frequency microphonea 

(fixed location) 

site #2 

Roving sound level meter location 

Community noise analyzer 

(CNA) location 

aSolid symbols indicate fixed 

measurements. 

Figure 4-1. Microphone Locations Used in High-Frequency-Range Measurements 

0 

OJ 

ro 

0 

0 

~;;:~ 

[ I 

~- ~ 
TR-3036 

Wind direction 

;;; 

co 

o 

o 

Figure 4-2. A-Weighted Sound Pressure Level Directivity Pattern for a 

Single MOD-W Turbine. Levels shown are in dB(A). 

4.2.1 Single Turbine Operation 

For the single turbine evaluation, CNA data were collected from two locations 

(shown in Figure 4-1). One location was at the geometric center of the threeturbine cluster 1 e 75D (160 m or 525 ft) from Turbine No .. 2" The other was 

off-site and downwind of Turbine No .. 1 at a distance of about 3D (274 m or 

900 ft). The majority of the data collected involved turbines cycling on and 

off during the recording periods, but enough information was obtained with a 

stable operational configuration (i.e., one, two, or three turbines constantly 

operating for the entire recording period) to summarize the results. 

The fall-off of SPL observed with distance from a single turbine is shown in 

Figures 4-3 and 4-4. The cumulative distributions of the A-weighted SPL measured at 1.75D, 3D, and 10D downwind distances are plotted in Figure 4-3 along 

with the equivalent SPL (LeQ) and a representative background distribution for 

a surface wind speed of 7-Y mps (15-20 mph). As shown, the observed levels 

remain essentially normally distributed (a linear decrease with increasing 

probabili ty level) at the 1 e 75D and 3D distances.. The effect of the background noise becomes evident at the lOD distance, however, as this cumulative 

distribution shape approaches that of the reference background. It is likely 

that the HF turbine noises from a single MOD-2 would be heard only intermittently at this distance, under the reference background conditions. 

Figure 4-4 plots the equivalent sound level or L as function of downwind 

distance. The L corresponds to the equivalent so~~d level of a steady sound 

which, over a gi~~n period of time, would contain the same noise energy as the 

time-varying sound as the fluctuating turbine or background sounds. The L 

eq 

62 
<{ 

co 

""0 

---- (f) 

Q) 

> 

~ 

Q) 

l... 

:::::l 

(f) 

(f) 

Q) 

l... 

CL 

""0 

c 

:::::l 

0 

(f) 

""0 

C 

:::::l 

o 

(f) 

+-' 

C 

Q) 

cti 

> 

:::::l 

0-

W 

70 

65 

60 

55 

50 

45 

40 

35 

30 ~ 

25 

103 feet 

0.1 

Rotor diameters 

10 

1.5 

30 50 70 

1.750 (WT2) 

Leq = 59 dBA 

-'-'-'_ 30 (WT1) 

'-

Leq = 52 dBA 

"" 100 (WT2) 

Leq 43 dBA 

-- Background 

Leq 42 dBA 

90 95 98 99 

Cumulative probability 

TR-3036 

N 

co 

;;; 

o 

o 

of SPL at Three Downwind 

MOD-2 Turbine 

3.0 6.0 12.0 24.0 

Downwind distance 

Emi Downwind of a Single 

63 
70 

« 65 II) 

"'0 

--en 60 (]) 

> 

~ 55 (]) 

I.-

::::I 

en 

en 50 (]) 

l.-

n 

"'0 45 c 

::J 

0 

en 40 

"'0 

2 

..c 35 Ol 

(]) 

~ 30 I 

« 

25 

'10 30 50 70 90 95 9899 

Cumulative probability 

WTS 1, 2 & 3 

operating 

Leq = 55 dBA 

WTS 2 & 3 

operating 

Leq = 51 dBA 

Background 

Leq = 42 dBA 

TR-3036 

"<T 

oJ 

oJ 

o 

o 

Figure 4-5. Cumulative Distributions of A-Weighted SPL at Site No.2 (see 

Figure 4-1) for Two and Three Operating MOD-2 Turbines 

1S a preferred noise level metric of the u.s. Environmental Protection Agency 

(EPA). The plot of Figure 4-5 indicates" on the average, the turbine emissions level and the background noise at the Goodnoe Hills Site become the same 

at a distance of about 1220 m (4000 ft) or 13+D downwind. This distance may 

be considered the average audible range for this site. The turbine emissionlevel decay curve of Figure 4-5 asymptotically approaches the mean background 

L instead of falling as the "r2u dependence shown. This is most likely the 

r~~ult of a varying background level and propagation effects. The departure 

from r2 dependence began about 5D or 457m (139 ft) downwind of the turbine. 

This means the detectability of HF-range emissions downwind in the outdoors is 

almost entirely controlled by the background level in the vicinity of the 

receiver, as indicated by the shallow LeQ slope. A polynomial that describes 

the curve of Figure 4-5 to better than 0.)% is 

L (A) = -3.89464 x4 + 46.67294 x3 - 191.884 x2 + 287.1514 x - 28.4 e eq . , (4-1) 

where x is the log10 of the downwind distance in feet. 

4.2.2 Multiple Turbine Operation 

The effect of multiple turbine operation at the CNA measuring point east of 

the site shown in Figure 4-1 is summarized in the L cumulative distribution 

plots of Figure 4-5. The effect on the local noise I~vel is raised by 3 dB by 

the operation of all three turbines over Turbine No.1 (WT1) operating 

alone. Similarly, the difference between Turbines No" 2 and No 0 3 operating 

and No.2 operating alone at this location is, on the average, +8 dB. Thus, 

the local noise environment is dominated by the closest turbine in the cluster, but the effects of multiple turbine operation are most noticeable when 

sounds are compared from turbines located nearly the same distance upstream 

from the observer. 

64 
TR-3036 

4,,3 

A total of nine data runs were selected from the two experimental years, seven 

from 1982 and the two available from 1983, to quantify the effects on turbine 

inflow on the HF noise level radiated from a single turbine rotor" With the 

exception of the two 1983 runs the were selected to cover a range 

of hub-height wind speeds, vertical stabilities, and all with the same microphone placemente Table 4~1 lists the inflow conditions and the on-axis and 

in-plane A-weighted values at the reference distance of 1.5D from the 

hub aX1S for these 

3 1 

Table 4-1 indicate , on 

on-axis and 

ditions lis 

low-wind flow (just 

vect unstable 

lable in Table 1 is 

a systematic variation in 

stabili ,or 

experiment avai lable we 

ferences in the on-axis 

tions installed between the 

Sound Variation 

that there is little difference 1n the 

levels under the range of inflow contable (Ri :::: +24), 

(well above rated), conThe information 

establish whether or not there is 

levels with wind speed, 

, with only two cases from the 1983 

any statistically significant difresult of the turbine modificaods" 

The standard deviation of the entries 1n Table 4-1 was found to be ±2 dB for a 

wide range of rotor e, wind range covering cut-in to well 

above rated conditions There is a noticeable increase in the L level 

(6 dB) at a slightly more than doubl of the speed" This wi~~ speed 

dependence is plotted 4-6 for the on-axis and in-plane microphone 

positions, The correlation coefficient for the in-plane data is 

o 981, with standard of iO 3 dB for a linear regression. Similarly, 

the correlation coefficient the s readings is 00889, with a standard 

Levels for 

Run Hub-Height Wind Richardson On~Axis In-Plane 

No Direction Number (A» <1 (A» Year 

(deg) ) t~B) 

23-1 267 6 3 24 5geO 59,,9 1982 

21-1 275 3 o 46 60 9 6183 1982 

19-1 266 10 0,,18 61 8 6265 1982 

17-2 2 8 11 ,,01 62 1 6285 1982 

26-1 12 o 13 6104 6205 1982 

18 2511 1 26 62 8 neaa 1983 

27 0 62 8 63 7 1982 

11 56 6 68 6208 nea" 1983 

25-1 274 14 5 ~ 92 63 0 64 .. 1 1982 

Means 264 11 60 62 0 62 .. 4 

65 
..-... 

co 

"0 

66 

64 

62 

60 

58 4 6 

---0 In-plane 

---6 On-axis 

8 10 12 14 

Hub-height mean wind speed (mps) 

l!) 

co 

ro 

o 

o 

16 

TR-3036 

Figure 4-6. Leq(A) Levels as a Function of Hub-Height Wind Speed 

error of ±0.7 dB. The regression relations describing the MOD-2 on-axis and 

in-plane <L (A» dependencies as a function of the mean hub-height wind speed eq (UH) over a range of 6 to 15 mps are 

<Leq(A» = 0.52005 UH + 56 

<L (A» = 0.51327 UH + 57 eq 

(on-axis) 

(in-plane). 

(4-2) 

(4-3) 

A closer examination of Table 4-1 reveals some correlations with the hubheight wind direction and vertical stability. Table 4-2 lists the correlation 

matrices for the on-axis and in-plane <L (A» values with the hub-height wind 

direction and speed and the rotor disk v~~tical stability. While of Table 4-2 

confirms that the <L (A» levels are indeed related to the wind direction and 

vertical stability, tHese parameters would be site-dependent. 

Table 4-2. Multivariate Regression Correlations of 

of MOD-2 <L (A» Levels with Hub-Height 

W1nd . Vector eq d . 1 bOlO an Vert1ca Sta 1 1ty 

Parameter 

Wind direction 

Wind speed 

Richardson number 

Correlation coefficient 

Standard error (dB) 

66 

On-Axis 

<L (A» eq 

Level 

-.468 

0 .. 889 

- .. 691 

0 .. 945 

0.61 

In-Plane 

<L (A» eq 

Level 

- .. 037 

0 .. 981 

-.784 

0.983 

0 .. 36 
TR-3036 

Because of this dependency, there appears to be little advantage to including 

the wind direction as a predictor of the <L (A» for the MOD-2 in view of the 

high degree of correlation 1n the windeqspeed (loading) and stability. 

However, the relationship to the stability expressed here may be valid only 

for the Goodnoe Hills site. Thus, we suggest that only the hub wind speed be 

used" We suggest that the on-axis <L (A» value at rated wind speed be 

considered as a HF ssions figure-~~-merit for comparing MOD-2 class 

machines" For the MOD-2, thi s value would be 63 dB at the rated hub wind 

speed of 12.5 mps (28 mph) .. 

An interesting characteris ic of the MOD-2 A-weighted levels is the very small 

temporal or sampl e ion under a given set of conditionse The 

frequency distributions of the <SPL(A» parameter for the highest and lowest 

wind runs (25-1 and 23-1) are plotted in Figure 4-7. As shown in the figure, 

the sample-to-sample variation is quite small in both cases, enabling us to 

support the statement that the MOD-2 HF ion is very steady (as opposed 

to that of the LF, which is discussed in Section 5.0). 

4 3 .. 2 Emi 

The spectral content of the emissions is important not 

only for its contribution to measures of community annoyance, such 

as the L (A) parameter, but whether there are potentially 

annoyinge~iscrete tone ions. presentation is also useful 

for evaluating the relative contributions of various aeroacoustic source 

mechanisms to the total emission levels Such knowledge may be 

very important if the emission levels are considered too great and 

some modifications are required 

Two general presentations are current in use: narrowband and 

1/3-octave" , high-frequency resolution of the narrowband 

spectrum is most useful for examining the turbine emissions for discrete tone 

noise. Because of the number of elemental frequency bands or spectral lines 

involved (typically anywhere from 200 to 800), narrowband spectra are not useful for statistical analysis procedures because of the large data volume associated with their use To make the storage of the spectral representation 

more tractable for statistical analyses on smaller computers, the 1/10- or 

1/3-octave bands are often employed. The elemgntal s pect7ral frequency band is 

then bounded by a frequency ratio of 21/ 10 or 21 3. The 1/3-octave 

bandwidths, besides having a smaller number of elemental bands to process and 

store, approximately correspond to the critical bandwidths used by psychoacoustlclans to assess tonal noise.. We have chosen to use the 1/3-octave 

resolution for statistical analysis of both the low- and high-frequency-range 

MOD-2 emissions" The HF-range analysis bands are listed in Table 2-4" In 

addition, we have employed narrowband spectral techniques to examine the MOD-2 

emlSSlons for the existence of discrete tonal noise. 

Figure 4-1 shows the 10cat ions of the sound level meters used in the 1982 

experimental series. One was aced 137 m (450 ft or 1e5D) upwind and along a 

line parallel to the rotor s. This measurement station, referred to as the 

on-axis location, makes a 24° angle with the rotor plane centered at the 

hub. The other microphone station, which will be referred to as the in-plane 

67 
TR-3036 

Level Distribution <0 

co 

00 

0 

0 

Run #1: 8251 AX 

Run #2: 8231AX ----

50 

........ 

~ ~ 40 0 

-

Q) 

() 'I c II Q) II 1.-

1.-

::J 30 , I () 

II () 

0 , , -0 I I ~ 20 () , I c , I Q) 

::J I 0- , 

I Q) 10 , 1.- , u.. \ I 

I 

0 

50 60 70 80 90 100 

A-weighted level (dB) 

level distribution statistics 

Run #1 Run #2 

Mean BPL (dB) :;:: 63.7 58.0 

Peak BPL (dB) 66.0 59.8 

Min. BPL (dB) :;:: 61.1 56.3 

Std. dev. (dB) :;:: 1.26 0.85 

Skewness coef. :;:: -0.50 0.00 

Kurtosis coef. :;:: -0.34 -0.33 

Mode (dB) 64 58 

Median (dB) :;:: 64 58 

Figure 4-7. Observed Frequency Distributions of A-Weighted SPL for Highest 

and Lowest Mean Hub-Height Wind Speeds 

68 
TR-3036 

location (as shown in Figure 4-1) was located 137 m or 1.SD perpendicular to 

the rotor axis and in its plane. 

4 .. 3.2 .. 2 Establi 

----------------------~------~------------------

To ensure that the are actually emi tted from the MOD-2 and not the 

result of wind-induced or "pseudo" noise, it is important to establish reference spectra for a range of conditions. We did this by including a 

10- to IS-minute recording wi th each data run wi th the turbine shut 

down.. Figure 4-8 shows the range of background acoustic spectra taken at the 

on-axis and in-plane positions for the upper and lower limits of the average 

hub-height wind observed the 1982/83 experimental periods. The 

corresponding wind speeds were 6 and 14 .. 5 mps at the 59-m level on the BPA 

tower. As we see in this f , more than a 20-dB range in background noise 

is typical for the winds encountered at this microphone location during the 

1982 experiment, with some differences in the two positionse The ensemble mean 

band pressure level (BPLs) wi th the turbine ing and the associated 

background for these two cases 4-9 Only the on-axis 

microphone position wa used for data runs during the 1983 

experiment.. Figure 4-10 s a 1982 on-axis background 

spectra with one from 1983 with identical average wind speeds. One 

notices the s at hi leSe vIe attribute these 

differences to the condition in April/May 1982 and 

and August 1983; e.g. 

60 

50 

Leq = 57 dBA 

40 14.5 mps 

30 Leq = 37 dBA 

20 ~--~,~~~~~~~~~~~~~ 

102 103 104 

1/3-octave center frequency (Hz) 

69 

teo 

eo 

o 

o 

and 
TR-3036 

70 (J) 

co 

c;; 

0 

0 

.......... 

CO 60 '"0 

- Turbine operating Q) 

> Leq = 60 dBA ~ 50 (]) 

lo... 

:J 

C/) 

C/) 

(]) 

lo... 

a. 40 

'"0 

c 

ccs 

.c 

c 30 Background ccs 

(]) Leq = 34 dBA ~ 

20 ",,"" 

102 103 104 

1/3-octave center frequency (Hz) 

(b) lowest wind speed observation (6 mps) 

Figure 4-9. Comparisons of Observed Mean SPL Spectra for Operating and 

Background Conditions in High (a) and Low (b) Winds 

70 
TR-3036 

70 0 

CJ) 

;;; 

0 

0 

(l) 

"0 60 ......... 

ID 

> 

~ 

(!) 50 I-

::l. 

CI) 

CI) 

(!) 

ICl. 40 

"0 

c: 

G:S 

.0 1982 c: 

G:S 30 

(!) 1983 ~ 

20 102 103 104 

V3-octave center frequency (Hz) 

403 203 Rotor Inflow Influence on Distribution 

In Section 40301 the on the mean wind speed was 

discussed One would a similar relationship to exist with 

the 1/3-octave spectral , since the A-weighted <Leq> value is a summation 

of these bands. 4-11 ots the ensemble-averaged 1/3-octave BPLs from 

the on-axis and in-plane positions for seven mean wind speeds from the 1982 

experiment. The wind-speed dependency is less clear in the on-axis spectra 

than the in-plane. Also, the high-frequency level (above the 2500-Hz band) of 

the on-axis spectra rises because the local background noise exceeds the turbine radiation level. A comparison is made between two 1983 data runs and 

three runs from 1982 at similar wind speeds in Figure 4-12. We see that the 

turbine emission levels were 2-3 dB higher in the 630- to SOOO-Hz bands in 

1983 for a mean=wind=speed range of 12-13 mps 

We attempted, as in the case of the <L q(A» values, to determine if other inflow parameters such as the rotor s~ vertical stability (and therefore the 

turbulence structure) had any influence on the MOD-2 mean and temporal characteristics of the HF acoustic spectral content. Unfortunately, the bulk of the 

data available was from the 1982 experiment, when we had only a minimal 

description of the inflow turbulenceo 

We first examined data runs in which the mean hub-height wind speeds were 

similar but the stability (turbulence characteristics) varied. Using 

Table 4-1 as a guide, we chose to compare 1982 Runs 19-1, 17-2, and 26-10 

These three runs involved mean wind speeds of 10.3, 11.7, and 1200 mps and 

71 
TR-3036 

60 

, .......... 

co 

"0 

- 55 

Q) 

> 

~ 

0) 50 lo-

::::l 

Cf) 

Cf) 

0) 

lo0. 45 "0 

C 

ro 

..c 

c 

ro 40 ,0) 

:2: 

35 

102 

Leq 

---. 

co 

"0 

'-'" 

0) 

> 

0) 

0) 

lo-

::::l 

Cf) 

Cf) 

0) 

lo0. 

"0 

C 

ro 

..c 

c 

ro 

0) 

:2: 

Run 21-1 

50 

45 

40 

35 

Run 23-1 

UhUb = 6.3 mps 

Ri. = +24 

--- On-axis 

= 60 dBA - -- In-plane 

30 ~--~--~~~~--~--~~~ 

102 103 104 

1/3-octave center frequency (Hz) 

Leq = 61 dBA 

" Run 19-1 

Leq = 63 dBA 

)I 

-...... .".-""', 

\ 

\ 

\ 

\ 

'\ 

" UhUb 9.3 mps ........ Uhub = 10.3 mps 

Ri. = +0.18 

..... 

Ri. = +0.46 

103 104 102 103 104 

1/3-octave center frequency (Hz) 1/3-octave center frequency (Hz) 

Figure 4-11. 1982 HF-Range Acoustic Spectral Distribution for On-Axis 

(solid) and In-Plane (dashed) Locations as a Function of 

Wind Speed and Stability 

72 

0; 

co 

o 

o 
Q) 

> 

.S!2 

Q) 

b 

::l 

en 

en 

Q) 

b 

0.. 

"0 

C 

ct1 

.c 

c 

ct1 

Q) 

:2 

60 

55 

50 

45 

40 

35 

Leq = 63 dBA 

Run 17-1 

Uhub = 11.7 mps 

Ri = -0.01 

Leq 63 dBA 

Run 26-1 

Uhub = 12.0 mps 

Ri =+0.13 

TR-3036 

N 

en 

ro 

o 

o 

102 103 104 102 103 104 

1/3-octave center frequency (Hz) 

55 

50' 

45 

40 

35 

102 

4~'11 

Leq 64 dBA 

Run 27 

Uhub = 13.3 mps 

Ri = +0.42 

103 104 

1/3-octave center frequency (Hz) 

1982 Acoustic 

(solid) and In-Plane ( 

Wind and Stabil 

73 

1/3-octave center frequency (Hz) 

Leq = 64 dBA 

Run 25-1 

Uhub 14.5 mps 

Ri = -0.92 

102 103 104 

1/3-octave center frequency (Hz) 

Distribution for On-Axis 

Locations as a Function of 

(concluded) 
---. 

III 

"'C 

.......... 

Q) 

> 

~ 

Q) 

I-

:::J 

(J) 

(J) 

Q) 

I0.. 

"'C 

c:: 

ro 

..c 

c:: 

ro 

Q) 

~ 

..-

III 

"'C 

.......... 

Q) 

> 

Q) 

Q) 

lo.... 

:::J 

(J) 

(J) 

Q) 

lo.... 

0.. 

"'C 

c:: 

ro 

..c 

c:: 

ro 

Q) 

~ 

60 

1982 --- C') 

0> 

00 

0 

UhUb = 12.0 mps 0 

55 Ri = +0.13 

= 61 dBA 

-"" \ 

50 ,,"""\ "-... .... ..... 

" " 1983 '\ 45 --- \ 

Uhub = 12.7 mps 

\ 

\ 

Ri = +1.26 \ 

'\ 40 Leq = 64 dBA , .... _ ..... 

35 

102 103 104 

1/3-octave center frequency (Hz) 

60~---~--~~----------~--rl~ 

55 

50 

45 

40 

1982---

Uhub = 13.3 mps 

Ri -= +0.42 

Leq = 63 dBA 

1983 --

Uhub = 13.3 mps 

Ri = +6.68 

Leq = 63 dBA 

1/3-octave center frequency (Hz) 

Figure 4-12. Comparison of the On-Axis Component of the HF Spectral 

Distribution between 1982 and 1983 under High Loading 

Conditions 

74 

TR-3036 
TR-3036 

Richardson numbers of 0 18, -0,,01, and 0.13, respectively" This range of 

Richardson numbers indicates a neutral to slightly stable surface layer which 

would be dominated by slightly buoyancy-damped, mechanically {shear} generated 

turbulence" Figure 4-13 presents the ensemble-averaged mean BPL spectra for 

the on-axis and in-plane measurement stations for these runse On the whole, 

these mean spectra show remarkably little run-to-run variation. The exception 

is the on-axis station of runs 17-2 and 19-1, where less acoustic energy is 

present in the bands of run 17-2 even though this run has the 

higher vllnd speed of the two. The ion of the 1/3-octave BPLs 

1S as plots of the ensemble 1%, 5%, 10%, 20%, and 50% BPL 

exceedence, or <L20>, and <L50> level s in Figure 4-14" 

These levels represent of time the randomly sampled BPL equaled 

or exceeded that f For example, a 1000-Hz, 1/3-octave BPL <L20> of 

55 dB means that 20% of the samples contained levels of 5S dB or more. The 

only obvious difference is the on-axis or 1% exceedence level, which is 

in the 2500-Hz band with the others 

Runs 19-1 and 1-1 

res ively), but 

opposed to 0 18 The 

60 

<D 55 > 

~ 

<D 50 ~ 

::J 

(J) 

(J) 

<D 

~ 

Cl. 45 

u 

c 

co 

.c = 10.3mps c 

co 

(l) 

::2: 

1nce ffered 

the latter was more 

BPLs plott 1n 

Run 17-2 Run 26-1 

vs. 9.3, 

ie, 0 46 as 

4-15 and the 

'3; 

00 

o 

o 

UhUb ::: 11.7 mps Uhub = 12.0 mps 

Ri = -0.01 

104 103 104 

1/3-octave center 

75 

Ri =+0.13 

103 

\ 

'\ 

104 

) OF 

Inflow 
en 

(J) 

> 

(J) 

(J) 

u 

c 

(J) 

"0 

(J) 

(J) 

u 

x 

w 

en 

(J) 

> 

(J) 

::::J 

(J) 

u 

c 

(J) 

"0 

(J) 

(J) 

u 

x 

w 

(f) 

(i) 

> 

(J) 

::::J 

(J) 

u 

c 

(J) 

"0 

(J) 

(J) 

() 

x 

W 

60 

55 

50 

45 

40 

-"" \ 

\.-, 

" UhUb = 11.7 mps ''-........ 

Ri. = -0.01 ", 

L(1) = 0 

L(s) = t::.. 

L(10) = 0 

L(20) = • 

L(so) = dashed line 

On-axis 

, 

35 l..----IL--..L-L-L...L..L--L_---'---'---I-..L...I.....L..II....II 

60 

55 

50 

45 

40 

35 

60 

55 

50 

45 

40 

1 02 1 03 1 04 1 02 

102 

L(1) = 0 

L(s) = t::.. 

L(10) = 0 

L(20) = • 

L(so) = dashed line 

103 

Uhub = 12.0 mps 

Ri.=+0.13 

L(1) = 0 

L(s) = t::.. 

L(10) = 0 

L(20) = • 

L(so) = dashed line 

(a) 

On-axis 

104 102 

(b) 

On-axis 

102 

103 

103 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

103 

\ 

~ 

\ 

\ 

TR-3036 

In-plane 

104 

In-plane 

104 

In-plane 

104 

(c) 

1/3-octave center frequency (Hz) 1/3-octave center frequency (Hz) 

Figure 4-14. Comparison of L1, LS' L10 , LZO ' and LSO 1/3-octave Level 

Spectra at 1.5D at the On-Ax1S and In-Plane Measurement 

Locations for 1982 Runs: (a) 17-2; (b) 19-1; and (c) 26-1 

76 

l{) 

(J) 

co 

o 

o 
Q) 

> 

(J) 

60 

55 

~ 50 ::l 

en 

en 

(J) 

b 

a. 

"0 45 c 

(1j 

.D 

C 

(1j 

(J) 

:2: 

40 

On-axis 

R19-1, Uh = 10.3, Ri 

1/3-octave center frequency (Hz) 

4-15" of 

9 3 

1 

10 

TR-3036 

R19-1 

R21-1 

103 

In-plane 

1/3-octave center frequency (Hz) 

104 

<0 

(J) 

0; 

o 

o 

exceedence levels 4-160 It is clear, under these conditions, that 

there 1S essent uniform BPL across the spectrum with the 

increase in wind and stabili Not much variation is evident in the 

on-axis exceedence plots, as shown in 4-16a0 There is a noticeable increase in peaking in the IOOO-Hz and 2500-Hz BPLs of the in-plane measurement 

(Figure 4-16b) in the , lower-stability case (run 19-1)0 

The increase in the peaking with exceedence or L-level noted above appears to 

associated with some form of oscillatory behavior of the acoustic emissions in 

these bands This peaking appears to be load-related and often more noticeable in the in-plane rather than in the on-axis measurements@ This apparent 

load dependency is shown in Figures 4-15 and 4-16 at both measurement 

stations. The on-axis, 2500-Hz band L-levels seem to be less affected than 

the in-plane exceedences, indicating that the process responsible radiates 

more strongly in the rotor plane. The in-plane peaking behavior broadens to 

include lower frequency bands @ The extent of this broadening appears to be 

associated with lower stabilities (Richardson numbers)e This is most noticeable in Runs 27 and 25-1 in Figure 4-170 We know, for example, that during 

Run 25-1, the turbine exhibited a very unstable behavior and finally shut itsel f down because of exces s i ve dri ve-t rain vi brat ion Very high level s of 

coherent, low-frequency emissions were also present during this run, which are 

discussed in Section 5 0 

A good example of the effect of wind speed or rotor loading for two runs under 

similar stability conditions is plotted in Figure 4-17.. The mean hub wind 

speed for Run 27 was 13 .. 3 mps and the Richardson number was 0 .. 42; the wind 

speed for Run 21-1 was 903 mps with a Richardson number of 0,,46. The on-axis 

station shows the typical upward shift in BPL at frequencies below 2500 Hz for 

77 
60 r 

co 55 "0 

......... 

(/) 

Q) 

> 

(]) 

(]) 

u 

c 

(]) 

"0 

(]) 

(]) 

u 

>< 

w 

50 

45 

40 

UhUb == 10.3 mps 

Ri.=+0.18 

L(1) 0 

L(5) =!::::. 

L(10) = 0 

L(20) = ., 

L(50) = dashed line 

On-axis 

Run 19-1 

'--

35~--~~~~ww~------~~~~~u 

102 

60 

..--... 

co 55 "0 

(/) 

(]) 

> 

(]) 50 

::J 

(]) 

u 

c 45 (]) 

"'0 

(]) 

(]) 

u 

>< 

w 40 

103 104 102 

1/3-octave center frequency (Hz) 

/-, 

/ '-, ,-- '--;' \ 

Uhub = 10.3 mps 

Ri.=+0.18 

L(1) = 0 

L(5) =!::::.. 

L(10) = 0 

L(20) = • 

L(50) = dashed line 

\ 

Run 19-1 

\ 

\ 

\ 

\ 

\ 

\ 

"-

" 

(a) 

In-plane 

Uhub = 9.3 mps 

Ri. = +0.46 

103 

TR-3036 

Run 21-1 

104 

1/3-octave center frequencX' (Hz) 

UhUb = 9.3 mps 

Ri. = +0.46 

Run 21-1 

t---

'" 00 

o 

o 

co 

'" 00 

o 

o 

102 103 104 

1 /3-octave center freq uency (Hz) 1/3-octave center frequency (Hz) 

(b) 

Figure 4-16. Comparison of L1 , LS' LID' LZD ' and LSD 1/3-octave Level 

Spectra at I.SD at the On-Ax1S (a) and In-Plane (b) 

Measurement Stations 

78 
co ----

~ 55 

(j) 

> 

(j) 

(j) 

I-

::::l 

C/) 

C/) 

(j) 

IQ. 

50 

45 

40 

On-axis 

Uh = 13.3 mps, Ri = +0.42 

~ R21-1, Uh ::: 9.3 ::: +0.46 

35 102 103 104 102 

1/3-octave center (Hz) 

4-11" 

TR-3036 

In-plane 

103 104 

1/3-octave center frequency (Hz) 

and 

the wind speed, but above that frequency band the levels become 

similar. There is a more or less uniform upward BPL shift in the rotor plane, 

except for the peaking or in the 1600 to 4000-Hz bands in the run with 

the higher rotor This difference is clearer in the peak BPL plots of 

Figure 4-18 To whether this fluctuating or as illatory condition 

is related to the inflow li, we looked at two high-wind runs (Run 25-1 

at 1405 mps and Run 27 at 1303 mps) with Richardson numbers of -0 .. 92 and 

o 42 e The mean BPL spectra assoc wi th these two cases were plotted in 

Figure 4-190 While both the on-axis and in-plane mean BPL spectra are very 

similar, the peak BPLs shown in Figure 4-20 for each are quite different in 

the 2500-Hz region The unstable case shows a greater tendency for peaking in 

the on-axis measurement (Figure 4-20a), and while the more stable one has the 

same tendency in the rotor plane measurement (Figure 4-20b)@ While it appears 

that rotor loading is the major factor determining whether or not highfrequency oscillatory behavior will occur, the stability (and therefore the 

turbulence structure) seems to influence the directivity of the acoustic radiation. We did not experience the degree of turbine operational instability 

under the stable atmospheric conditions of Run 27 that we observed during 

Run 25-1 e 

We attempted to compare the two available data runs of on-axis measurements 

taken in 1983 (with the modified turbine) with 1982 runs made under conditions 

as similar as possible. Run 27 (1982) and Run 11 (1983) had the same mean 

hub-height wind s (mean rotor loading), but in the latter inflow was much 

more stable; ieee, 0042 in 1982 and 6068 in 19830 The closest corresponding 

1982 run to 1983 i S Run 18 was Run 26-1 e Run 26-1 had a mean wind speed of 

12 mps and a Richardson number of 0 e 13, compared wi th 1207 mps and 1 e 26, 

respectively, for Run 18 Figures 4-21 and 4-22 plot the ensemble mean BPL 

and spectra for these two caseso The mean and <L20> BPL spectra of 

79 
60 

co 55 "'0 

(/) 

(!) 

> 

~ 50 

:J 

(!) 

() 

c 45 (!) 

"'0 

(!) 

(!) 

() 

w 

>< 40 

Uhub = 13.3 mps 

Ri = +0.42 

L(1) =0 

L(5) = b. 

L(10) = 0 

L(20) = • 

L(50) = dashed line 

On-axis 

Run 27 

UhUb = 9.3 mps 

Ri = +0.46 

TR-3036 

Run21-1 

o 

o 

N 

co 

o 

o 

102 103 104 

co 

"'0 

(/) 

(!) 

> 

(!) 

....J 

(!) 

() 

c 

(!) 

"'0 

(!) 

(!) 

() 

>< 

w 

60 

55 

50 

45 

40 

1/3-octave center frequency (Hz) 

Uhub = 13.3 mps 

Ri = +0.42 

L(1) =0 

L(5) = b. 

L(10) =0 

L(20) = • 

L(50) = dashed line 

Run 27 

1/3-octave center frequency (Hz) 

(a) 

In-plane 

102 

(b) 

1/3-octave center frequency (Hz) 

Uhub = 9.3 mps 

Ri +0.46 

103 

Run21-1 

1/3-octave center frequency (Hz) 

104 

Figure 4-18. Comparison of L1 , LS' L10 , L20 , and LSO 1/3-0ctave Level 

Spectra for High and Moderate Wind Speeds under Stable Inflow 

Conditions. On-axis measurements are shown in (a) and inplane measurements in (b). 

80 
(l) 

> 

(l) 

(l) 

~ 

::J 

(f) 

(f) 

~ 

0-

-0 

C 

CI:l 

..0 

c 

CI:l 

(l) 

~ 

On-axis 

55 

50 

45 

TR-3036 

In-plane 

;; 

N 

00 

o 

o 

35 L-__ L-__ ~-L-LL-__ ~ __ ~~~ 

102 103 1 04 102 103 104 

1/3-octave center frequency (Hz) 1/3-octave center frequency (Hz) 

Mean 1 for a Mean Wind 

mps and Stable (Run 21) and a Mean 

Wind of 14@5 mps and Unstable Conditions (Run 25-1) 

81 
CI) 

(!) 

> 

(!) 

(!) 

() 

c 

(!) 

"'0 

(!) 

(!) 

() 

>< 

w 

a:.l 

"'0 

--CI) 

(!) 

> (!) 

::J 

(!) 

() 

c 

(!) 

"'0 

(!) 

(!) 

() 

>< 

w 

60 

55 

50 

45 

40 

35 102 

60 

55 

50 

45 

40 

TR-3036 

On-axis 

N 

o 

N 

co 

o 

Uhub = 13.3 mps 

Ri = +0.42 

L(1) =0 

L(5) = b. 

L(10) = 0 

L(20) = • 

L(50) = dashed line 

Run 27 

1/3-octave center frequency (Hz) 

(a) 

In-plane 

Run 27 

Uhub = 13.3 mps 

Ri = +0.42 

L(1) =0 

L(5) = 1:!. 

L(10) = 0 

L(20) = • 

L(50) = dashed line 

1/3-octave center frequency (Hz) 

(b) 

102 

UhUb = 14.3 mps 

Ri = -0.92 

103 

Run 25-1 

1/3-octave center frequency (Hz) 

104 

Run 25-1 

Uhub = 14.5 mps 

Ri = -0.92 

102 103 

1/3-octave center frequency (Hz) 

o 

Figure 4-20 .. Comparison of Ll , LS' LlO ' L20 , and LSO 1/3-0ctave Level 

Spectra for High-Wind, Stable vs. High-Wind, Unstable Inflow 

Conditions. On-axis measurements are shown in (a) and inplane measurements in (b). 

82 
o 

N 

...J 

60 

50 

40 

30 

1/3-octave center frequency (Hz) 

mean BPL 

-- R27 (1982), Uh = 13.3 mps, 

Ri. = +0.42 

A 11 (1983), Uh = 13.3 mps, 

Ri = +6.68 

20b---~--~--~~~--~--~--~~ 

102 103 104 

1/3-octave center frequency (Hz) 

BPL 

4-21., Mean and BPL for 1982 Run R21 and 1983 Run A-II 

83 
TR-3036 

60 LO 

0 

~ 0 

0 

...-

co 

u 55 '-" --..... Q) ........... > '-., ~ 

Q) 50 , l>- , :J , (J) 

(J) 

Q) \ l>-

Cl. 

45 \ u \ c: 

ctl \ 

..0 \ 

c: \ ctl 

Q) 40 \ ~ \ 

\-

35 102 103 104 

1/3-octave center frequency (Hz) 

(a) mean 1/3-octave BPL spectra 

60 <0 

0 

N 

co 

0 

0 

...-.. 

CO 55 --..... .... U ...... _"'"'\ ......... 

Q) \ 

\ > \ ~ 50 \ Q) 

l>-

:J \ (J) A26-1 (1982), \ (J) 

Q) \ 

45 Uh = 12.0 mps, l>- , Cl. \ u Ri = +0.13 \ c: \ ctl ---- A18 (1983), \ ..0 

0 \ N 40 Uh = 12.4 mps, , -' Ri +1.25 '- 35L---~---L~~~--~--~--~~ 

102 103 104 

1/3-octave center frequency (Hz) 

(b) L20 BPL spectra 

Figure 4-22. Mean and L20 BPL Spectra for 1982 Run 26-1 and 1983 Run A18 

84 
TR-3036 

Figure 4-21 show excellent agreement for the bands below 1600 Hz for the two 

years under the same mean loading conditions. Figure 4-22 shows the expected 

higher mean and <L28> levels for the 1983 run (since it has a higher wind 

speed) below the 160 Hz band. What is different is the sharp spectral cutoff 

above the l600-Hz band in both 1983 runSe There does seem to be a hint of the 

oscillatory peaking seen in the data for the 1982 higher wind-speed run, particularly for run 11, as shown in the L-Ievel summaries of Figures 4-24 and 

4-25.. In the 1983 runs, there appears to have been a downward shift in the 

frequency band, in which the peaking occurs, from 2500 Hz in 1982 to 1000 Hz 

in 1983 .. 

4 .. 4 

Averaged narrowband (25-Hz resolution) spectra were computed over a frequency 

range of 100 to 10,000 Hz for a 2-minute period located midway in each data 

run listed in Table 4-1 The purpose was to locate and identify the possible 

source(s) of any discrete tonal components found in the MOD-2 emissions .. 

Figures 4-26, 4-27, and 4-28 present a sample of the resulting narrowband onaxis and in-plane spectra for Runs 23-1, 17-2, and 25-1 from the 1982 experiment (1mfl, moderate, and high wind regimes). Figures 4-29 and 4-30 present 

the on-axis spectra from 1983 runs 11 and 18 in moderate to high winds. No 

significant, steady tone noise components were found This indicates that the 

mechanical noise sources associated with the drive train are well controlled, 

and there appear to be no discrete aeroacoustic sources of consequences 

70 to 

N 

co 

1983 RunA18 0 

1983 Run A11 0 

en 

u Uhub = 13.3 mps Uhub = 12.7 mps 60 

m 

> 

~ 

Q) 50 l... 

::l Turbine Turbine en 

en 

Q) 

l... 

a.. 40 

"0 

c 

co 

..0 

c 30 Background 

co 

Q) Background 

:2: 

20 102 103 104 102 103 104 

1/3-octave center frequency (Hz) 1/3-octave center frequency (Hz) 

4-23 Lom~lat~lson of Acoustic at I. 

Measurement Location with and without 

under Wind 

85 
60 

-III 55 

"'0 

CJ) 

Q) 

> 

Q) 50 

:::J 

Q) 

() 

~ 45 

"'0 

Q) 

Q) 

() 

~ 40 

UhUb = 13.3 mps 

Ri = +0.42 

L(1) = 0 

L(5) = 0 

L(10) = b.. 

L(20) = • 

L(50) = dashed line 

1982 Run 27 

35~--~~~~~~~--~~~~~~ 

102 103 

1/3-octave center frequency (Hz) 

102 

UhUb = 13.3 mps 

Ri = +6.68 

103 

TR-3036 

1983 Run A11 

104 

1/3-octave center frequency (Hz) 

Figure 4-24. Comparison of Ll , L5 , LIO ' L20 , and L50 On-Axis 1/3-0ctave 

Level Spectra for Ident1cal Mean Wind Conditions during 1982 

and 1983 Experiments 

60 

EO 55 

"'0 

CJ) 

Q) 

~ 50 

Q) 

g 45 

Q) 

"'0 

Q) 

Q) 

() 

~ 40 

Uhub = 12.0 mps 

Ri=+0.13 

L(1) = 0 

L(5) = 0 

L(10) = b. 

L(20) =. 

L(50) = dashed line 

1982 Run 26-1 

1/3-octave center frequency (Hz) 

102 

Uhub = 12.7 mps 

Ri = +1.26 

103 

1983 Run A18 

1/3-octave center frequency (Hz) 

O"l 

o 

N 

co 

o 

o 

104 

Figure 4-25. Comparison of Ll , L5, LI0 , L20 , and L50 On-Axis 1/3-0ctave 

Level Spectra for S1milar Wind Speed Regimes in 1982 and 1983 

86 

co 

o 

~ o 

o 
4-26 .. 

4-27 

TR-3036 

10 ~ ____ ~ ____ ~ __ -L __ L-~ ____ -L ______ ~ __ ~~~ 

102 103 104 

Frequency (Hz) 

Narrowband = 12 .. 5 Hz) 

50 

40 

30 

20 

1982 Run 17-2 

Uhub = 11.7 mps 

Ri = -0.01 

On-axis 

Background 

10L-----~-----L---~~~L-----~-----L--~~~ 

102 103 104 

Frequency (Hz) 

1982 Narrowband 

Wind Conditions 

87 

for Moderate 
<l) 

> 

<l) 

60 r-----.------.--.--.-.----__ ~----~--~~ N 

N 

co 

o 

o 

50 

40 

30 

20 

10 102 

1982 Run 25-1 

Uhub = 14.5 mps 

Ri = -0.92 

On-axis 

103 

Frequency (Hz) 

Background 

104 

TR-3036 

Figure 4-288 Representative 1982 HF-Range Narrowband Spectra for AboveRated, Unstable Inflow Conditions 

<l) 

> 

<l) 

<l) 

l.... 

::l 

(/) 

(/) 

<l) 

l.... 

0.. 

"0 

C 

a:l 

OJ 

60 

50 

40 

30 

20 

1983 Run A 11 

Uhub = 13.3 mps 

Ri = +6.68 

Background 

source 

\ 

10L-----~----~--~--~~----~------~--~~~ 

102 103 104 

Frequency (Hz) 

(") 

N 

co 

o 

o 

Figure 4-298 1983 On-Axis HF-Range Narrowband Spectra for Above-Rated, 

Stable Inflow Conditions 

88 
60 

50 

co 

"0 

Q) 

> 40 ~ 

Q) 

~ 

::J 

(f) 

(f) 

Q) 

~ 30 CL 

"0 

C 

co 1983 Run A18 co 

20 Uhub = 12.7 mps 

Ri = +1.26 

10 

102 

Figure 4-30. 1983 On-Axis 

Stable Inflow 

103 

Frequency (Hz) 

89 

TR-3036 

'<t 

N 

co 

0 

0 

source 

\ 

104 

for Near-Rated., 
TR-3036 

5eO CHARACTERISTICS OF LOW-FREQUENCY-RANGE EMISSIONS 

In this section, we discuss the acoustic emissions characteristics of the 

MOD-2 design in the low-frequency-range (LF), nominally S-lOO Hz. Coherent or 

impulsive emissions in this frequency range were responsible for annoyance 

problems in sever~l households within 3 km of ~he MOD-l turbine near Boone, 

N.C., reported on in Refs. [1] and [19]. The acoustical loading of the homes 

affected by the MOD-2 LF emissions and subsequent internal acousto-mechanical 

interactions were responsible for the annoyance of the residents. Therefore, 

it is quite important not only to assess the MOD-2 emissions under a range of 

inflow conditions to determine the degree, if any, of similar characteristics, 

but also to identify the inflow properties responsible" In the MOD-l case, 

the inflow was responsible for exciting Strouhal-type vortex shedding from the 

support tower legs, the ultimate cause of the coherent radiation. Since the 

MOD-2 rotor is upwind of its support tower, one would expect the source of 

coherent LF noise radiation to reside in the inflow structure itself. The 

question then arises of whether or not combinations of inflow structure and 

coupled rotor aerodynamic response could be responsible for coherent noise 

levels similar to that of the MOD-l. 

In this section, we use both direct and ensemble statistical approaches to 

identify efficient inflow predictors for correlating the radiated 1/3-octaveband spectrum levels. We also apply a number of statistical measures of 

coherent noise radiation developed for characterizing the MOD-l to the MOD-2 

data and compare the results. Finally, we use available blade surface pressures, aeroelastic measurements, and radiated acoustic pressure fields to 

define the physical space scales ln the inflow responsible for LF noise 

produced by the MOD-2 rotor. 

5.1 Influence of Rotor Inflow Structure on LF Noise Spectra 

5.1.1 MOD-2 Aeroacoustic Response Function 

An objective of this study was to determine, by experimental methods, the 

observed aeroacoustical response function of a MOD-2 turbine. This, of 

course, assumes that the turbulent inflow structure is the sole excitation and 

that it can be quantified in some manner in order to be successfully related 

to the radiated acoustic pressure spectrum. We have taken two approaches to 

this task. One, the direct approach, was based on taking the power spectra 

ratio of simultaneously measured inflow turbulence and radiated acoustic 

signals. In the other approach, we used the statistics of randomly sampled 

ensembles of the 1/3-octave acoustic spectra as the dependent variables and 

five bulk or characteristic properties of the inflow as the independent variables ln a multivariate regression model" Both approaches are discussed 

below. 

S.l.l.i Direct Measurement Approach 

During our 1983 data Run No. AOS, the tethered balloon ins trument package, 

with the hot-film anemometer installed, was flown in the Turbine No. 2 inflow 

at approximately hub height.. The orientation of the hot-film sensor (a 

O.lS-mm diameter, quartz-coated wire) in the flow was such that the electrical 

output was proportional to the vector sum of the local longitudinal and vertical (upwash) components of the turbulent wind. The nonlinear electrical 

90 
TR-3036 

signal was transmitted via a digital FM radio telemetry link to a ground 

receiver, where the signal was re-converted to an analog voltage, linearized, 

and recorded on FM magnetic tape. The dynamic range of the radio telemetry 

link was better than 70 dB with a data bandwidth of 125 Hz. The bandwidth of 

the final FM tape recording was limited to 100 Hz, however. 

Because of the nonstationary nature of the inflow, the 3D-minute data run was 

divided into six 5-minute segments. The particular 5-minute period discussed 

here was chosen because it exhibi ted minimal variation in height of the 

anemometer (a standard deviation of ±6 m) and contained substantial levels of 

high-frequency turbulence. Table 5-1 summarizes the pertinent operational and 

inflow characteristics associated with this run segment. 

We chose a vertical window of ±10 m from the mean hot-film anemometer height 

as the region of the rotor disk from which we would correlate the acoustic 

output at the 1.5D measuring station" This choice resulted in defining the 

two disk segments pictured in Figure 5-1 By delaying the data conversion and 

Fourier transformation a fixed amount from the time the rotor blade was parallel to the tower base, we could start the Oe5-second conversion period to 

coincide vlith the windows shown in Figure 5-1" The inflow turbulence signal 

was delayed in time equivalent the mean convection speed, 7 .. 4 mps. The 

acoustic spectra were delayed an additional O.S-second with respect to the 

turbulence signal to allow for the sound propagation (retarded) time from the 

disk segment to the microphone array. The resulting averaged spectra were 

calculated from ten 4-second records containing 80 blade passages and 81,920 

data points. 

The measured LF acoustic response (spectral ratio) in Pa/mps is plotted in 

Figure 5-2 over a frequency range of 2-100 Hz with an effective bandwidth 

(resolution) of 2 Hz.. The abscissa has also been scaled in units of the 

reduced frequency parameter k def ined by k:= ncf /U, where cis the chord 

length and U the relative blade speed at 80% span. An interesting result is 

achieved if the response spectrum of Figure 5-2 is plotted against the 

turbulent wavelength (assuming the frozen turbulence hypothesis) as ln 

Figure 5-3, in which the location of the integral scale is also shown. Since 

one interpretation of the integral scale is that it represents the largest 

Table 5-1 .. Turbulence Excitation and Turbine Operation 

for the of Run A05 

Turbulent Layer Structure Parameters 

Mean measurement height (above the tower base) 

Mean horizontal wind speed 

Turbulence intensity 

Layer turbulence integral scale length 

Turbine Operating Angles 

Indicated mean blade angle (ref at StaG 1260) 

Calculated mean angle of attack (Sta .. 1164) 

Calculated mean angle of attack (Sta. 1562) 

91 

78 ±6 m 

7.39 mps 

8.0% 

2 .. 89 m 
TR-3036 

Measurements taken 

in these sectors 

l' / 

I" {. \ 

/ I '" ,/ \ \ 

I --I ,--__ " ,/ __----, ~ \ 

~ Blade sta. 116:5%)~ Blade sta. 1562 (87%) 

Facing downwind 

Figure 5-1. Relationship of Location of Measurements on Blade No.1, 

Rotor Disk Geometry, and Height of Elevated Turbulent Layer 

c 

o 

t) 

C 

:J 

"4-

~ 10-1 

C 

o 

Q. 

CJ) 

9:_ 

.~ in 

(j) E :J ____ 10-2 

o C\l 

uCL 

C\l ........... 

>-

U 

C 

Q) 5- 10-3 

Q) 

lo-

"4-

~ o 

~ 

<D 

N 

00 

o 

o 

10-4 ~----~----~--~ __ ~~ ____ ~ ______ ~ __ L--L~ 

100 (10- 1) 101 (100 ) 

Frequency (Hz) 

102 (101 ) (k) 

Figure 5-2. Measured Low-Frequency Acoustic Response Function 

92 

l{) 

N 

00 

o 

o 
c 

o 

+"' 

() 

C 

:J 

-

10 8 I'-

N 

- co 

o 

o 

TR-3036 

CG 10- 1 

u; 

C 

C'Cl 

l-

+-' 

5-3 

Correlated turbulent eddies 

10-4 ~ __ -L ____ ~-L_~J-~~ ____ ~~~~ ____ ~ __ -L~~~ 

10-1 

Measured 

Turbulent 

10 0 101 

Turbulent wavelength (chords) 

Acoustic 

102 

as Function of 

correlated eddy size in the flow, it is clear from Figure 5-3 that the 

characteristics of the radiated spectrum appear to change at that boundary. 

Parameter tivariate Hodel 

As explained in Section 204, ensemble statistics (up through and including the 

4th moments) of the LF emissions spectra in 1/3- and octave-band resolution 

listed in Table 2-2 were compiled from 100 random samples of the MOD-2 farfield acoustic signals for each of the three, 10-minute segments of each data 

rune The resulting statistics were then averaged together to form the 

aggregate statistical record for the run.. A similar approach was used for 

background measurements, but it was only 10 minutes in length and no averaging 

was necessarYe Measures of the inflow bulk or characteristic scaling 

parameters (e.g@, a reference-height mean horizontal wind speed UH, the rotor 

disk gradient Richardson number Ri, and various turbulence length scales 

measured at a reference height) were compiled as discussed in Section 2.4.2. 

ficance of Observed Run-to-Run lons 

Before attempting to correlate the observed run-to-run variations ln the 

measured far-field acoustic emissions spectral statistics with inflow 

characteristics, we compared the observed variations to what would be expected 

for completely random fluctuations Figures 5-4a, b, c, and d plot the 

observed run-to-run variations for the 1/-3 octave band means and variance, 

skewness, and kurtosis coefficientsa Also included in these figures are the 

variations which would be expected from a purely random noise process at the 

93 
14.0 

CO 

~ 

.§ 12.0 ..., 

(1j 

.;:: 

~ 10.0 

V 

:> 

(]) 

...:l 8.0 

S 

;:J 

s... ..., 

() 6.0 (]) 

0... 

(f) 

'"d 4.0 1=1 

(1j 

iIl 

1=1 

(1j 2.0 (]) 

:::::1 

0.0 

10 

1=1 

:§ 

(1j 

.;:: 

(1j 

:> 

~ (]) 

~ (]) 

0 

U 

Ul 

rn 

Jl) 

1=1 

;;: 

Q) 

.!:<l 

(f) 

v 

:> 

(]) 

...:l 

'"(j 

1=1 

(1j 

iIl 

10 -1 

1 

\ 

\ 

--.... 

mbsl-rangQ-comp 

observed variation range 

co 

N 

co 

g 

expected range (99%-level) 

I 

I 

k=1 

- .... 

-- -- ......... " 

I I " I I I I I II i 

10 10 2 

1/3-0ctave Center Frequency (Hz) 

(a) mean BSL comparison 

i III 

10 J 

bl-BI:QW-rangQ-C017\p 

0 

observed variation range gJ 

gs 

0 

expected range (99%-level) 

_J 

I 

I 

I 

I 

I 

I 

I 

I 

I \ I 

" 

k=1 

10 10 2 10 J 

1/3-0ctave Center Frequency (Hz) 

1=1 

o 

:;:::; 

0.30 

.~ 0.25 (1j 

:> 

..., 

1=1 

(]) 

~ 0.20 

(]) 

o 

u 

(]) 0.15 (.) 

1=1 

(1j 

.;:: 

~ 0.10 

V 

:> 

(]) 

...:l 

'"d 0.05 1=1 

(1j 

iIl k=1 

\ ,. '" \ 

\ 

\ 

\ 

\ 

\ 

\ 

TR-3036 

bl-covaT-Tangfl-C0771P 

observed variation range 

expected range (99%-level) 

\ ,-

'--"- "" 

(J) 

N 

co 

g 

10 10 2 

1/3-0ctave Center Frequency (Hz) 10 J 

(b) variance coefficient comparison 

10 2 

1=1 

.8 

..., 

(1j 

.;:: 

(1j 

:> 

..., 

1=1 

(]) 10 'u 

S 

Jl) 

0 

U 

.~ rn 

0 

..., 

s... 

;:J 

:::.::: 

v 

:> 

(]) 

...:l 

'"d 

s:l 

(1j 

iIl 

10 -1 

1 

bl-kurf-rang8-comp 

observed variation range 

expected range (99% -level) 

k=1 

10 10 2 

1/3-0ctave Center Frequency (Hz) 

N 

N 

~ o 

10 J 

(c) skewness coefficient comparison (d) kurtosis coefficient comparison 

Figure 5-4 .. Comparisons of the Observed Run-to-Run Variations of the BSL 

Mean and First Three Statistical Moments and the Variations 

Expected from a Purely Random Process. 

94 
TR-3036 

99% confidence level. These plots show that significant run-to-run variations 

did occur in the four statistical moment distributions. The k = 1 line, shown 

in these figures, represents the acoustic (cyclic) frequency corresponding to 

an aerodynamic reduced frequency, k, of unity where k = n Iv (c is the chord 

length and Vc the convection velocity at an effective span~is~ radius of 75%. 

Choos Multivariate tic Inflow Predictors 

In the specification of a multivariate regression model, one strives to choose 

independent variable predictors. While being sensitive to the predicted quantity, these variables are uncorrelated with one another; i.e., to minimize the 

degree of multicollinearity presente Unfortunately, in dealing with atmospheric inflow variables, this is often an impossible task. Under homogeneous 

flow conditions, the Monin-Obukov similarity theory gives us an independent 

set of turbulence predictors. However, for inhomogeneous flows, like those at 

the Goodnoe Hills site, the degree of collinearity between predictor variables 

can be minimized but not eliminated entirely. 

Ideally, we would like to arrive at a set of independent variables which would 

adequately describe the inflow state in order to predict the ensemble statistical behavior of the observed spectral characteristics of the far-field 

acoustic emissions. We know from the work of Homicz and George (see 

Section 3.1) that these predictors should be representative of a mean axial 

convec tion veloci ty, V ') and the accompanying turbulence scales and intensities. Coupling thi~ requirement with the influence of surface layer 

similarity (see Section 3 .. 2), we saw that the hydrodynamic stability has a 

strong influence on these parameters. Thus, a set of inflow state predictors 

should include parameters which serve as a representative measure of the 

mean inflow velocity; 

disk layer hydrodynamic stability; 

• turbulence intensities; and 

• characteristic turbulence scales. 

From Table 2-5, available measures of mean inflow (axial convection velocity) 

included observations near the hub-height (59 m) and at the hot-film anemometer, located at a 45-m elevation on the BPA Met Tower. The variation of 

mean wind speed with height was shown in Section 3.2.2. This parameter varys 

as a function of the surface shear-stress or friction velocity, U~IO and 

roughness (fetch) or z and stability" Measures of the inflow stability 

included the disk gradi~nt Richardson number, Ri, the Monin-Obukov length, L, 

and the z/L parameters. The vertical variation of turbulence intensi ty was 

shown in Section 3.2.3 It varys as a function of stability, mean wind speed, 

and U*0 Observations of the amount of small-scale turbulence present at the 

45-m height are available as the 1-10 Hz-band mean square values for the 

longitudinal and vertical components 0 Available characteristic turbulence 

length scale measures include 45-m elevation observations of 

• the I x, I x, and I z integral scales; and u . w . w • the long1tud1nal and vert1cal reduced frequency spectral peaks, f and f • 

mu mw 

The degree of bivariate correlation for the turbulence intensities and scales 

versus the representative mean velocity, stability, and roughness parameters 

are shown in Table 5-20 The cross-correlation matrix for the stability, 

95 
TR-3036 

velocity, and roughness scales is shown in Table 5-3. As is apparent by the 

content of these tables, there are few inflow characteristic scales which are 

completely independent of one another, thus, raising the degree of multicollinearity in multivariate models where they are used as independent variables. 

Inflow Stability 

Because of the lack of independence in most of the available inflow characteristics scales, we have more thoroughly examined the inter-relationships 

between these parameters" For example, using one-way analysi s of variance 

(ANOVA) techniques, we examined the percentage of observed variation in the 

longitudinal and vertical 45-m level turbulence components (u t , Wi), explained 

by a multivariate model consisting of the 

Table 5-2" Correlation Coefficients for Inflow 

Turbulence Scales and Intensities versus Stability, 

Velocity, and Roughne s s 

a 

I x I x I z f mu f mw 

<S >2 <S >2 

u w w u w 

Stabilitl 

Ri .332 - .. 277 -,,492 - .. 455 .. 170 - .. 249 -.604 

z/L - .. 200 -.848 - .. 932 .132 -,,207 - .. 217 -.791 

L -.225 -.248 - .. 134 .. 267 .342 ,,639 .937 

Velocitr 

U45 .586 -.538 - .. 428 -.609 .202 .598 - .. 222 

UH .593 -.541 -.365 -.502 .317 .697 -.084 

u* .. 562 - .. 263 -0044 -.604 - .. 617 .. 733 .. 269 

Roughness 

Zo .397 - .. 577 -.732 -.648 .116 -.191 - .. 710 

a<S > = u,W 1-10 Hz band mean square value. 

Table 5-3 .. Cross-Correlation Matrix for Stability, Velocity, and 

Roughness Predictors 

Ri z/L L U45 UH U,t(' Zo 

Ri .. 174 -.121 .. 118 - .. 017 .097 .517 

z/L .374 .443 .297 .319 .103 .523 

L -0121 .443 .. 175 .239 ,,475 .266 

U45 .. 118 .297 .175 .981 .. 814 .460 

UH -,,017 ,,319 .239 .981 .816 .403 

U i ,: ,,097 e103 .475 .814 .816 .576 

Zo .517 .523 .266 .. 460 .403 .576 

96 
100 

'"C1 90 

OJ 

.:: 

'(ij 80 

P.. 

~ 

rx:l 70 

.:: 

,~ 60 ...., 

ro 

'C 50 ro 

:>-

(ti 40 ...., 

0 

E-< 30 '+-< 

0 

..., 20 .:: 

OJ 

<:.) 

... 10 OJ 

0... 

-10 

10 -I 

TR-3036 

o M-O length scale, L; 

o disk Brunt-Y~is~l~ period, TBy ; 

o roughness length, z ; and the 

o 45-m mean wind spee~, U45 0 

Figures 5-5a and b demonstrate that, on average, 90% of the observed spectral 

varianqe in these turbulence components is related to changes in the inflow 

stability. We have found that the hydrodynamic stability of the rotor disk 

layer, as expressed by the Richardson number parameter, strongly varies with 

the time-of-day at the Goodnoe Hills site. This can be seen for the period of 

1600-2400 h local standard time (LST) in Figure 5-6. In the figure, the 

30-minute disk Richardson number is plotted for 25 data runs from both the 

1982 and 1983 experimental periods. The shift from unstable to stable inflow 

conditions is seen to take place rather abruptly in the vicinity of 1630 

hours" The 1983 data set contained much more stable runs than those of 

1982" These runs are ted as a function of time 1n Figure 5-7 with the 

most stable conditions occurring after 1930 h L8T. 

uctnnp-anO'VR we omp-a,,01J a 

'" 100 

~ '" 

0 

0 90 

PERCENTAGE OF OBSERVED VARIANCE 

'"C1 

'(ij 

P.. 

.:: 

Q) 

80 

PERCENTAGE OF OBSERVED 

~ VARIANCE 

EXPLAINED BY THE: ~ 70 EXPLAINED BY THE: 

o M-O length scale, L rx:l o M-O length scale, L 

tJ B-V period, Tbv .:: tJ B-V period, Tbv 

Do Roughness length, Zo Do Roughness length, Zo :3 60 <> 45m. mean wind speed, U <> 45m meon wind speed, U ro .. Unexplained residual .. Unexplained residual 'C 50 (\j 

:>-

(ti 40 ...., 

0 

E-< 30 

'+-< 

0 

~ 

10 -1 1 10 

0... 

H 

<:.) 

OJ 

OJ 

-10 

20 

10 

10 -. 

~ 10 -1 1 

Cyclic Frequency, f (Hz) Cyclic Frequency, f (Hz) 

) vertical or component 

Figure 5-5 .. ANOVA Results for the 45-m 

or In-Plane Turbulence 

97 

~ 

'" 00 

0 

0 

10 
TR-3036 

I 

~ 

17 18 19 20 21 22 23 

Local Standard Time (h) 

Figure 5-69 Observed Variation of the Disk Gradient Richardson Number 

• ..-1 

0:: 

as a Function of the Time-of-Day for the Combined 1982/83 

Experimental Periods 

12 

~~ 10 Q) 

..0 

El 

:;j 

Z 8 

J:1 

0 

rIl 

'"d 

~ 6 <tl 

..cl 

CJ 

• ..-1 

0:: 

-+-' 4 

J:1 

Q) 

• ..-1 

'"d 

<tl 

~ 2 d 

A ~ 

0 

-+-' 

0 A 0 0:: 

L[) 

N 

N 

<Xl 

0 

0 

-24-----------.----------,----------~----------r_--------~ 

14 16 18 20 22 

Local Standard Time (Hours) 

Figure 5-1. Observed Variation of the Disk Gradient Richardson 

Number for the Combined 1982/83 Experimental Periods 

98 

24 

24 
TR-3036 

Influence of Stability on Mean Wind Speed 

The change from unstable to stable inflow conditions was found to have a pronounc~d effect on the character of the mean wind speed profile. Figure 5-8 

plots the combined 1982/83 hub-height mean wind speed values (UH) as a function of the disk, Ri, between -1 and +2. The tendency for a step-up in speed 

under stable flow conditions is apparent. One reason why there are not many 

data points in the unstable region is that prior to about 1600 h the winds 

were generally below the turbine cut-in speed (~ 6 m/s). Figure 5-9 shows the 

1982/83 variation of UH with the time span corresponding to our data collection period. While at first glance there appears to be considerable scatter, 

a closer inspection reveals the tendency for a dual grouping of points into 

high- and low-speed sets in the stable flow, present after about 1900 h. The 

least-squares trend line reveals the tendency towards maximum UH values in the 

vicinity of 2000 h and a trend towards another maximum after 2230 h. The high 

mean wind point in the unstable region prior to 1630 h was related to a 1982 

case in which a passing upper atmosphere disturbance was influencing the 

normal diurnal wind speed variations 

14 

'"'d 

~ 12 

p.. 

rn 

'"'d 

.,..,. ~ 

!3: 

~ 10 Q) 

:21 

I/). il , 

I 

<D 

C\J 

C\J 

co 

o 

o 

6~--~--~----~--4----r--~----r---~---r--~----r-~ 

-1.00 -0.50 0.00 0.50 1.00 

Disk Gradient Richardson Number, 

5-8.. Observed Mean Hub-Height Wind Speed as a 

99 

Richardson Number for the 

Periods 
-rn 

14 

"-.. 13 S 

.......... 

'"d 

~ 12 

0.. 

rn 

] 11 

~ 

~ 10 

Q) 

::2l 

+J 9 ..d 

I:l.O • ..-! 

Q) 

tt:: 

I 8 

..a 

::J 

::r: 

7 

6 

15 

\ 

\ 

\0 

\ 

\ 

\ 

\ 

\ 

\ 

"-

D 

unstable 

16 17 

Ri=O 

I 

I 

10 

I 

-1/ 

I 

01 

o 

1:1 

/ 

o 

/ 

/ 

/ 

/ 

/ 

0 

o 

o o 0 

o 

...... ""'-

0 

0 0 

stable 

o 

~ 

, 

\ 

\ 

0 

0 

18 19 20 21 

\ 

Local Standard Time (h) 

TR-3036 

o 

o 

o 

o 

I 

\ 

, 

I 

/ ....... 

o 

o 

22 23 

Figure 5-9. Observed of the Mean Hub-Height Wind Speed as a 

Function of the Time-of-Day for the Combined 1982/83 

Experimental Periods 

The 1983 variation of the observed height of the PNL Tower peak mean wind 

speed is plotted as a function of the disk Richardson number in Figure 5-10. 

The amplitude at this height and time of occurrence are shown in Figures 5-11 

and 5-12, respectively. The horizontal dashed lines in the latter two figures 

correspond to the turbine cut-in (6 m/s) and cut-out (20 m/s) velocities. The 

variation of the mean wind speed vertical profile ,U(z}, as a function of the 

Richardson number is displayed in Figure 5-13. 

Influence of Stability on Characteristic Turbulence Scales 

The relative sensitivity of the 45-m mean wind speed (U45 ) and the 

characteristic turbulence scales to the disk Richardson number are shown in 

Figures 5-14 and 5-15" The relationship between U45 and the 45-m level 

longitudinal and ve:tical turbulence integral .scal~s lux and Iwz are plotted 

in Figure 5-14. F1gure 5-15 plots the relat10nsh1p for U45 and the reduced 

frequency peaks associated with the longitudinal and vertical component 

spectral peaks, f and f • The observed relationship between the component mu mw integral scale lengths (I x and I z) and the spectral peak reduced frequencies u w 

are plotted in Figures 5-16a and b. While the expected inverse relationship 

between the longitudinal characteristic scales seems to exist, the scatter in 

the vertical component scales resists specific classification for this small 

sample. 

Influence of Stably-Stratified Inflow on Turbulent Energy Distribution 

The inflow structure at the Goodnoe Hills site follows a pronounced diurnal 

cycle, particularly in the summer month's when the peak energy production was 

achieved. Figure 5-17 plots the hourly mean wind speed normalized by the 

100 

r--

N 

N 

00 

o 

o 

24 
-

" :-. (( 

~ ;p -

120 

..---

S 

'--J 

S 100 

::1 

S 

...... 

~ 

cO 80 ::2l 

'"d 

Q) 

Q) 

~ 

en 60 

'"d 

d 

...... 

~ 

'H 

0 40 

+> 

..c: 

blJ 

...... 

Q) 

~ 20 

J 

--~----------------

co TR-3036 N 

N 

co 

o 

o 

O~------~--------~------~--------~------~--------.-------~ 

-2 0 2 4 6 8 10 12 

Rotor Disk Gradient Richardson Number. Ri 

Figure 5-10. Observed of the Height of the Mean Wind Speed 

22 

] 18 Q) 

~ 

en 16 

'"d 

d 

~ 14 

~ 12 Q) 

::2l 

~ 10 CIl 

i5 8 

Maximum as a Function of the Gradient Richardson Number 

for the 1983 Experimental Period 

o 

Rotor Disk Gradient Richardson Number. Ri 

a> 

N 

1il 

o 

o 

Figure 5-11. Observed Rotor Disk Maximum Mean Wind Speed as a Function 

of the Richardson Number for the 1983 Experimental Period 

101 
22 

] 18 OJ 

0.. 

(f) 16 

"d 

d 

~ 14 

~ 12 OJ 

::2l 

~ 10 rn 

...... 

Q 8 

J-t 

o 

b 6 ~ 

~ 4 ttl 

OJ 

~ 

2 

TR-3036 

o 

(') 

~ o 

o 

O~--------~----------'--------'-----------'----------I 

Figure 5-12 .. 

18 

"d 

~ 14 

0.. 

(f) 

"d 

.S 12 

!3: 

6 

14 16 18 20 22 24 

Local Standard Time (Hours) 

Observed Rotor Disk Maximum Mean Wind Speed as a Function 

of the Time-of-Day for the 1983 Experimental Period 

PNL Tower 

Measurement Elevations 

107 Tn G---€) 

11 4 Tn !3----£1 

61 Tn __ 

311 Tn b:------A 

15Tn~ 

4 ~-~--...----,----,-- I I I I I 

-2 0 2 4 6 8 10 12 

Disk Gradient Richardson Number, Ri 

Figure 5-13. Observed Variation in the Mean Wind Speed at the Five 

Levels of the PNL Tower as a Function of the Gradient 

Richardson Number for the 1983 Experimental Period 

102 
450 

....-

!400 

~ 1-1 

w350 .... 

ttl 

() 

m 300 

~ 200 0) 

d 

& 150 

S 

o 

u 100 bD 

d 

o 

.....:l 50 

o 

~ 0.20 '+-I 

0.18 

1'Il- 0.16 0) 

• ..-1 

() 

~ 0.14 

& 

~ 0.12 ~ 

"d 0)0.10 

(J 

~ 

"d 

W 0.08 

0:: 

,!:cI 

ttl 0.06 

W 

~ 

Cd 0.04 

f..I ..., 

(J 

~0.02 

m 

0.00 

20 

0) 

Cd 15 () 

m 

.... 

ttl 

J..t 

b.O 

0) 

..., 

d 

- 10 ..., 

d 

0) 

d 

o 

0.-

S 

o 

u 5 

161 ~16l 

! -

:::> 14 

..a 

0) 

0) 

0.-

m 12 

"d 

d 

~ 

d 10 

ttl 

0) 

:::21 

S 8 I.f.) 

"I':!'" 

6 

u 

/' 

/ 

Iu 

"- \ 

\ 

\ 

\ 

\ 

\ 

\ 

TR-3036 

N 

C') 

N 

co 

o ,0 

o 44---~~--~--~--'--'--~--'--.---.--.---r--.--.---.--. 

-2 o 246 B 

Disk Gradient Richardson Number. Ri 

5-14 .. Variation Wind 

the (Iwz) Length Scales a Number 

fmw C') 

0.006 C') 

.... -.... N 

'" co .; 0 / \. 0 

/ \ 

/ \ I \ I \ 

~I \ 

\ Z I \ I \ 0.005 I \ I 

I \ 

I \ I:. I \ 

I \ 

I \ I \ I \ I "'- I 

~ \ 0.004 ........ I \ ttl "- I \ I I'll "- I 

I 

)6 ·PI 

cd " :::> I "- / \ I I \ ..., / "- / \ d / 

/ ~ 0 "- "- .; 

"- ~ / ¢ ..... _- ,-

tIl 0.003 0 ~ / 

" / ,!:cI 

• ..-1 

I'll -jmu - 0 

0.002 

6 B 10 

Disk Richardson Number. Ri 

5-15 .. 45-m Horizontal Wind 

(f ) • lOW 

as a Funct10n of the 

103 
350 

300 

50 

TR-3036 

18 

IJ 16 

IJ 

IJ 

IJ 

IJ 

4+---~--~--~--~----~--~--~---jr----~~i 

0.04 0.08 0.12 0.16 0.20 0.00 0.04 0.08 0.12 0.16 0.20 

U-Comp Reduced Frequency Spectral Peak, fmu W-Comp Reduced Frequency Spectral Peak, fmw 

(a) Variation of lux Versus fmu (b) Variation of lwz Versus fmw 

Figure 5-160 Observed of the 45-m Horizontal and Vertical 

Length Scales as Functions of the Frequency 

Reduced 

daily or diurnal mean for the summer or peak wind period (May through August) 

and winter or minimum wind period (November through January) of the 1985/86 

wind season. The same data is plotted in absolute units in Figure 5-18 to 

show the relationship to the turbine cut-in speed. 

The above figures clearly demonstrate that the maximum energy availability 

occurs between the hours of 1700-0600 1ST, during non-disturbed conditions" 

Even in winter, with its short days and low sun angle, the thermally-driven 

cycle is clearly discernible in Figure 5-17. From Figures 5-6 and 5-7, it is 

obvious that the period of maximum energy availability also coincides with a 

disk-layer inflow, which is stable and becomes increasingly so as the night 

progresses. Peak energy availability (at the hub elevation) typically occurs 

between 2000 and 2100 LST or 3-4 hours after sunset.. From Figures 5-6 and 

5-7, the corresponding disk Richardson number ranges from about +0.5 to +7.0 

for a geometric mean located between +300 and +4.0. 

The thermal and kinematic fields are nearly in phase, indicated by the plots 

of the Brunt-Vaisala frequency (N) and the 45-m mean wind speed, as a function 

of the disk Richardson number in Figure 5-19. The phase relationship between 

the Brunt-Vaisala frequency and the characteristic turbulent scales (I x, I z, 

f ,and f ) in terms of the Richardson number are shown in Figures ~-20 ~nd my mw 5-l1. 

From the foregoing series of graphs, it is clear that the structure of the 

stably-stratified inflow at Goodnoe Hills responds or tracks the disk gradient 

Richardson number. As the stability increases, an internal boundary layer 

104 
'"C 

w 

w 

P-t 

1.30 

1.20 

rn 1.10 

'"C 

d 

• ..-1 

13: 

d 1.00 

ro 

w 

::2! 

.-I 0.90 

ro 

d 

J..4 

::l 

Q 0.80 

~ 

!>. 

.-I 

J..4 

2: 0.70 

::c: 

5-11@ 

TR-3036 

summer 

winter 

/ ..... 

I " " 

-, 

\ 

Wind 

Summer and Winter of 

S'Um.:rner '85 

winter '85-86 

<D 

i:] 

OJ 

o 

o 

3~~--~~~--~~~--~~~--~~~--~~~--~~-'--'-~-'--'-' 

o 2 4 6 8 10 12 14 16 18 

Local Standard Time (h) 

5-180 Observed Diurnal Variation of Actual 

for Summer and Winter of the 1985 

105 

20 22 24 

Mean Wind 

Season 
0.006 18 TR....;3036 

--- "- l!\ I.'l 

t:d --- "- "- ......" ~16 

"/ 

I 

I "-

450 

400 

::J 

1-4 

W 350 

-ItS 

() 

rn 300 

-ItS 

~ 

~250 ~ 

d 

1-4 

~ 200 d 

(!) 

d 

& 150 

S 

0 

u 100 

b.O 

d 

0 

....:I 50 

0 

Z S 

......" 

~0.005 

CJ o 14 

d rei (!) 

& (!) 

(!) 

(!) p.. s.. rn 12 13:.. 

!! 0.004 

"d 

d 

ItS • .-4 

fIl 13: • .-4 10 ItS d :> ItS I (!) 

~ :::2l d , 

::J S 8 

cE 0.003 li.) 

~ 

~ 

fIl 

• .-4 

q 6 

0.002 4 

-2 o 

Figure 5-19 .. Observed 

and the 

of the 

20 0.006 

---E! --- ......" I.'l 

t:d ~ ......" 

1-4 

W Z 

td 15 ~0.OO5 CJ CJ 

rn d 

(!) 

- & ItS 

~ 

b.O (!) 

(!) ~ 

~ 13:... 

d 

1-4 10 !! 0.004 ~ ItS 

d fIl 

(!) • .-4 

ItS d :> 0 

p.. I 

~ Ei d 

0 ::J 

u 5 ~ to 0.003 

-() 

ItS ~ 

fIl • .-4 • .-4 ~ q ~ 

(!) 

:> 

0 0.002 

-2 o 

I 

I 

, 

I 

I 

I 

I 

I 

I 

't:. 

I 

I 

I "- I "- "- I , I 

u 

, 

',N , 

, 

, 

" , 

"- 't\, 

2 4 6 8 10 12 

Disk Gradient Richardson Number, Ri 

of the 45-m Mean Wind Speed (H) 

Frequency (N) as a Function 

Richardson Number 

N 

I'lL ,. "- \ 

\ 

\ 

\ 

\ 

\ 

Iw \ 

2 4 6 B 10 12 

Disk Gradient Richardson Number, Ri 

Figure 5-20. Observed Variation of the Brunt- Vaisala Frequency (N) 

and the Horizontal (1 x) and Vertical (1 z) length Scales . u.. Til 

as a Funct10n of the D1Sk R1chardson Number 

106 

co 

C') 

~ o 

o 

14 

O"l 

C') 

N 

co 

0 

0 

14 
~ E 0.20 18 

/ .... --" TR-3036 "H u ~ 0.18 ,....... 

"H ~ 16 

rnO.16 E Q) 

...... --- u :::> 14 & 0.14 

::s ..a 

tr Q) I 

I \ 

I 

I 

I 

I 

/' , / \ 

/ fmw " , \ 

9' \ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

~ 0.12 Q) 

D.t I \ \ 

~ (f) 12 

"g 0.10 "d 

c:l u .,.., 

::I ~ "d ' 10 Q) 0.08 c:l ~ ro 

Q) 

.!:d ::21 

ro 0.06 

S Q) 8 

O-t UJ 

0.04 -.;fi 

6 

0.02 

0.00 

\ 

\ 

/"'\ 

\ 

\ 

- "- I 

I 

I 

I 

I 

I 

"- I 

~ \ /;( "-

¢ \ / "-

, / "-

0'--// "- ~ 

'-

/ 

/ 

4 

Disk Gradient 

10 

Richardson Number. Ri 

Peak Reduced 

(;OI!Up4)nE~n t) and 

\ 

\ 

\ 

\ 

\ 

\ I 

)~ 

forms within the rotor disk, accompanied by a wind speed maximum and increased 

vertical mean shear Under these conditions, as indicated by the peak in the 

Brunt-Vaisala frequency near an Ri of +4,,0, shear flow instabilities become 

well developed, including the possibili of breaking waves {Kelvin-Helmholtz 

instability)e 5-22 and 5-23 present an ANOVA analysis of the observed 

run-to-run variat ion of the longi tudinal and vertical 45-m elevation turbulence components These provide a crude presentation of the turbulent 

energy generation balance for each component" Roughly two-thirds of the 

observed run-to-run changes in the turbulent energy spectrum (at space scales 

of the rotor disk and smaller) can be explained from variations in the shearstress {local shear generation)e The bulk of the remainder are due to 

variations in flow stability (buoyancy). 

Inflow Predictors for Far-Field 

A serles s t ic scales, which would serve as independent 

variables the MOD-2 turbine far-field acoustic spectral 

response Vla a linear multivariate model, were evaluated using ANOVA 

techniquese Three ctor {characteristic} scales, which exhibited maximum 

independence in Tables 5-2 and 5-3, were identified for evaluation. These 

included 

M-O scaling length, L 

disk gradient Richardson number, Ri 

45-m mean wind speed U45 

• Vertical turbulence scale length, Iwz 

Richardson number, Ri 

45-m mean wind speed, U45 

o 

'<t 

~ o 

o 
-

'" ~ 

II I 

~-~ 

TR-3036 

d 100 0 

• .-1 

~ 

ttl 

• .-1 90 &-t 

ttl 

:>- 80 

d 

:::1 

p:: 70 

I 

d 

:::1 60 p:: 

"lj 

Q) 50 

d 

• .-1 

ttl 

.-I 40 0.. 

~ 

t':iI 

- 30 0 

Q) 20 bD 

ttl 

~ 

d 10 Q) 

() 

&-t 

Q) 0 P.. 

-10 

10 -1 

Figure 5-22 .. 

d 100 0 

• .-1 

~ 

ttl 90 • .-1 

&-t 

ttl 

:>- 80 

d 

:::1 

p:: 70 

I 

d 

:::1 60 p:: 

"lj 

Q) 50 

d 

• .-1 

<d 

.-I 40 0.. 

~ 

t':iI 

- 30 0 

Q) 20 bD 

<d 

~ 

d 10 Q) 

() 

~ 

Q) 0 P.. 

-10 

10 -1 

total 

u-ccnnp. an01Ja 

I 

9 0 0 

u 0 

lit 0 

0 72J16 0 

shear-stress 

0 generated 

t:. 

26% 

buoyancy 

generated 

1 10 10 III 

Rotor Disk Normalized Wave Number, Dn/U 

ANOVA Analysis of the 

Longitudinal or Axial 

total 

U 

lit 

N 

L 

U 

o 

t:. 

0 

0 

o 0 

o 9 

0 A 

01 0 0 

& I $ & 

Observed Run-to-Run Variation in the 

(u) Turbulence Component 

o o 0 0 

8 !;;I 

8· 0 

~ 0 ¢ 

o 

o 

0 

0 

0 0 

w-ccnnp. an01Ja 

o 

~ 

o ~ 62J16 

shear-stress 

o 0 0 generated 

33% 0 buoyancy 0 

0 generated 

Q 0 o~ 

<!Ie 0 ~ 

1 10 10 III 

Rotor Disk Normalized Wave Number, Dn/U 

Figure 5-23.. Same as Figure 5-22 but for the Vertical or In-Plane (w) 

Turbulence Component 

108 

N 

<t 

N 

co 

o 

o 

::;: 

N 

co 

0 

0 
TR-3036 

• M-O scaling length, L 

Richardson number, Ri 

Vertical turbulence scale length, Iwz 

45-m mean wind speed, U45 0 

The ANOVA results are presented in Figures 5-24 and 5-25 for the multivariate 

model predicting the far-field, mean 1/3-octave band spectrum levels or 

<m(n)l> where <ml> is the ensemble mean.. It is clear from Figure 5-24, the 

addition of a measure of the turbulent scale I z explains the variation in 

mean acoustic output above lO-Hz or k = 1 (r~ferenced at 75% span)" A 

slightly better prediction is achieved if both the Land Ri parameters are 

included in addition to U45 and the Iwz scalese The relative contribution to 

the total variation expla1ned by each of the predictors is shown in Figure 

5-25. Below the lO-Hz band, changes in mean wind speed explain more than 90% 

of the observed MBSL variation. Above lO-Hz, all four predictors help explain 

the variation, the vertical length scale Iwz becoming dominate in the 50-Hz 

band and above .. 

The performance of the three models in explaining the observed run-to-run 

variation in the MBSL second moment, <b2(n» = <m(n)2/m(n», or variance 

coefficient is plotted 1n Figure 5-26. The ensemble varlance 1S denoted 

<m(n)2>. While not explaining 100% of the observed variation, the fourpredictor model does much better than the other two across the frequency band 

100 

(j) 

"d 90 0 

::2! 

:>-. 

..0 

80 

"d 

(J) 70 Q 

'Cd 

.-< 

0.. 60 ><: 

w 

Q 50 ,£ 

...,J 

(1j 

's:: 40 

(1j 

:> 

'H 30 0 

...,J 

Q 20 (J) 

u 

~ 

(J) 

P-. 10 

0 

1 

L=1 

PREDICTORS EMPLOYED IN THE 

MODEL: 

C') 

..;-

C\J 

co 

o 

o 

M-O I!waling length. L 

Richardson number. Ri 

45m mean wind speed, U 

Vertical scale length. Iwz 

Richardson number. Ri 

45m mean wind speed, U 

M-O scaling length. L 

Vertical sca.le length. Iwz 

Richa.rdson number. Ri 

45m mean wind speed, U 

10 10 10 :I 

1/3-0ctave Center Frequency (Hz) 

5-24@ ANOVA of Multivariate Models 

a Series of for the Observed Run-to-Run 

Variation of the Acoustic MBSL Spectra 

109 
100 

......t 

(I) 

'U 90 0 

::2t 

~ 80 

,0 

'U 

(I) 70 r:: .,-4 

cd 

......t 

0.. 60 ~ 

IiJ 

r:: 50 0 

.,-4 

+J 

cd 

.,-4 40 s... 

cd 

:> 

'H 30 0 

+J 

r:: 

(I) 20 CJ 

s... 

(I) 

P-. 10 

0 

Figure 5-26 .. 

mbsl-3Omin 

I I I I I II I I I I I II 

10 10 2 

TR-3036 

PERCENTAGE OF OBSERVED VARIANCE 

EXPLAINED BY THE: 

o Richardson number, Ri 

o M-O length scale, L 

A Reference wind speed, U 

o Vertical turbulence scale, Iwz 

.. Unexplained residual 

I II 

10 :3 

1/3-0ctave Center Frequency (Hz) 

ANOVA Analysis of the Observed Run-to-Run Variation 

of the Acoustic MBSL Spectra as a Function of Four Inflow 

Predictor Scales 

o 

1-= 1 

o 

o 

l:::. 

PREDICTORS EMPLOYED IN THE 

MODEL: 

M-O scaling length, L 

Richardson num.ber. Ri 

45m. m.ean wind speed, U 

Vertical scale length, Iwz 

Richardson num.ber. Ri 

45m. m.ean wind speed, U 

M-O scaling length, L 

Vertical scale length, Iwz 

Richardson num.ber, Ri 

45m. m.ean wind speed, U 

10 1011 10 3 

1/3-0ctave Center Frequency (Hz) 

Same as Figure 5-24 but for the Observed Variation 1n the 

Acoustic BSL Variance Coefficnent <b2> Spectra 

110 

"'l" 

~ co 

o 

o 

lJ) 

"'l" 

N 

co 

o 

o 
TR-3036 

of interest" The performance of all three models. tends to drift downward 

above k = 1. The relative variation explained by each of the four predictors 

is shown in Figure 5-27. In contrast to the MBSL situation, in which changes 

in the mean wind speed explained the bulk of the observed variation at 10 Hz 

and below, all four predictors are necessary to cover the entire 2-160 Hz band 

range for the MBSL second moment. 

Figures 5-28 and 5-30 present the performance of the three models in 

explaining the observed run-to-run variations in the MBSL third and fourth 

statijtical moments (skewness and kurtgsis coeffigients), or <b3(n» = 

<m(n) >/«m(n)2»3 and <b4 (n» = <m(n) >/«m(n)2».. Again, the fourpredictor model performs better than the other two, particularly in explaining 

the spectral variation of the MBSL fourth moment or kurtosis coefficient co 

Figures 5-29 and 5-31 display the relative variation explained by each of the 

four predictors for the skewness and kurtosis coefficients, respectively. 

Again, all four predictors are necessary to explain the bulk of the run-to-run 

variation in the 1/3-octave bands of interest. 

Spectral Sensitivity to Inflow Characteristic Scales 

The four predictors evaluated above were employed as independent variables in 

a linear, multivariate spectral regression model. In this model the response 

or dependent variables were the four statistical moments (mean and variance, 

skewness, and kurtosis coefficients) of the mean 1/3- and octave-band spectral 

levels (MBSL) for each data run. The model for the BSL mean is of the form 

'+-< 

o 

100 

blc'IJ-30min 

90 

80 

70 

60 

50 

40 

30 

20 

10 

o 

PERCENTAGE OF OBSERVED VARIANCE 

EXPLAINED BY THE: o Richardson number, Ri 

D M-O length scale, L 

lJ. Reference wind speed, U 

(; Vertical turbulence scale, Iwz 

.. Unexplained residual 

, <D 

~ 

C\J 

co 

o 

o 

-10~--~--~~~~~----~~~~~~----~-'-'~~rn 

1 10 10 2 10 3 

1/3-0ctave Center Frequency (Hz) 

5-27® Same as but for the Observed Variation in the 

<b2> 

III 
TR-3036 

100 

W 

'"d 90 0 

:::!! 

l>. 80 

,.c 

'"d 

(l) 70 s:l 

..... 

ro ...... 

0.. 60 ~ 

!:il 

s:l 50 0 

..... 

+.l 

ro ..... 40 J... 

ro 

:> 

"H 30 0 

+.l 

s:l 

(l) 20 C) 

J... 

(l) 

0... 10 

0 

1 

000 

o 

D. 

o 

Iv= 1 

I 0 

I 

o 

o 

PREDICTORS EMPLOYED IN THE 

MODEL: 

G----€l 

M-O scaling length, L 

Richardson number, Ri 

45m mean wind speed, U 

Vertical scale length. Iwz A- - D. Richardson number. Ri 

45m mean wind speed, U 

M-O scaling length, L 

Vertical scale length, Iwz 

Richa.rdson number, Ri 

45m mean wind speed, U 

10 10 II 10 3 

1/3-0ctave Center Frequency (Hz) 

Figure 5-28. Same as Figure 5-24 but for the Observed Variation in the 

"H 

o 

100 

90 

80 

70 

60 

50 

40 

30 

o+..l 20 

s:l 

(l) 

~ 10 (l) 

0... 

o 

Acoustic BSL Skewness Coefficient Spectra 

bUik-30m:i:n 

PERCENTAGE OF OBSERVED VARIANCE 

EXPLAINED BY THE: o Richardson number, Ri 

tl M-O length scale, L 

A Reference wind speed, U 

o Vertical turbulence scale, Iwz 

'* Unexplained residual 

-10~----~--~~~~~~----'---'-'1-'1-'1 ~I'I'I~I-----.--'--'-'.-rrlnl I 

1 10 10 2 10 3 

Figure 5-29 

1/3-0ctave Center Frequency (Hz) 

5-25 but for the Observed Variation 1n the 

BSL Skewness Coefficient <b3> Spectra 

112 

co 

'<t 

N 

co 

o 

o 
TR-3036 

100 

o .-I 

OJ 

'"'(j 90 0 

::a 

t>. 80 

..0 

'"'(j 

OJ 70 d 

.~ 

cd 

.-I 

0. 60 ~ 0 

CxJ 

d 50 .S 

o4-l 

cd 

.~ 

~ 40 

cd 

:> 

"H 30 0 

o4-l 

d 20 OJ 

u 

~ 

OJ 

P-t 10 

0 

o 0 

t;,. 

t;,. 0 

o 

(/il 0 

0 

0 0 t;,. 

I t;,. ~ 

t;,. 

o 

o 

k-= 1 

10 

1/3-0ctave Center 

80 0 

~ t;,. 

PREDICTORS EMPLOYED IN THE 

MODEL: 

(Hz) 

M-O seeding length, L 

Richardson n'L/:mber. Ri 

45m. m.eol.n wind speed, U 

Vertical scale length, Iwz 

Richardson num.ber. Ri 

45m. m.ean wind speed, U 

M-O scaling Length, L 

Vertical scale length, Iwz 

Richardson num.ber, Ri 

45m. m.ean wind speed, U 

Figure 5-30. Same as 5-24 but for the Observed Variation 1n the 

100 Acoustic BSL Kurtosi 

(J) 

"0" 

N 

<Xl 

o 

o 

PERCENTAGE OF OBSERVED VARIANCE 

90 blkt-30m.in EXPLAINED BY THE: Fcj 0 Richardson number, Ri (l) 0 M-O length scale, L d 

• .-1 80 t;,. Reference wind speed, U cd 

.-I 0 Vertical turbulence scale, Iwz ~ 

~ 

~ 70 .. Unexplained residual 

d 

0 60 • .-1 

~ 

cd 

• .-1 

50 S-t 

cd 

:> 

.-I 40 cd 

~ 

0 

~ 30 '+=t 

0 

~ 20 

d 

(l) 

CJ 

S-t 10 

(l) 

p... 

0 

-10~--~---~~~~~·----~~~~-1-'~~--~--~~~1In 

1 

Center (Hz) 

Figure 5-310 Same as 5-25 but for the Observed Variation 1n the 

Acoustic BSL Kurtosis Spectra 

113 

g 

~ 0 

0 
TR-3036 

where n is the band center frequency and E(n) is the residual error between 

the observed and predicted values.. The relationship for the second, third, 

and fourth statistical moments [b2(n), b3 (n), and b4 (n)] would be similar but 

with a different coefficient vector [ao ,,,.,,,a4] for each. 

501.1.3 Model Interpretation 

The models discussed above allow us to examine not only the relationship of 

the mean acoustic spectral response to changing inflow conditions, but the 

variations of that response (the higher-order statistical moments) as well .. 

The elements of the model coefficient vector [al,ooe,a4] for each 1/3-octave 

frequency band listed in Table 2-2 reflect the change in the mean response per 

unit increase in the associated predictor variable, with the remainder of the 

predictors held constant or 

a<sl 3(n» 

aR. ~ 

If each coefficient ~s normalized by the predictor population mean or 

(5-2) 

then the relative spectral sensitivity of each inflow predictor scale can be 

measured on each frequency band response and the results compared" Thus, we 

can use these models to identify the characteristic scales or predictors which 

have the most influence over the statistical distributions of the MOD-2 LF 

emissions spectra. Those predictor scales, having substantial slopes relative 

to the remainder, can be considered indicative of the dominant physical 

processes responsible for acoustic radiation in a particular frequency band. 

Mean Acoustic Band Spectrum Level. A plot of the normalized coefficients 

(response slopes) for the expected or mean BSL <Sl/3(n» model is shown in 

Figure 5-32. In this diagram the reference wind speed (U45 ) and disk gradient 

Richardson number (Ri) have the most influence on the average spectral farfield acoustic radiation below 10 Hz (k = 1).. Above that frequency, the 

vertical turbulent length scale, I z, gradually becomes more important. It is 

also interesting to note the cha~ge in sign for this parameter; i"e .. , band 

output increased with decreasing scale length below 10 Hz (k = 1) and the 

inverse occurred above 10 Hz .. 

BSL Variance Coefficient.. The influence of the four inflow characteristic 

scales on the second moment (variance coefficient <b2» of the observed BSL 

distributions is shown in Figure 5-33" The <b2> statistic is a measure of the 

width or range of levels observed in a particular 1/3-octave band, relative to 

the band mean or expected level. Large values of <bZ> correspond to a very 

wide probability distribution, indicating the assoc~ated processes may be 

wideband random. Similarly, small <b2>s may indicate that some form of a 

narrowband random process (or processes) is responsible for the observed 

variation. A large run-to-run range of <b2> values are noted in the 3 .. 15, 10, 

and 12.S-Hz bands, as is indicated Figure 5-4b. The sensitivity plot of 

114 
.......... 0.15 N 

::c ,~ 

III 

"d 

IZl 

(l) 0.10 

04 

0 

...-I 

en 

,k\ 

I ~ 0.05 

'{1 

H 

0 

+> 

u 

• ..-1 0.00 

"d 

(l) 

H 

Cl.i 

"d 

(l) 

N -0.05 ·M 

.--l 

<d 

S 

H 

0 

Z 

-0.10 

0.005 

IZl 

(l) 

04 

0 

.--I 

en 

H 0.000 

0 

+> 

U 

'M 

'"d 

(l) 

H 

Cl.i 

'"d 

(l) 

N 

...... 

(d -0.005 

S 

H 

0 

Z 

1 

1 

m.br!JT30 

TR-3036 

L[) 

(\J 

OJ 

o 

o 

.~~~~~~~~~~--------------------. 

Iv= 1 

CHARACTERISTIC INFLOW SCALE 

PREDICTORS: 

o Richardson number, Ri 

D Monin-Obukov length scale, L 

t;. Reference wind speed, U 

o Vertical turbulence scale length, Iwz 

(Hz) 

for Mean Band 

CHARACTERISTIC INFLOW SCALE 

PREDICTORS: 

o Richardson number, Ri 

o Monin-Obukov length scale, L 

t;. Reference wind speed, U 

Iv= 1 0 Vertical turbulence scale length, Iwz 

10 10 2 

1/3-0ctave Center Frequency (Hz) 

for BSL 

115 

10 :'I 

(\J 

L[) 

(\J 

OJ 

o 

o 
TR-3036 

Figure 5-33 indicates the TJ45 and Ri (atmospheric structure) have the most 

influence in these bands with a moderate contribution from the vertical scale 

length I z. This time the <b2> sensitivity to the vertical turbulent scale 

remains ¥nverse below 12.5 Hz and becomes essentially zero until it achieves 

dominant influence and positive correlation with and above the 80-Hz band. 

BSL Acoustic Skewness Coefficient. The relative spectral sensitivity of the 

characteristic inflow scaling parameters on the BSL distribution third moment, 

or skewness coefficient <b3>, is plotted in Figure 5-34. The skewness 

coefficient is a measure of the lack of symmetry in the sample probability 

density function. A normal or Gaussian distribution is symmetrical about the 

mean; therefore, the skewness is zero. A positive skewness coefficient in a 

particular frequency band reflects a predominance of large BSL values (peaks) 

over smaller ones; therefore, the distribution is "positively skewed .. " The 

run-to-run variation plot of <b3> in Figure 5-4c indicates a multi-band 

structure, with large variations in the 2-5, 8-20, 40, and 80-160 Hz bands. 

Figure 5-34 indicates parameter sensitivities similar to those that were found 

for the variance coefficient <b2>. 

BSL Acoustic Kurtosis Coefficient.. The normalized BSL sensitivity of the 

fourth moment <b4>, or kurtosis coefficient, to the set of four inflow 

characteristic scales is shown in Figure 5-35. The kurtosis coefficient, or 

"flatness factor," is a relative measure of the magnitude of the BSL values in 

rtl 

Q.) 

P-t 

o 

..-t 

(() 

I-l 

o 

o+=l 

() 

:B 

Q.) 

I-l 

P-t 

'"d 

Q.) 

N 

....... 

blBkr30 

0.005 

0.000 ~~-s-=--t1~~~~~~~~~~--------------------. 

] -0.005 

CHARACTERISTIC INFLOW SCALE 

PREDICTORS: 

I-l 

o 

Z 

-0.010 

1 

Iv= 1 

o Richardson number, Ri 

o Monin-Obukov length scale, L 

ll. Reference wind speed, U 

o Vertical turbulence scale length, Iwz 

10 10 2 

1/3-0ctave Center Frequency (Hz) 

I I I 

10 3 

Figure 5-34& Characteristic Scaling Sensitivities for BSL Skewness 

Coefficients 

116 
TR-3036 

the observed band probability distribution.. The kurtosis coefficient for a 

normal distribution is 3.. An observed BSL distribution void of sharp peaks 

will have a kurtosis of 3 or will possibly be negative (flat). A high value 

of <b4> in a particular frequency band indicates a substantial percentage of 

large values (peaks) relative to the mean band level and a distribution peaked 

at the mean. The band structure noted in Figure 5-4c of the <b3> statistic 

has become more discrete in the plot of <b4> in Figure 5-4d. The largest 

variations of <b4> occur in the 3.15-5, 8, 20, 40, and 100-160 Hz bands. The 

parametric sensltivity plots of Figure 5-35 indicate that atmospheric 

structure, as indicated by the Richardson number, is the dominant influence on 

<b4> below 10 Hz and inversely correlated (low stability)" High values of 

<b4> in the 100-160 Hz bands are positively correlated with very stable flows. 

Empirical Agreement with the Theory of Homicz and George.. In general, there 

is excellent agreement between the theories of Homicz and George [10,11] and 

our measurement of the MOD-2 wind turbine. They identified the axial convection velocity, V , the upwash or in-plane turbulence component intensity, and 

scale I z as t~e primary inflow properties that influence the mean band 

spectrumw<S (n» level emissions We found that the dominant characteristic 

inflow scal~~ were wind , U, the upwash or vertical turbulent 

scale, I z, measured within the rotor disk, and the gradient Richardson number 

stabilit; parameter measured across the vertical extent of the rotor disk. We 

agree with the axial flow component, since the axial and convection velocity 

0.5 

0.4 

fIl 

Q) 

~ 0.3 0 

,.....{ 

('f) 

~ 

0 0.2 +J 

u 

...... 

'"d 

Q) 

~ 0.1 Cl.t 

'"d 

Q) 

N 

...... 0.0 ,.....{ 

cO 

El 

~ 

~ -0.1 

-0.2 

-0.3 

1 

CHARACTERISTIC INFLOW SCALE 

PREDICTORS: 

o Richardson number, Ri 

tJ Monin-Obukov length scale, L 

/:;. Reference wind speed, U 

blktr30 

L=1 \) Vertical turbulence scale length, Iwz 

10 10 2 10 3 

l/3-0ctave Center Frequency (Hz) 

Sensitivities for BSL 

117 

'OJ" 

l{) 

(\J 

Q) 

o 

o 
TR-3036 

are approximately related by V = 0 .. 8U EO And, since I z is proportional to 

S (ni' as well as to the mean-~quare value of the integrated value of S (n) 

o~ w , again there is agreement. Including the Richardson number paramete~ is 

necessary to account for the seemingly unique, stratified flow structure 

present at the Goodnoe Hills siteo We must also include the M-O length scale, 

L, to provide the diabatic surface-layer scaling in the rotor disk discussed 

in Section 3 .. 2. These adjustments would not, however, be necessary in the 

homogeneous isotropic inflow turbulence assumed by Homicz and George in their 

models. Thus, including them is a concession to the real atmosphere .. 

501.1.4 Case Studies of the Role of Inflow Structure on Radiated Spectral 

Characteristics 

A series of case-study comparisons were made among the six usable data runs of 

1983. Tables 3-3a, b, and c show that four inflow regimes were available for 

comparison: (1) low wind speed, low stability (LWLS); (2) low-to-moderate 

wind speed, low-to-moderate stability (LMWLS); (3) low wind, high stability 

(LWHS); and (4) high wind, high stability (HWHS). These cross-comparison run 

combinations and associated run data are listed in Table 5-4. 

Low Wind, High Stability versus High Wind, High Stability Inflow Conditions. 

The normalized, longitudinal (axial) and vertical (upwash) 45-m level turbulence spectra for Runs A05 and Al8 are plotted 1n Figure 5-36" The corresponding mean BSL <Sl/3(f», <b2(f», <b3(f», and <b4(f» acoustic spectra 

are shown in Figures 5-37a, b, c, and d. The lower abscissa is scaled in 

terms of the convection wave number, normalized by the rotor disk dimension, 

Comparison 

Category 

LWHS (3) 

HWHS (4) 

LMWLS (2) 

HWHS (4) 

LMWLS (2) 

HWHS (4) 

LWLS (1) 

LMWLS (2) 

LWLS (1) 

LWHS (3) 

HWHS (4) 

HWHS (4) 

LMWLS (2) 

LMWLS (2) 

Table 5-4. Inflow Structure Comparisons Data 

Run 

Numbers 

A05 

A18 

A14-1 

A18 

A14-2 

All 

A03 

A14-1 

A03 

A05 

Al8 

All 

A14-1 

A14-2 

7.3 

12.4 

9.2 

12 .. 4 

9 .. 9 

13 .. 6 

8 .. 1 

9.2 

12 .. 4 

13.6 

9 .. 2 

9 .. 9 

Ri 

11.7 

1 .. 25 

0 .. 13 

1 .. 25 

0026 

6068 

-0.12 

0 .. 13 

-0.12 

11 .. 7 

118 

1 .. 25 

6.68 

0 .. 13 

0 .. 26 

L 

(m) 

10 .. 7 

10 .. 7 

183 

10,,7 

10 .. 7 

10 .. 7 

-508 

183 

-508 

10 .. 7 

10 .. 7 

10 .. 7 

183 

10 .. 7 

136 

125 

157 

125 

86 .. 5 

336 

210 

157 

210 

136 

125 

336 

157 

86 .. 5 

7 .. 5 

8 .. 8 

16.5 

8 .. 8 

6 .. 1 

6 .. 8 

13 .. 3 

16 .. 5 

13 .. 3 

7 .. 5 

8.8 

6 .. 8 

16.5 

6 .. 1 
~ ::J 

~ 

C 

'-' 

::J 

Cf) 

C 

..... 

C 

(l) 

C 

0 

a. 

E 

0 

() 

cti 

C 

-0 

::J 

.~ 

0) 

c 

0 

-l 

100 

10-1 

10-2 

10-1 100 101 

Reduced frequency (f nz/U) 

t2 

N 

co 

o 

o 

102 

(a) longitudinal (axial) turbulence component normalized spectra 

100 

ill 

II) 

N 

co 

0 

0 

N* -2/3 slope ::J 

----- C 

:;: 10-1 

Cf) 

C 

C 

(l) 

c 

0 

a. 

E 

0 10-2 () 

cti 

() 

:;:::; 

~ 

(l) 

> 

I I 

100 101 102 

Reduced frequency (f = nz/U) 

(b) vertical turbulence component normalized spectra 

TR-3036 

Figure 5-360 Normalized 45-m Turbulence Spectra for Runs AlB and A05 

119 
- /, ~ 

II J 

~ ~ -

80 1 ~I aill/OS mbl ........ N 1'-

81 l{) ::c SI ~ "'- 0 

~I ~ 0 III °1 Pt:/ 

......... 1 .......... 70 ........ 

d ......... 

0. 

«I 

rn 

......... 

..... 

III 60 

I> 

III 

..J 

a 

:;:I 

s.. 50 ., 

() 

III 

0. 

rn 

Pt:/ 

d 

«I 40 run AtS III 

d run AOS ______ IBIIli> __ 

«I 

III 

~ 

30~----r--r-'-'I1I1'----.--.--r'-ITTT'----'--'-'-Tl-'rn 

.......... 

........ 

d 

......... 

N 

e 

i 

1 

1.0 

d 0.8 III 

..... 

() 

• .-4 

<1M 

<1M 

III 

0 

u 0.6 III 

() 

d 

«I 

..... 

s.. 

«I 

:::> 0.4 

..J 

rn 

III 

III 

., ~ 0.2 () 

0 

I 

(I') 

'-..... 

..... 

0.0 

1 

1 

1 

10 10 III 10 III 

1/3-0ctave Center Frequency (Hz) 

I I I I I II I I iii i I I I Iii 

10 10 III 10 III 

~I 

~I I 

I 

I 

I 

I 

I 

I 

I 

... 

Rolor Disk Normalized Wave Number, Dn/Ub 

mean band spectrum levels 

"', /\ , , \ " \ 

\ ...... " ... , 

'--... 

ru.n AtS 

run .AOS -- ------

10 10 III 

1/3-0ctave Center Frequency (Hz) 

10 

Rotor Disk Normalized 'Wave Dn/Ub 

(b) band spectrum level var1ance coefficient 

10 III 

co 

l{) 

N 

co 

o 

o 

TR-3036 

Figure 5-37. Ensemble Statistical Moments of l/3-0ctave Acoustic Band 

Spectral Levels for Runs AlB and A05 

120 
_2.5 

~ t'J 

e 

+' 

~ 2.0 ..... 

C) 

..... 

--

Q) 

0 

u 

(/) 1.5 (/) 

Q) 

d 

~ Q) 

~ 

rf.) 1.0 

....:I 

rf.) 

r:o 

Q) 

Pro 

+' 0.5 C) 

0 

I 

t'J 

~ 

rl 

0.0 

1 

I 

1 

14 

-,-, 

d 

~ 

e 12 

+' 

d 

Q) 

..... 

C) 

..... 10 

--

Q) 

0 

U 

.~ 8 

(/) 

0 

+' 

I-. 

:::1 

~ 6 

....:I 

rf.) 

r:o 

Q) 4 

P 

ro 

+' 

C) 

0 2 I 

t'J 

~ 

..-t 

0 

1 

I 

1 

) 

,\ 

I \ 

I \ 

, \ 

, \ " \ '''' I \ '\ 0 ... I , 

... '\' ... '\ ", \' ......... I \ 

_ 1_ ~ - '- L - - _ ... - -'....-

~' , 

run A 

run A05 

'" 

I 

, 

I 

, 

I ,~ , " , ,\ , I ' , 

, \, 

I \ , 

, '! I 

I 

10 10 III 

en 

l{) 

N 

00 

o 

o 

1~ significance 

level 

10 s 

1/3-0ctave Center Frequency (Hz) 

I I I Iii I I i I I II I I 

10 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

(c) band level skewness 

ill 

10 II 

m18/05 Jd 

;1 "dl run A18 1; 0 

~I '5 1 <D 

N 

run A05 ea~""",""""~_ 

- 00 

1 0 

1 0 

, 

, 

I 

I 

I 

I 

, I I 

" I 

1\ I 

1\ I 

i I I 

, I I 

I I I 

I 

, I' , I 

I 

" I 

" I f 

, 

I 

I 

, 

I 

significance 

level 

'" '" 

10 10 III 10 3 

1/3-0ctave Center Frequency (Hz) 

I I I I I II I I i I I I I i I i I 

10 10 II 10 s 

Rotor Disk Normalized Wave Number, Dn/Ub 

band level relative kurtosis 

TR-3036 

Figure 5-31$ Ensemble Moments for l/3-0ctave Acoustic Band 

Levels for Runs Al8 and A05 

121 
TR-3036 

or Dn/Ub , where Ub is the relative rotor velocity at the effective radius of 

75% span" The span and chord dimensions, in terms of the wave number, are 

indicated on the plots" The mean BSL plot of Figure 5-37a shows the influence 

of the higher axial wind speed below 20 Hz, but above that the two spectra are 

essentially the same. A similar situation exists for the <b2(f» spectra of 

Figure 5-37b, though the lower wind run becomes more discrete below 20 Hz .. 

The plots of the <b3(f» and <b4 (f» spectra in Figures 5-37c and d indicate 

some spectral shifts in the bands for the two runs.. The clear difference, 

however, is the strong peaking behavior being exhibited at 80 Hz and above in 

the A05 run data. We believe this to be associated with the intense vertical 

stratification present during this run as indicated by the detailed profiles 

measured by the tethered balloon shown in Figure 5-38. 

Low-to-Moderate Wind, Low Stability versus High Wind, High Stability Inflow 

Condi t ions.. The 45-m axial and upwash spectra are shown in Figure 5-39 for 

Runs A14-l and A18. The mean BSL spectra of Figure 5-40a show a much sharper 

difference in the spectral slopes below 20 Hz than was true for the previous 

comparison, even though the lower wind run here has a higher mean velocity" 

Above 20 Hz, the curves again nearly overlap" The variance coefficient 

spectra of Figure 5-40b indicate less variation in the lower speed run in the 

bands between 6.3 and 25 Hz. The skewness and kurtosis spectra in 

Figures 5-40c and d indicate a much more discrete behavior in the bands below 

10 Hz for the lower speed, lower stability run, though a harmonic relationship 

in the Al8 run does seem to e~iste 

The A14-2 and All runs each fall into similar classifications in Table 5-4, 

but the inflow conditions are quite different from those of the previous 

pair. For example, both the axial and upwash turbulence spectra of run All 

exhibit a discrete feature near f = 0.6. This can indicate that atmospheric 

wave motions are present at the anemometer height.. Even though the hub-height 

mean wind speed of the All run is almost 50% higher than that for run A14-2, 

there is more turbulent energy in the inflow of the latter, as indicated in 

Figure 5-41. This seems to be reflected in the steeper slope of the mean BSL 

curve in Figure 5-42a and the variance coefficient spectra in Figure 5-42b" 

The skewness and kurtosis coefficient spectra shown in Figures 5-42c and dare 

also quite different from those of the A18 and A14-l runs. There is much more 

evidence of spectral peaking in the more turbulent A14-2 run below 20 Hz. The 

All run, however, shows evidence of peaks occurring often in the bands above 

63 Hz, probably as a resul t of the discrete peak in the inflow spectra in 

Figure 5-410 

Low Wind, Slightly Unstable versus Low Wind, Slightly Stable Inflow 

Conditions. The axial (longitudinal) and upwash (vertical) reduced-frequency 

spectra for Runs A03 and A14-l are compared in Figure 5-43" Again, even 

though the A14-l run has a higher associated mean wind speed, there is more 

turbulent energy in the A03 run's unstable inflow. This underscores the need 

to consider the stability characteristics of the inflow as well as the velocity magnitudes. As before, this excess turbulent energy is reflected in the 

mean BSL spectra in Figure 5-44a as larger low-frequency emission levels and a 

steeper slope with increasing frequency. Above the 10-Hz band, however, the 

slightly stable A14-1 run emits slightly higher levels. The variance 

coefficient or <b2> spectra of Figure 5-44b show essentially the same behavior 

for this pair of runs" The effects of increased stability and a slightly 

greater mean wind speed in Run A14-1 are demonstrated in the <b3> and <b4> 

122 
100 100 1.:i;::;~ 

~ I I 

co ~ :Y 0 - 0 

• • • • 80 • • 80 

• • • • • • • • 

Hub height Hub height 

60 60 

E E ...... ....... .!: 

O'l • .r::. 

Q) O'l 

Q) I I .-. 40 40 N 

W 

20 20 

o 

4 5 6 7 8 9 10 11 12 o 4 5 6 7 8 9 10 11 12 

Wind speed (m/s) Wind speed (m/s) 

5-38a. Smoothed and of 

Wind Speed for Run A05 
E 

....... 

..c 

OJ 

(l) 

I 

...... 

N 

.p.. 

100 

80 

Hub height 

60 

40 

20 

o 

180 

----- -----

• 

• 

Wind direction (deg) 

100 

80 

Hub height ----- -------- 60 

E 

...... 

..c 

OJ 

om 

I 40 

20 • 

o ~--~--~--~----~--~-~----~--~--~ 

180 220 260 300 340 

Wind direction (deg) 

Figure S-38b. Saoothed and High-Resolution Vertical Profiles of 

Direction for Run AOS 

N /.0 - ~ 

~ I J 

0 ~ - ~ 0 
E 

...-

..c 

CJ) 

CJ.) 

I 

I-' 

N 

V1 

100 

80 

60 

40 

20 

o 

16 

100 

80 

Hub height 

60 

E 

....... 

..c 

CJ) 

CJ.) 

I 40 

• 

20 

17 18 19 20 21 22 

Temperature (0 C) 

Figure 5-38c. Saoothed and 

Temperature for Run A05 

Hub height 

17 18 19 20 21 22 

Temperature (0 C) 

Profiles of 

C") 

(!) 

C\J 

co 

0 

0 

r-3 

l=l:1 

I 

W 

o 

W 

0\ 

h;;;:~ 

I I 

~ ~ -
~ ::J 

::::::. 

c 

::J 

Cf) 

C 

+-' 

C 

(]) 

C 

0 

Q. 

E 

0 

() 

qs 

C 

""0 

::J 

+-' 

O'l 

C 

0 

.-J 

100 

10-1 

10-2 10-1 

Reduced frequency (f = nz/U) 

"" <0 

N 

co 

o 

o 

(a) longitudinal (axial) turbulence component normalized spectra 

+-' 

C 

(]) 

C 

o 

Q. 

100 

~ 10-2 

() 

qs 

() 

+-' 

lo-

(]) 

> 

10-3 

-2/3 slope 

10-1 100 101 ' 

Reduced frequency (f = nz/U) 

If) 

<0 

N 

co 

o 

o 

102 

(b) vertical (upwash) turbulence component normalized spectra 

TR-3036 

Figure 5-39. Normalized 45-m Turbulence Spectra for Runs AlB and A14-l 

126 
S 

::1 

b 50 o 

Q) 

0. 

{'f) 

"d 

d 

~ 40 

d 

cd 

Q) 

~ 

run ,,(18 

run ,,(14-1 ---------

,,'11/14-' -millill 

<0 

<0 

N 

co 

o 

o 

30~----~--~~~~~~----~~~~~~~----~--~~~~~~ 

I 

1 

~ O.B ()) 

:§ 

'H 

'H 

()) 

o 

U 

()) 

CJ 

d 

cd 

.~ 

cd 

:> 

o-:l 

(f) 

III 

()) 

l> 

cd ...., 

CJ 

0 

I 

C'J 

~ 

0.6 

0.4 

0.2 

0.0 

I 

1 

1 10 10 II 

1/3-0ctave Center Frequency (Hz) 

iii iii Iii iii Iii 

10 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

mean band spectrum levels 

10 II 

iii 

10 :II 

a1B/14-1 C01J 

~I t:S 

~I 

I 

run A18 

~I o 

~I 

I 

run A 14-1 -- - -- - -- -- -- -- Q 

10 10 a 

1/3-0ctave Center Frequency (Hz) 

I I I I I I I I I I I I I I I 

10 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

(b) band spectrum level variance coefficient 

r-

<0 

N 

co 

o 

o 

10 3 

I II 

10 3 

TR-3036 

Figure 5-40. Ensemble Statistical Moments of 1/3-0ctave Acoustical Band 

Levels for Runs AlB and A14-l 

127 
--. 2.5 

3 

C'J 

.e 

.p) 

§ 2.0 .,... 

o 

.,... 

..... 

..... Q) 

o 

u 

Q) 

:> 

cd 

.p) 

0 

0 

I 

C'J 

~ 

.-t 

0.5 

0.0 

I 

1 

14 

1 

~I 

~I 

1 

-1-

't31 

~I (,) 

1 

aIB/14-1 sk 

00 

<0 

N 

00 

o 

o 

1% significance 

level 

run A 18 

run A 14-1 - -

I I I I I I 

10 10 1II 

1/3-0ctave Center Frequency (Hz) 

I iii II I I I I Ii I I I 

10 10 1II 

Rotor Disk Normalized Wave Number, Dn/Ub 

(c) band level skewness coefficient 

"I 

10 :I 

"I 

10 :I 

atB/14-t kt 

run A 18 

run A 14-1 - - - - - - - -

,~ 

" ,I 

I ' ~ I' , I 'I 

\ I I 

I I I 

\ , I 

I I ~ 

" I 

" I \' I 

f I 

I 

I 

\ 

\ 

\ 

OJ 

~ 00 

o 

o 

- - - 1% significance 

level 

O~----.---.-.-"~,,.-----r--.-.~=r""~~-'--'--'-'lIrrn 

I 

1 

1 

(d) band 

10 10 1II 

1/3-0ctave Center Frequency (Hz) 

I I I I II I I I I II I I I 

10 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

10 :I 

"I 

10 II 

level relative kurtosis coefficient 

TR-3036 

Figure 5-40.. Ensemble Statistical Moments of 1/3-octave Acoustical Band 

Spectrum Levels for Runs A18 and A14-1 

128 
c 

::J 

(f) 

c 

.....-

c 

(l) 

c 

10-1 

8. 100 

E 

o 

u 

~ 

C 

"0 

::J 

...... 

Q) 

c 

o 

...J 

N* 

::l 

--...... 

C 

:;: 

en 

c 

C 

(l) 

c 

0 

0.. 

E 

0 

u 

~ 

U 

:.;::; 

I-

(l) 

> 

10-2 

10-1 

-10° 

10-1 

10-2 

A11 

10-3 

10-1 

) vertical 

Reduced frequency (f nz/U) 

) turbulence normalized 

10° 101 

Reduced frequency (f nz/U) 

~ co 

0 

0 

102 

o 

I'-

N 

co 

o 

o 

turbulence component normalized spectra 

TR-3036 

5-416 Normalized 45-m Turbulence for Runs A14-2 and All 

129 
......... 80 N 

::c 

""-

III 

"d 

.........-

.......... 70 ......... 

J:l .........-

~ 

cd 

rIJ 

........... 

.-4 

Q) 60 

> Q) 

....l 

S 

::; 

M 

o+J 50 () 

Q) 

~ 

rIJ 

"d 

J:l 

cd 40 III 

J:l 

cd 

Q) 

~ 

30 

1 

I 

1 

1.0 .......... 

......... 

E.-

N 

e 

~ 0.8 Q) 

'0 

..... 

'1-4 

'1-4 

Q) 

0 

u 0.6 Q) 

() 

J:l 

cd ..... 

M 

cd 

::> 0.4 

....l 

rIJ 

III 

Q) 

> cd 

o+J 0.2 () 

0 

I 

CO') 

""- .-I 

0.0 

1 

I 

1 

I ~I cd 1/14-2 -ml!lIIll 

51 ~I ~ C\J 

I'- UI C\J 

co 

0 .. ... I 0 

"-... I .... .... .... I .... .... "- ... I ... , 

, I .... .... I ... .... ... ... ... ... .... ...... ...... ... ~ , 

I .... "-... ... .... ... .... runAt1 .... .... ... run A 14-2 --------- ... 

10 

1/3-0ctave 

10 II 10 I 

Center Frequency 

I I i i j i I i I I I II 

10 10 II 

Rotor Disk Normalized Wave Number, 

(a) mean band spectrum levels 

~I 

~I I 

I 

I 

run Att 

run A14-2 ---------

~I 

~I U 

I 

I 

I 

(Hz) 

I i 

Dn/Ub 

ill 

10 I 

C') 

I'-

C\J 

co 

o 

o 

10 10 II 10 I 

1/3-0ctave Center Frequency (Hz) 

j j j I j j I j j j j j I j j j i I 

10 10 II 10 I 

Rotor Disk Normalized Wave Number, Dn/Ub 

(b) band spectral level variance coefficients 

TR-3036 

Figure 5-42. Ensemble Statistical Moments of l/3-octave Acoustical Band 

Spectrum Levels fDr Runs A14-2 and All 

130 
/'- 2.5 

:3 

C") 

e 

+l 

~ 2.0 

:§ 

'H 

'H 

Q) 

o 

u 

CIl 1.5 CIl 

Q) 

s:: 

~ Q) 

~ 

[f) 1.0 ....:l 

[f) 

c:o 

Q) 

Pcd 

+l 0.5 u 

0 

I 

C") 

~ 

..-I 

0.0 

14 /'-

~ 

3 

~ 

e 12 

+l 

s:: 

Q) 

:§ 10 'H 

'H 

Q) 

0 

u 

.~ 8 

CIl 

0 

+l 

s.. 

::l 

~ 6 

....:l 

[f) 

c:o 

Q) 4 

Pcd 

+l 

U 

0 2 I 

C") 

~ 

..-I 

0 

1 

~/ 

~/ / 

1 

-1-

run A 11 

run A 14-2 

Rotor 

) band 

AU 

I 

I 

I 

I 

" 

'"d/ 

~/ (.) 

/ 

" '\ 

'\ 

- ~/-

l\.a"- '-""""', 

Normalized Wave 

\ 

\ 

\ 

A14-2= ==----

'"t:l/ S 

~/ 

/ 

10 10 II 

(Hz) 

0.11/14-2 Ilk 

1% significance 

level 

10 :I 

10 ::I 

Dn/Ub 

(2,11/14-2 kt 

L() 

J'-. 

N 

00 

o 

o 

- - - - 1% significance 

level 

10 :I 

1/3-0ctave Center Frequency (Hz) 

10 

Rotor Disk Norma.lized Wave Dn/Ub 

band level ku.rtosi 

TR-3036 

Acou.stical Band 

and All 

131 
100 <D 

f'-

N 

~ co N* 0 

0 

::J 

~ 

C 

--- en 

c 

+-' 

C 

(J.) 

C 

0 10-1 

0.. 

E 

0 

() 

n:s 

c A14-1 

'"0 

::J 

.-=: 

0) 

c 

0 

-l 

10-2 

10-1 100 101 

Reduced frequency (f = nz/U) 

(a) longitudinal (axial) turbulence component normalized spectra 

+-' 

C 

(J.) 

C 

o 

0.. 

E 

o 10-2 

() 

aj 

() 

+-' 

I-

~ A14-1 

Reduced frequency (f = nz/U) 

(b) vertical (upwash) turbulence component normalized spectra 

TR-3036 

Figure 5-43. Normalized 45-m Turbulence Spectra for Runs A03 and Al4-l 

132 
- ~ ~ 

II I 

~ - ~ TR-3036 ,-.... 80 ~I m03/14-t ....,-nhl 

N 

ttl &1 '-..... ~ 

o::l (JI co 

~ r--

N 

I co 

0 

0 .---.. 70 ........ 

.E.. 

0.. 

cd 

IZl 

.......... 

-(J) 60 

t> 

(J) 

...:I 

El 

::l 

J.4 

+J 50 0 

(J) 

Q. 

IZl 

'U 

s:l 

cd 40 ru1l o::l 

s:l run A14-1 -

(Ij 

(J) 

::!! 

30 

1 10 10 I 10 I 

1 

Rotor Norrnalized 

band levels 

1.0 0,03/14-1 CIW 

~ ~I 't:!1 .::1l t 

C\1 ~I 'fll (J) 

e I I r--

N 

co 

0 

~ I 0 

s:l 0.8 (J) 

:§ 

'+-1 

'+-1 

(J) 

0 

u 0.6 (l) 

0 

s:l 

(Ij 

.~ 

(Ij 

:> 0.4 ....:l 

IZl 

o::l 

(l) 

t> 

(Ij 

~ 0.2 run A03 (J 

0 run A 14-1 I 

(iJ 

'-..... 

..-l 

0.0 

10 10 D 10 I 

1/3-0ctave Center Frequency (Hz) 

1 10 

Rotor Disk Normalized Wave Dn/Ub 

coefficients 

Moments 1 Acoustic Band 

Runs A03 and Al/",-l 

133 
__ 2.5 ,...... 

E., 

C"J 

e 

...., 

~ 2.0 ..... 

C) 

..... 

..... 

..... 

w 

0 

U 

rn 1.5 rn 

w 

d 

~ w 

,!:d 

U) 1.0 

....:l 

U) 

CIl 

w 

t> 

a:I ...., 0.5 C) 

0 

~ I 

C"J 

"" .-4 

0.0 

1 

I 

1 

~I i:S 

~I 

I 

I 

# 

I' f I' ,\ , ,. , \ 1\ ,. , \ I \ , , 

, \I \ , , 

, .. " , , '" ~, 

, , , , 

I , 

I , I , 

, 

, 

, 

, 

, 

'ttl S 

~I 

I 

- 1- - - - - - - - -

run A03 

run A 14- 1 - - -------

I I I I I II II I I I II 

10 10 II 

1/3-0ctave Center Frequency (Hz) 

a03/t4-t iliA: TR-3036 

o 

co 

C\J 

co 

o 

o 

19S significance 

level 

I II 

10 II 

I I I I I II I I I I II I i I II 

10 10 III 10 :I 

Rotor Disk Normalized Wave Number, Dn/Ub 

(c) band spectrum level skewness coefficients 

...., 

d 

w 

..... 

C) 

..... 

..... 

..... 

w 

o 

u 

14 

10 

8 

6 

4 

2 

~I 

~I 

run A03 

run A 14- 1 - - ------

~ I, 

I, 

I, 

" , I. , \ I' I , 

, , I , I , 

I ,I , I I 

g , , , , , I , 

, 

, " , , " I 

, " I 

, " , • I 

I I , , I \ I \ 

I \ 

\ 

\ 

... 

m03/14-1 A:t 

00 

iil 

o 

o 

- - - 19S significance 

level 

O~----'--'--~~~~-----'--~~~~~~~-.---.-.-..-"n 

I 

1 

1 10 10 II 

1/3-0ctave Center Frequency (Hz) 

I I I I I II I I I I II I I 

10 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

10 1\ 

I i I 

10 :I 

(d) band spectrum level relative kurtosis coefficients 

Figure 5-44. Ensemble Statistical Moments of 1/3-octave Acoustic Band 

Spectrum Levels for Runs A03 and A14-1 

134 
TR-3036 

spectra of Figures s-44c and de The frequency shift and characteristics are 

very noticeable. The distinct harmonic nature of the bands in which peaking 

occurs in the unstable A03 run gives way to very strong, but much more bandlimited behavior in the slightly stable A14-1 run. 

Low Wind, Unstable Inflow versus Low Wind, Very Stable Inflow Conditions. The 

normalized spectra associated with the two inflow components are shown 1n 

Figure 5-45. A -2/3 spectral slope at high frequencies is indicative of the 

turbulent inertial subrange III As one would expect, there is much less turbulent energy present in the very stable inflow. The slight discreteness in 

the band between f = 0.06 and 0.6 in the axial spectra of Run ADS hints that 

waves may be present The plot of the mean BSL spectra in Figure 5-46a is 

surprising, however. Even though Run AD5 has a lower mean wind speed and less 

energy in the turbulent inflow, its mean LF radiated acoustic levels are 

essentially identical to those of Run AD3 

The reason for this behavior becomes apparent when we compare the detailed 

inflow structure The first hint is that the upwash scale length I z is 7.5 m 

3 .w for Run ADS, versus 13" m for run AD3 The transfer funct10n plot of 

Figure 5-4 shows ive slope (almost 2 dB/Hz/m in the 2-Hz 

1/3-octave band) below 12 .. 5 Hz; i .. ee, the smaller the I z, the • w greater the mean BSL below 12.5 Hz. Second, the peak wlnd speed occurs at a 

height of 45 m (as listed in Table 3-3c) with its attendant strong shears and 

just below the hub of the turbine., Finally, these features are clearly illustrated in the detailed tethered balloon prof of Figure 5-47. The strong 

shear layer near 45 m is di tinctly evident.. It is the A05 run we discussed 

in connection with the direct measurement of the MOD-2 acoustical response 

function in Section 5 .. 1.1 

The <b2> spectra of Figure 5-46b shows that the acoustic levels of Run AOs are 

related to a more narrowband random process than those of Run A03 below about 

2D HZe While some frequency shifting has taken place in the <b3> and <b4> 

spectra of Figures 5-46c and d, the low-stability run exhibits the distinct 

harmonic structure while the high-frequency peaking is characteristic of the 

very stable case. Again these two runs underscore the role of vertical stability in determining the aeroacoustic response of a large wind turbine. 

High Wind, Moderate-to-High Stability vs. High Wind, Very-High-Stability 

Inflow Conditions. Here we compare the effects of increased stability under 

high wind conditions. Figure 5-48 plots the normalized axial and upwash turbulence spectra for Runs All and AlB. The discrete peak in the All spectra is 

evident and not present in the Al8 run data" The mean BSL spectra of 

Figure 5-4980 for both runs are almost linear in log acoustic pressure (dB) 

versus log cyclic frequencye Furthermore, only the effect of the increased 

wind speed for the All run appears to be responsible for the slightly higher 

levels associated with it" The variance coefficient spectra in Figure 5-49b 

are essentially the same, indicating that the processes operating are dynamically similar The skewness and kurtosis spectra of Figures 5-49c and d below 

about the 31 5-Hz band are very similar" We believe the peaking activity in 

the All data is a result of the high-frequency discreteness of the turbulence 

inflow spectra in Figures 5-48" This is reflected somewhat in the smaller 

upwash length scale for the All run" 

135 
100 

~ :J 

........ 

'2 

'-' 

:J 

Cf) 

C 

......-

+-' 

C 

Q) 

C 

0 10-1 

Q.. 

E 

0 

() 

cti 

c 

-0 

:J 

+-' 

O'l 

C 

0 

...J 

10-2 

-2/3 slope 

A05 

10-1 100 101 

Reduced frequency (f nz/U) 

N 

co 

N 

co 

o 

o 

102 

TR-3036 

(a) longitudinal (axial) turbulence component normalized spectra 

100 C') 

co 

~ 0 

0 

;* 

........ 

c 

;;: 10- Cf) 1 

C 

+-' 

C 

Q) 

C 

0 

Q.. 

E 

0 

() 10-2 

~ () 

+-' 

I.-

m 

> 

10-3 

10-1 100 101 102 

Reduced frequency (f nz/U) 

(b) vertical (upwash) turbulence component normalized spectra 

Figure 5-45. Normalized 4S-m Turbulence Spectra for Runs AO) and AOS 

136 
TR-3036 

80 flJJ3/06 mfull ..-... 

N "d 

::tl 

""- ~ III u '<t 

00 "d N ........ 00 

0 

0 

- 70 ..-... 

d 

........ 

e:lt 

td 

rtl 

........... 

.-4 

II) 60 

I> 

II) 

...::! 

E 

::1 

M 

o+J 50 0 

III 

e:lt 

rn 

"d 

d 

td 40 CO 

d run ADS 

td 

II) 

:::!I 

30 I 1 j I( i j i I 

1 10 III 10 III 

(Hz) 

1 10 ::l 

Rotor Wave Dn/Ub 

) mean band levels 

1.0 a03/06 CO'll 

-----. ;1 "dl --3 t:i ~I ~I L() C\l 00 

N e I I 00 

0 

0 

o+J 0.8 d 

Q) 

.... 

. ~ 'PI 

'PI 

Q) 

0 

u 0.6 Q) '" '" " CJ " I \ 

d " \ I \ 

ttl , \ 

'J:: \ ...... " ttl \ 

::> 0.4 \ 

, ...::! --, rn 

co 

Q) 

I> 

ll:l run AD3 o+J 0.2 CJ 

0 run AOS ""''''''''''''''''''''"--'''''''''''''''' I 

c::l 

""- M 

0.0 

1 10 10 I 10 3 

1/3-0ctave Center Frequency (Hz) 

I I i i I I II I I i I II i I I I I 

1 10 10 II 10 3 

Rotor Disk Normalized Wave Number. Dn/Ub 

( band level 

Figure 5-46 Ensemble Stati Moments of Acoustic 1/3-0ctave Band 

Levels for Runs AO) and AOS 

137 
.......... 2.5 m03/05 sk TR-3036 ......... 

EM 

..a 

.........-

..., 

d 2.0 Q) 

.... 

0 

.... 

..... 

..... 

Q) 

0 

u 

fI) 1.5 fI) 

Q) 

Q) 

~ 

.!4 

Cf.l 1.0 

....:J 

Cf.l 

o::l 

Q) 

t> 

<t:I ..., 0.5 0 

0 

I 

M 

'-.. 

..-f 

0.0 

I 

1 

;1 

~I 

-1-

, I, .. 1 , 

~/- \ 

run A03 

"t:31 

~I (.) 

1 

1 , 

, 

, 

, 

, ~ , , \ , ,\ , , , , , " , " , '! 

, 

I 

, 

, 

run AOS ---------

10 10 I 

1/3-0ctave Center Frequency (Hz) 

1% significance 

level 

10 :I 

I Iii I iii iii iii i I II 

10 10 10 :3 I 

Rotor Disk Normalized Wave Number. Dn/Ub 

(c) band spectrum level skewness coefficients 

14 m03/05 kt 

.......... Sl "t:31 ......... run A03 E- ~I ~I ~ -.::f4 run AOS (.) N 

e 12 I ..... _------ I co 

0 

0 

..., I . d I Q) , .... I 0 

...... 10 I ..... 

..... I Q) I 0 I U , 

1\ I 

.~ 8 " , 

" I fI) I I , 0 

..., " I 

s... , , I 

:;:I 6 , , I 

~ I I' 

....:J I 

Cf.l I " 

o::l I " , 

4 I 

(I) I 

t> I 

ltS , ..., I 0 

0 I 

I 2 I 

~ N 

co 

o 

o 

C'J 1

,

:::"-:.:,,.'_ 

~ - - - 1% significance 

..-f level 

O~----'---'-'-TlrrTT'-~~'--T~~-'''''~---'--~~~~~~ 

I 

1 

1 

(d) band 

10 10 II 

1/3-0ctave Center Frequency (Hz) 

iii iii Iii iii Iii 

10 10 I 

Rotor Disk Normalized Wave Number. Dn/Ub 

level relative kurtosis coefficients 

10 :I 

iii 

10 :3 

Figure 5-46. Ensemble Statistical Moments of Acoustic 1/3-octave Band 

Spectral Levels for Runs A03 and A05 

138 
E 

...... 

..c 

OJ 

100 

80 

60 

I 40 

20 

o 

180 

Run A03 

Ri -0.12 

220 260 300 340 

Wind direction (deg) 

S-41ae Detailed Profiles 

100 

80 

E 60 

...... 

..c 

0) 

Q) 

I 40 

20 

o 

-

7 

for Runs A03 and AOS 

Run A03 

Ri = -0.12 

8 9 10 11 12 13 

Wind speed (m/s) 

Figure S-41be Detailed 

139 

14 

TR-3036 

co 

] Wave layer 

~ 0 

0 

1 Wave layer 

Run AOS • 

Ri =+11.7 

200 240 280 320 360 

Wind direction (deg) 

• 

Run AOS 

Ri=+11.7 

S 6 

of Wind 

of Wind Direction 

m 

co 

N 

co 

0 

0 

7 8 9 10 11 12 

Wind speed (m/s) 

for Runs A03 and AOS 
20 

o 

301 

• 

302 303 304 

Potential temperature (K) 

TR-3036 

305 297 298 299 300 302 

Potential temperature (K) 

Figure 5-41c. Detailed Vertical Profile of Thermal Structure (Potential 

Temperature) for Runs A03 and A05 

Low-to-Moderate Wind, Slightly Stable versus Low-to-Moderate Wind, Slightly 

More Stable Inflow Conditions. Finally, Runs Al4-1 and A14-2 give us the 

opportunity to examine the acoustic effects of inflows with similar wind 

speeds but with subcritical (Ri <0.25) and above-critical stabilities. The 

normalized axial and upwash spectra are shown in Figure 5-50. The mean BSL 

spectra of Figure 5-51a shows what may be described as a rotation about the 

10-Hz band; i.e., the A14-2 run emits higher levels below 10 Hz but less than 

the A14-1 run above 10 Hz. The characteristic inflow length scales (I x, I x, 

and I Z) listed in Table 5-4 indicate that the lengths associated ~ith ~he 

more ~table run (A14-2) are approximately half those of the less stable run. 

In particular, the in-plane scale length Iwz is only 6 .. 1 versus 16.5 m, 

respectively. Thus, this demonstrates that the effect of increasing the 

stability above the critical is to decrease the characteristic turbulence 

scales. This has the effect of increasing the LF acoustic output below 10 Hz 

while decreasing it above 10 Hz. 

The <b2> spectra, plotted in Figure 5-51b, indicate that some form of a wideband dynamic process is present at the lower stability which produces harmonic 

aeroacoustic excitation. The frequency shifts in the <b3> and <b4> spectra in 

Figures 5-51c and d are quite evident" The shift toward smaller turbulence 

scales appears to be illustrated by the growth of the peak in the 20-Hz band 

and the decrease in those below 10 Hz for the more stable run" A comparison 

of these two runs has given clear evidence of the role of varying stability 

conditions under similar mean loads. 

140 
TR-3036 

100 

a; 

N 

~ co 

0 

0 

::l 

~ 

c -2/3 slope ----::J 

(J) 

C 

--- +-' 

c· 

(l) 

C 10-1 0 

Q. 

E 

0 

() 

Reduced frequency (f = nz/U) 

100 

N 

(J) 

N 

co 

0 

0 

N* 

::l 

......... 

.--. 

C 

3: 10-1 

(J) 

C 

+-' 

C 

(l) 

C 

0 

Q. 

E 

0 10-2 () 

cti 

() 

:;::; 

\,... 

(l) 

> 

10-3

10_1 

Reduced frequency (f = nz/U) 

vertical turbulence 

Figure 5-48., 45-m Turbulence for Runs All and A18 

141 
,...... 

N 

::r: 

"'--

III 

"d 

'-' 

,...... 

-

~ '-' 

Po. 

tt1 

e 

..... 

(l) 

~ (l) 

~ 

8 

:;j 

J.4 

~ 

0 

(l) 

Po. 

rn 

"d 

d 

tt1 

III 

d 

tt1 

(l) 

::!I 

-,...... 

.E.. 

C\l 

e 

80 I fll.1II/tt ""1111 TR-3036 ~I al ~I C') 

~I OJ (.)1 N 

co 

0 

0 

1 

70 1 

I 

I 

60 

50 

40 run A 18 

runAH -----"""" ..... _-

304-----~~_.~" .. rr----.__._.II_rTlTl----.__.--II_.TT~ 

I 

1 

1.0 

1 

~I 

~I 1 

10 10 II 10 II 

1/3-0ctave Center Frequency (Hz) 

I i I Iiiii I II I iii iii 

10 10 II 10 II 

Rotor Disk Normalized Wave Number. Dn/Ub 

(a) mean band spectrum levels 

fll.tII/1 t C011 

d 0.8 1 

'<t 

OJ 

N 

co 

o 

o 

(I) 

..... 

0 

..... 

--(I) 

0 

u 

(I) 

0 

d 

tt1 

'i:: 

ro 

:> 

~ 

U) 

CIl 

(I) 

Pro 

~ 

0 

0 

I 

C"1 

"'-- ..--i 

0.6 

0.4 

0.2 

0.0 

1 10 10 II 

1/3-0ctave Center Frequency (Hz) 

r-- I I I I I I I I I I I II I I 

1 10 10 II 

Rotor Disk Normalized Wave Number. Dn/Ub 

(b) band spectrum level variance coefficients 

10 l! 

I II 

10 l! 

Figure 5-49. Ensemble Statistical Moments of Acoustic 1/3-0ctave Band 

Spectrum Levels for Runs All and Al8 

142 
Figure 

.-.. 2.5 

£ 

t"J 

e 

...., 

~ 2.0 

'0 

..... 

..... 

..... 

cu 

o 

u 

I'Jl 1.5 I'Jl 

cu 

; 

cu 

~ 

rn 

cu 

Pcd ...., 

(.) 

0 

I 

t"J 

""'- .-I 

1.0 

0.5 

0.0 

I 

1 

14 .-.. 

.--

3 

-.:J4 

e 12 

...., 

t:l 

II.l 

'0 

• .-4 10 ..... 

..... 

II.l 

0 

U 

.~ 8 

rtl 

0 

...., 

s... 

::1 6 ~ 

~ 

rn 

CO 

II.l 4 :> 

«:I ...., 

CJ 

0 2 I 

C'J 

""'- .-I 

0 

I 

1 

1 

) 

1 

mtl/lt 11111: TR-3036 

51 

~I 

I 

I 

~I 

~I (.) 

I 

- 1- - - - - - - - - - - -

run AtB 

run At t 

10 10 I 

1/3-0ctave Center Frequency (Hz) 

l() 

(J> 

N 

co 

o 

o 

1~ significance 

level 

10 :II 

j I I I I I I I I I II I I I I II 

10 10 10 :II I 

Rotor Disk Normalized Wave Number. Dn/Ub 

band level skewness 

m18/11 11:'1 

51 1!1 , run AtB ~I I 

~I , (.) 

I run At t ----- - I I , 

, 

I 

I 

, 

, 

f 

, 

, 

, 

• I 

, 

I 

I 

I 

I 

I 

I 

, 

I 

I 

I 

I 

I 

I 

I 

, 

<0 

(J> 

N 

co 

0 

0 

, significance 

level 

10 10 II 10 :II 

1/3-0ctave Center Frequency (Hz) 

I I I I I I I I I I I I I i I I II 

10 10 II 10 :I 

Rotor Disk Normalized Wave Number, Dn/Ub 

) band level coefficients 

Ensemble Moments of Acoustic Ill-Octave Band 

Levels for Runs All and AlB 

143 
;;-~ 

II , 

~-~ 

N* 

:::l 

......... 

C 

::J 

(f) 

c 

+-' 

C 

Q.) 

C 

0 

a. 

E 

0 

() 

~ 

c 

"0 

:::l 

."!:: 

0) 

C 

0 

-l 

100 

I'-

en 

~ 0 

0 

-2/3 slope 

10-1 

A14-1 

10-2 

10-1 100 101 102 

Reduced frequency (f = nz/U) 

(a) longitudinal (axial) turbulence component normalized spectra 

c 

of 10-1 

C 

+-' 

C 

Q.) 

C 

o 

a. 

E 8 10-2 

~ () 

+-' 

~ 

Q.) 

> A14-1 

-2/3 slope 

00 

en 

~ o 

o 

10-3L-__ L-__ ~-L-L~ __ ~ __ -L~~~ __ ~ __ ~ __ ~~ 

10-1 100 101 102 

Reduced frequency (f nz/U) 

(b) vertical turbulence component normalized spectra 

TR-3036 

Figure 5-500 Normalized 45-m Turbulence Spectra for Runs A14-l and A14-2 

144 
1 

80 

70 

60 

I 

I 

I 

'" I 

~I 

~I 

°1 

I 

I 

I 

I 

I 

I 

TR-3036 

0> 

0> 

C\J 

eX) 

o 

o 

.304---~~-r~·~·~~'---·~--~~'-~~.----r--.-.-~~~ 

1 

1.0 

1 

1 10 II 

) 

145 

o 

o 

C') 

eX) 

o 

o 

Band 
_2.5 

3 

C"J 

e 

...., 

~ 2.0 .~ 

(J 

.~ 

OM 

OM 

Q) 

o 

u 

Q) 

:> 

cd ...., 

(J 

0 

I 

C"J 

0.5 

'---. 

...... 

0.0 

I 

1 

(c) 

14 

-----... 

~ 

e "'" 12 

...., 

d 

Q) 

.~ 

(J 

.~ 10 OM 

OM 

Q) 

0 

U 

fIl 8 .~ 

fIl 

0 

...., 

I-. 

;::I 

~ 6 

o-J 

(f) 

OJ 

Q) 4 

:> 

cd ...., 

(J 

0 2 I 

C"J 

'---. 

...... 

~I 

~I 

"t91 

~I (.) 

1 1 

-1- ------

run A14-1 

run A 14-2 ---------

1 10 10 II 

1/3-0ctave Center Frequency 

I I I I "I I I I I II 

10 10 II 

Rotor Disk Normalized Wave Number, 

TR-3036 

0.14-1/14-2 1111: 

1S!S 

(Hz) 

I I 

Dn/Ub 

o 

C') 

co 

o 

o 

significance 

level 

10 II 

I II 

10 II 

band spectrum level skewness coefficients 

0.14-1/14-2 kt 

el "t91 run A14-1 S ~I ~I N 

0 

run A14-2-------- C') 

1 1 co 

0 

0 

1 

I 

~ 

" 

" 

" I, ' , I , I , I , I I I , I I 

I 

, 

I 

I - - - - - 1S!S significance 

level 

O~--~--~~~~~-----'--'-~~TTnr~--'--'-''-ITTn 

I 

1 

) band 

1 10 10 II 10 :I 

1/3-0ctave Center Frequency (Hz) 

I I I I II I I I I /I I I I I I I 

10 10 II 10 II 

Rotor Disk Normalized Wave Number, Dn/Ub 

level relative kurtosis coefficients 

Figure 5-51. Ensemble Statistical Moments of Acoustic l/J-octave Band 

Spectrum Levels for Runs A14-1 and A14-2 

146 
5 .. 1 .. 1 .. 5 

TR-3036 

Relationship of Inflow Spectral Characteristics to the Mean LF 

Acoustic Spectrum 

We attempted to relate the spectral characteristics of the reference level 

(45-m) axial (longitudinal) and upwash (vertical) turbulence to the shape of 

the acoustic mean BSL curves.. One immediate problem confronting such a comparison is that the acoustic sources,. while perhaps generated by a linear 

dynamic process, are rotating within the surface boundary layer. The turbulence measurements, on the other hand, have been made at a fixed location, 

although within the rotor disk layer0 What is needed is to provide a scaling 

quantity to allow conversion from the fixed position to the rotating plane. 

We suggest the following ing space-frequency transformation: 

(5-3) 

where n' is the estimated cyclic frequency in rotating space (as seen by the 

rotor), n is the frequency space coordinates, n is the rotor rotation 

rate, Ro is the effect span), and I Z is the vertical or upwash 

turbulence Equatl'on 5-3 adjusts the fixedposition cyclic the former by the 

equivalent of the time span and quasi-horizontal) 

to traverse the stic vertical turbulent scale.. This transformation 

factor averages about 6 .. 4 for a mean Iwz of 9.8 m for the 1983 experimental 

data set.. Thus, a feature occurring at a cyclic frequency of 1 Hz in 

fixed coordinates would be est at 6.,4 Hz at span in the rotating 

frame for Iwz of 9.,8 m .. 

The approximation of allows us to plot the acoustic 

pressure and inflow turbulence on the same with abscissa values 

of n (acoustic) and n' (transformed turbulent) 5-52 through 5-57 

plot each of the six mean, acoustic densities observed in 

1983, <S (n», with the corresponding 45-m axial, S (n e ), and upwash or inplane, Safn'), mean turbulence spectra in logarithmi~ coordinates. A second 

abscissa w is also which i scaled in terms of the rotor convection 

wave number normalized the disk ion of Dn/Ub , where D is the rotor 

diameter and Ub the rotor veloci at the 75% span station 

(63 mps).. The wave number to the chord dimension at this span 

location is on the plots.. A line with a -5/3 slope is also included 

in these plots as a reference to the spectral slope as v1ith the 

turbulent inertial A turbulence spectrum with a -5/3 slope is 

necessary, but not to the isotropy of the inertial 

range.. An ion of these reveals, in , that 

(1) There 1S a small t 

spectra in the vicinity of the rotor 

also seems to coincide with the 

(indicated the two turbulence 

parallel) and 

in the mean acoustic pressure 

chord length which 

1 turbulence region 

component spectra becoming 

(2) The roll-off of the mean acoustic pressure spectrum approximates a 

-5/3 slope for reference wind speeds below about 10 mps. 

147 
/,-~ 

II I 

\l_~ 

TR-3036 

10 

, -5/3 slope 1 , , , 

, 

, , tj 

" , 10 -1 ~ , 

, , , , 

til 

4 

Ul 

I:l 

10 -I ~ 

~ 

l!1 ' Sap(n) 

~I 

10 -'~--~~~~~~--~--~~~~~--~~~~~T---~~~~~~10-4 

10 -1 1 10 10 II 10 :I 

Cyclic Frequencies, nand n' (Hz) 

I 10 -1 

I II1II iii I i III I I I i I II i i 

1 10 10 III 

Rotor Disk Normalized Wave Number, Dn/Ub 

iii I 11 

10 :I 

.....;. 

(") 

o 

(") 

<Xl 

o 

o 

Figure 5-52 .. Relationship of Mean Acoustic Pressure 

Sap{f) to Su{f') and Upwash 

Co~ponents for Run A05 

Distribution 

Turbulence 

Su(n') 

, -5/3 

, 

" , 

"-

" 

10 

1 

, tj 

" " 10 -1 ~ 

" " " " " 

til 

4 

Ul 

I:l 

10 -1~ 

~ 

I 

I 

I 

"ttl 

~I CJ 

Sap(n) 

10-·~--~~~-rrrnT---'--'-r1~In.---.-'-~rrrn~--'--'-'1I11~10-4 

10 -1 1 10 10 III 10 II 

Cyclic Frequencies, nand n' (Hz) 

I 10 -1 

1111'1 iii 1111( I Ililif I I 

1 10 10 III 

Rotor Disk Normalized Wave Number, Dn/Ub 

'" "1 10 II 

.....;. 

Figure 5-53.. Relationship of Mean Acoustic Pressure Spectral Distribution 

S ~(f) to 45-m Axial S (f') and Upwash S (fU) Turbulence a u w Co ponents for Run A03 

148 
'N'10 -1 

::r:: 

~ Cd 

& 

,........10 -II 

-E.. 

Q, 

Cd 

rr.l 

W S 10 -3 

(Il 

(Il 

W 

f... 

0.. 

C,) 

:;j 10 -4 

(Il 

;:1 

o 

C,) 

-< 

"d W 10 -II 

~ 

I 

f... 

Cd 

~ 

I 

10 -1 

'N'10 -1 

::q 

~ a:J 

e 

11 

10 -8 

(!) 

~ 10 -3 

CIl 

CIl 

(!) 

J... 

~ 

:3 10 -~ CIl 

::1 

o 

CJ 

<I! 

"t:I W 10 -8 

i;;:: 

I 

J... 

a:J 

(::L, 

Su(n') 

Sw(n') 

1 

Cyclic 

I IIIII 

1 

Rotor Disk 

" -5/3 slope 

" " " " " " " " " 

10 10 II 

Frequencies. n and n' (Hz) 

I I i I II I i I I j I II 

10 10 II 

" 

run a14-t 

" " " 

Sap(n) 

i j jiill 

10 :I 

Normalized Wave Number. DnjUb 

Acoustic Pressure 

I) and 

run 0.14-2 10 

TR-3036 

10 

1 

t::J 

10 -1 ~ 

l'Jl 

..... 

("+ 

'<l 

CJ.l 

~ 

10 -Il~ 

t:3 

.....;.. 

su(n'~ 

" -5/3 slope 

to 

o 

(') 

CX) 

o 

o 

" " " " " " Sw(n' 

" 

~I o 

~I 

" " " " " " 

Sap(n) 

t:I 

10 -! ~ 

f!!. 

f'4" 

'< 

CJ.l 

~ 

10 -8 ~ 

'S' 

o 

10 -B4----r-.-,,,"T1---r-.-.,-nnTl---.--rl~TTnT--_,--,_ITTTrrlr10 -. 

5-55 

10 -I 1 10 lOa 10 3 

Cyclic Frequencies, nand n' (Hz) 

I 10 -I 

i i j Iii iii III1I i I I i ill I i Ii iij 

1 10 lOs 10 3 

Rotor Disk Normalized Wave Number, Dn/Vb 

Acoustic Pressure Distribution 

) and Turbulence 

for Run 

149 
TR-3036 

"'N10 -1 1""W"\ DotB 1:10 

tt: t .po. ............ f- c.n ('q :3 a:S .' 

~ Su(n') ()) ~ ,.........10 -II \. -5/3 slope "0 

(l) 

.E- o '- ("+ 

0.. \. '"'1 Id \. 

\. e'.. rf) 

'- \. 0 (!) 

\. S 10 -3 10 -1 ~ '- tf) \. ~. \. tf) ("+ 

(!) '- '<l I-. '- 0.. \. ()) 

() ~ 

:;j 10 -4 10 _si tf) P :::1 --.::- 0 

() 

-< S 

"d "0 W 10 -II 10 -5 ~ 

~ I N 

............ I ~I Sap(n) ::r: I-. 

~I N Id 

t::<.. C,) 

10 -8 10 -4 

10 -1 1 10 10 8 10 II 

Cyclic Frequencies, n and n' (Hz) 

r I IIIII I I I I I I II I Ii IIII I I i I III 

10 -1 1 10 10 II 10 s 

Rotor Disk Normalized Wave Number, Dn/Ub 

Figure 5-56. Relationship of Mean Acoustic Pressure Spectral Distribution 

S (f) to 45-m Axial S (f') and Upwash S (f') Turbulence ap u w Components for Run Al8 

\. -5/3 SlOp63 

\. 

\. 

\. 

\. 

\. 

\. 

\. 

\. 

\. 

I 

\. 

\. 

\. 

\. 

~I Sap(n) 

~I C,) 

10 

o 

10 -1 ~ 

~. ("+ 

'<l 

Ul 

~ 

10 -lI-~ 

::;-

--.::-

S 

"d 

10 -9~ 

N ............ 

::r: 

N 

10 -·~--~--~~~~,---.--r-r",,"---.--r-~IITr,---.--,-rTllTrr10-4 

1 0 -I 1 10 10 II lOll 

Cyclic Frequencies, nand n' (Hz) 

I 

10 -I 

11111/ I 1111111 "lil'l I I 

1 10 1011 

Rotor Disk Normalized Wave Number, Dn/Ub 

I i I'I 

10 a 

Figure 5-51. Relationship of Mean Acoustic Pressure Spectral Distribution 

S (f) to 45-m Axial S (f') and Upwash S (f') Turbulence ap u w Components for Run All 

150 
TR-3036 

5 .. 2 

As previously stated, we at to compare the characteristics of the MOD-2 

LF emissions before and after major modificat were installed. The changes 

that occurred between our 1982 data collection and that done in 1983 included 

modifying the schedule and talling vortex near the 

leading 70% the rotor span The primary objective of both 

modif 1 turbulent loads Unfortunately, 

we did not , hot-film anemometer data lable to us 

1n ,. of the trumentation was the same for both 

years" Furthermore, the l!May 1982 runs tended 

to be more and those encountered in August 1983, as 

documented in Table 3-1 of overlap we needed to 

compare the 

The lack 

new set 

From our 

wind 

leas 

be a trend 

modified 

5 .. 2 .. 1 Mult 

trend 

to hi 

We appl ied the same 

reduced 1982 and 

5-5 as seal 

ANOVA analysi 

Richardson number 

inflow ctorso 

emission response 

1982 the to-run 

band of interes 

A different 

Hz (k = 1), 

changes 1n 

variations are 

u' and the mean 

us to identify a 

1982 information" 

li ,reference 

but only the varused as a measure 

information about the 

and no inforBecause the overlap in 

limited, we were able to identify 

Table 3-2 lists the 

mean wind speed, 

Richardson number and the 

list the ranges of the 

wind speed variance 

values observed, and 

condit listed in 

in 5-58, with a 

table show, there appears to 

mean emission levels above 10 Hz (in 1983) with the 

I 

1 1 

Section 5 .. 1,.1,,2 to the 

listed in Table 

and b present the results of an 

run-to-run ions using the 

)l UH, and variance, u', as 

that the LF acoustics 

than it was 1983 In 

, over the entire 

inflow structure (Ri)" 

1983 Below a frequency of 10 

MBSL is explained by 

Above 10 Hz, the 

with the variance, 
TR-3036 

Table 5-5. Overlap Ranges of Reduced 1982/83 Inflow Data Sets 

Parameter 1982 Ranges 1983 Ranges 

Richardson number -0 .. 01 to +0 .. 75 -0.12 to +0 .. 71 

z/L parameter -0 .. 01 to 5 .. 62 -0 .. 12 to 5 .. 62 

Hub-height wind -1 speed (ms ) 8.1 to 11 .. 9 8 .. 1 to 11.1 

Hub-height variance (ms-1)2 0.24 to 3 .. 45 0.5 to 1 .. 31 

M-O length L (m) 9 .. 5 to 638 -444 to 160 

The relative spectral sensitivities to these three predictors for 1982 and 

1983 are shown in normalized form in Figures 5-60a and be The coefficients of 

multiple determination, or r2' are also plotted as an indication of the 

applicability of the models in explaining the observed variation. It is quite 

clear from both figures that, at least for this choice of predictors, the 

vertical structure of the atmospheric inflow has the greatest influence on the 

spectral characteristics of the expected LF acoustic emission levels.. The 

1983 MOD-2 configuration, with the incorporated pitch schedule and vortex 

generator modifications, displays a pronounced emissions level sensitivity to 

Table 5-6 .. Comparison of Mean 1/3-0ctave Band Spectral Levels for the 

Conditions in Tables 3-2 and 5-9 for 1982 and 1983 Runs 

Band 1982 1983 Center 1983-1982 

Frequency <8(f» <8(f» Difference 

(Hz) (dB) (dB) (dB) 

2.00 74 .. 2 73.8 -1.4 

2 .. 50 72.4 72 .. 2 -0.2 

3 .. 15 70 .. 9 70 .. 8 -0 .. 1 

4.00 69 .. 0 68 .. 8 -0.2 

5 .. 00 67 .. 7 67 .. 6 -0 .. 1 

6.30 65 .. 2 65 .. 2 0 

8.00 63.4 63 .. 0 -0 .. 4 

10.0 61.4 61 .. 1 -0 .. 4 

12.5 59 .. 0 59 .. 2 0.2 

16.0 56.9 57 .. 6 0 .. 7 

20 .. 0 54.8 55.6 0 .. 8 

25.0 53 .. 2 53 .. 7 0 .. 5 

31 .. 5 55 .. 2 54 .. 5 -0 .. 7 

40.0 52 .. 1 52 .. 4 0 .. 3 

50 .. 0 49.7 50.2 0 .. 5 

63.0 47 .. 2 47.7 0 .. 5 

80.0 43 .. 6 45 .. 4 1 .. 8 

100 41 .. 7 43 .. 3 1,,6 

125 39 .. 1 41.3 2 .. 2 

160 37 .. 9 39 .. 2 1 .. 3 

Mean: 0 .. 4 

152 
.--.. 

II) 

-0 

-(J) 

() 

c 

(J) 

b 

(J) 

-

~ 

-0 

...J 

3 

2 

(f) 0 

II) 

C 

ct'l 

(J) 

E -1 N 

co 

en 

T""" 

c0 -2 co 

en 

5-58 

10 102 

1/3-octave center frequency (Hz) 

Q) 

o 

C') 

co 

o 

o 

103 

TR-3036 

Band Level for 1982 and 1983 

less stable flows below 10 Hz = 1) and just the opposite above that 

frequency to increasingly table flowse The 1982 conf ion, on the other 

hand, showed a lower overall stability sensitivity. In this case higher mean 

emission levels were as with less stable flows, reaching a peak in the 

8-Hz frequency band. 

The relative 1982/1983 normalized spectral sensitivities to the Ri, UH, and u' 

predictors are presented in Figures 5-61, 5-62, and 5-63, respectively. The 

much increased LF acoustic sensitivity to the Ri (atmospheric structure) in 

1983 over that in 1982 is quite obvious in the presentation of Figure 5-61, 

suggesting a dramatic change in the responsible aeroacoustic mechanisms. The 

spectral sensitivities to the mean axial wind component, shown in Figure 6-62, 

supports the statement that the 1983 turbine was acoustically more sensitive 

to the mean attack angle below 10 Hz and less sensitive above that range. The 

plot of the variance sensitivities (k = 1) in Figure 6-63 seems to suggest 

that the 1983 rotor experienced a more stable boundary layer. This is indicated by the low turbulence sensitivity resulting in increased lift and higher 

radiated mean acoustic levels below 10 Hz over the 1982 data for the same 

inflow conditions. 

5,,3 son of MOD-2 with MO~l LF Emi 

The most dominant (and annoying) characteristic of the MOD-2 LF emlSSlons was 

their coherence or impulsiveness. We have developed several measures of the 

degree of coherence in wind turbine radiated emissions [1], including 

The root-mean-square (rms) correlated 8- and 16-, 16- and 31.5-, and 31.5-

and 63-Hz octave spectrum levels (CBSL) and corresponding correlation coefficient (Ccoef) 

153 
- /- "- II I 

~ ~ - TR-3036 

100 

'U 90 

Q) 

d 80 • ..-4 

It! 

~ 

Q.. 

w 

>< 70 

d 

0 60 • ..-4 

~ 

It! 

• ..-4 

l-4 50 It! 

:> 

(tj 40 ~ 

0 

E-4 30 'H 

0 

~ 20 

d 

Q) 

CJ 

l-4 10 Q) 

p... 

0 

-10 

100 

'U 90 

Q) 

d 80 • ..-4 

It! 

~ 

Q.. 

M 

w 70 

d 

0 60 • .-4 

..., 

It! 

• .-4 

~ 50 It! 

:> 

...-4 40 It! 

..., 

0 

E-4 30 ..... 

0 

~ 20 

d 

Q) 

CJ 

l-4 10 Q) 

p... 

0 

-10 

1 

I. 

1 t3---B---8-"9-SD DD 

i=1 1 

PERCENTAGE OF OBSERVED VARIANCE 

EXPLAINED BY THE: 

o Richardson number, Ri 

o Hub-height mean wind speed, U t:. Hub wind speed variance, u' 

.. Unexplained residual 

10 10 III 10 a 

1/3-0ctave Center Frequency (Hz) 

(a) 1982 turbine configuration 

Iv= 1 1 

PERCENTAGE OF OBSERVED VARIANCE 

EXPLAINED BY THE: 

o Richardson number, Ri 

o Hub-height mean wind speed, U t:. Hub wind speed variance, u' 

.. Unexplained residual 

10 10 III 10 II 

1/3-0ctave Center Frequency (Hz) 

(b) 1983 turbine configuration 

Figure 5-59. Results of ANOVA Analysis for Mean Band Spectrum 

Level Inflow Parameter Sensitivities 

154 

o 

~ co 

o 

o 

~ co 

o 

o 
20 

10 

o 

... .s -10 

o 

..... 

"d 

(j) 6:: -20 

"d 

(j) 

.!::l -30 ...-4 

td 

e 

... 

~ -40 

-50 

2 

R 

u 

1.00 

0.75 

0.50 

0.25 

-60·~----~--~~~~~+-----~~--~~~~----~---.-.-.~~~0.00 

250 

200 

rfl 150 (l) 

0.. 

0 

fjj 100 

... 

0 

-t=J 50 (J 

:.a 

(l) 

... 0 a.. 

"d 

(l) -50 N 

...-4 

ttl 

e -100 ... 

0 

Z 

-150 

-200 

-250 

10 10 II 10 :II 

Center (Hz) 

) 1982 

1.00 

2 

R 

0.75 

0.50 

0.25 

~----~--'-'-'-""+-----'--'--IIIT"nn-----.---.-.-..-,,~o.oo 

1 10 10 

1/3-0ctave Center Frequency (Hz) 

1983 

Inflow 

155 

Predictor 

Levels 

10 II 

for 

TR-3036 

N 

;;; 

co 

o 

o 

~ cry 

co 

o 

o 
250 

200 

rJl 150 Q) 

0.. 

0 

rn 100 

l-t 

0 

~ 50 CJ 

• ..-1 

'"d 

Q) 

l-t 0 0... 

'"d 

Q) -50 N 

• .-4 

...... 

«:I 

El -100 l-t 

0 

Z 

-150 

-200 

-250 

1983 Ri 

1982 Ri 

I 'k=t 

I I I I I I Iii i I 

10 10 I 

l/3-0ctave Center Frequency (Hz) 

TR-3036 

I I I I 

10 :II 

"<t 

;; 

co 

o 

o 

Figure 5-61. Relative MBSL Spectral Sensitivity to Inflow Stability 

(Richardson Number) for 1982 and 1983 Configurations 

2.00 

1.50 

rJl 

Q) 

0.. 1.00 0 

...... 

[/) 

l-t 

0 0.50 ~ 

CJ 

• ..-1 

'"d 

Q) 

l-t 0.00 0... 

'"d 

Q) 

.~ -0.50 ...... 

«:I 

S 

l-t 

~ -1.00 

-1.50 

-2.00 ---I I I I 

I 'k=1 

I I I I I I II I 

10 10 I 

1982 Uh 

1983 Uh 

l/3-0ctave Center Frequency (Hz) 

I I I 

10 :II 

l!) 

;; 

co 

o 

o 

Figure 6-62. Relative MBSL Spectral Sensitivity to Inflow Stability 

for Mean Hub-Height (UH) Wind Speed 

156 
We 

I..t 

o 

.pi 

(J 

• pol 

'1::::J 

CLl 

10.00 

5.00 

0.00 

rt -5.00 

'1::::J 

CLl 

N 

'M 

Cd -10.00 

S 

I..t 

o 

Z 

15.00 

Plot 

octave BSL 

The observed 

and loS-Hz 

increased in 

octave band 

phase relat 

to-band correlation or 

of annoyance of the affected 

structures simultaneous 

the same 18 on the 

conditions 1n 

consequences 

5., .. 1 

Table 

to 

1983 1.4.' 

1982 1.4.' 

w ) 

TR-3036 

10 

CD 

~ <Xl 

o 

o 

ions of the three 

for the 8~, 16-, 

of lveness or coherence 

ion of the adjacent 

Thi indicated that a specific 

pressure field. The bandrespons for the degree 

it acoustically loaded the 

Therefore, we performed 

em1S 10ns over the lable range of inflow 

them with those of the MOD-l and their known 

octave band correlations for the available 

the reference 23- and 35-RPM 

levels (CBSL) for the 

very three adjacent band 

coefficient (Ccoef), however is not parblade-to-blade passage ion in the 

In 23-RPM case, on the other hand, 

the lower in frequency 

sage )0 The coherency 

157 
TR-3036 

Table 5-1 .. Statistical of Correlated or Coherent Octave Band 

Spectrum Levels Measured On-Axis at 1 .. 5D for 1982/83 MOD-2 

Data Runs Compared with the MOD-1 Turbine at 35 and 23 RPM 

Correlated Octave Band Spectrum Level 

Run 

MOD-2, 

8T23-1 

8T21-1 

ST19-1 

ST17-2 

8T26-1 

ST27 

ST25-1 

MOD-2 

A05 

AD3 

A14-1 

A14-2 

A18 

All 

MOD-l 

35 RPMc 

23 RPMd 

1982 

8/16Hz 

CBSLa/Ccoefb 

(dB/Hz) 

44 .. 1/0 .. 645 

57 .. 7/0 .. 464 

5906/0 .. 705 

7407/0 871 

65.8/0,,768 

6708/0 .. 776 

75.2/0 813 

55.1/0 .. 358 

58.9/0,,712 

52.3/0 .. 330 

54.3/0 .. 466 

65.6/0 .. 780 

6607/0 776 

70.8/0 .. 540 

71.2/0 .. 825 

16/31.5-Hz 

CBSL/Ccoef 

(dB/Hz) 

39.1/0 .. 768 

53 .. 4/0 .. 564 

48.9/0.258 

69.4/0.869 

5601/0.412 

6009/0 .. 677 

7004/0.735 

51 .. 3/0.407 

47.6/0 .. 270 

46.8/0.174 

46 .. 3/0.260 

57 .. 3/0.631 

59.2/0 .. 723 

72.9/0.663 

66.2/0.773 

aCorrelated band spectrum level. 

bBand pair correlation coefficient .. 

31.5/63-Hz 

CBSL/Ccoef 

(dB/Hz) 

33.4/0 .. 865 

47 .. 0/0 .. 525 

43 .. 5/0 .. 292 

59 .. 9/0 .. 733 

49,,3/0 .. 469 

50 .. 4/0 .. 375 

61.8/0.751 

40 .. 5/0 .. 097 

41.6/0 .. 194 

46.1/0.352 

41 .. 7/0 .. 273 

47.6/0 .. 347 

49 .. 0/0 .. 409 

67.7/0.571 

58 .. 1/0 .. 809 

CExtreme level of interior annoyance in homes at 1 kIn. 

dModerate level of interior annoyance at 1 kIn .. 

Hub Wind 

Speed 

(mps) 

6.3 

9.3 

10 .. 3 

11.7 

12.0 

13.3 

14.2 

7.3 

8.1 

9.0 

9.9 

12.4 

13.6 

17.0 

13.9 

Richardson 

Number 

24 

0,,46 

0.18 

-0.01 

0 .. 13 

0,,42 

-0 .. 92 

11 .. 7 

-0 .. 12 

0 .. 13 

0.26 

1.25 

6.68 

Stable 

Stable 

actually exceeds (as indicated by the much greater correlation coefficients) 

that of the 35-RPM case, with slightly more correlated energy in the 8/16-Hz 

band pair than for the higher rotation rate.. We believe this is the reason 

that slowing down the rotor did not give the affected residents complete 

relief from the impulsive annoyance .. 

We found that the annoyance threshold was reached when the impulse, measured 

outside an affected horne, in the 8/16-, 16/31.5-, and 31 .. 5/63-Hz CBSL simultaneously exceeded 52, 47, and 35 dB/Hz, respectively, with associated correlation coefficients of 0 .. 200 or greater [1]" These figures correspond to 

CBSLs of 73, 68, and 56 dB/Hz at the reference distance of 1.5D (91.5 m) from 

the turbine, assuming only geometric spreading for propagation. We know that 

sometimes the combination of terrain and atmospheric focusing raised the level 

at one or more of the homes as much as 25 dB above the equivalent value that 

158 
would be expected from 

MOD-1 to 

other subsequent 

5 3,,1.2 

While not a 

use of 

poses" 

environment 

es 1n 

function of 

band rs 

..... 

c 

~ 60 (J) 

.!: 8 55 (j) 

"'0 

C 

a:J 

.0 

159 

TR-3036 

Thus, we can use the documented 

annoyance factor of the MOD-2 and 

s or coherency, the 

comparati ve purstatistics: (1) the 

(2) level expressed in terms 

ime criterion 

The percentile 

values to identify 

values, which 

Thus, the LID 

time The difthe impulsiveness or 

contribute to the noise 

as a 

adjacent octave 

corresponding MOD-l 

rpm 
75 

(j) 

> 

~ 70 

E 

::l 

I- 65 +J 

l) 

(1) 

Q.. 60 en 

+J 

c,.-... 

55 (1) N I-I 

(1) .......... 

.em 

0-0 50 l) ........ 

en 

"0 

c 45 Cd 

..Q 

N 40 I 

,-

CV) 

.......... 35 <0 

30

4 

Figure 5-65 .. 

1982 

70 

(j) 

> 65 ~ 

E 

::l 60 I-

+J 

l) 

(1) 

Q.. 55 en 

+J ___ 

C N 50 ~I (1) .......... 

.em 

0-0 45 l)----

en 

"0 40 C 

(\j 

..Q 

N 35 

I 

CV) 

<0 30 .......... 

,-

CV) 

25 4 

TR-3036 

/ 

/ e 35 rpm co 

/ ;;; 

D./' co 

0 

° "/ 0 

/ 

/~23 rpm 

/ 

,/' 

,/' 

,/' 

,/' 

,/' 

D,/' / 

6 L0 

6 

/' 

,/' 

/ / 1982 0---

,/',(j 1983 6 

/ MOD-1 E9 / 

6 8 10 12 14 16 18 

Hub-height wind speed (mps) 

son of .. 5-Hz CBSLs va .. Hub-Height Wind Speed for 

and 1983 MOD-2 Configurations and MOD-1 Turbine 

/ / 

/ 

./' 

/0 

./ 

~/ 

/ 

6 8 

1982 ° --- 1983 6 ---

MOD-1 E9 

Hub-height wind speed (mps) 

5-66 of 31 .. 5/63-Hz CBSLs vs. Hub-Height Wind Speed for 

1982 and 1983 MOD-2 Configurations and MOD-1 Turbine 

160 
values for 23~ 

plot" We bel 

reflects 1n 

vortex generators and the 

emissions are les 

coherent below that 

were , the 1 

the vortex 

values 

are 

1 

TR-3036 

also included in each 

of the two trend lines 

c response caused by the 

It is clear that the 

mps and slightly more 

of whatever instabilities 

to be much 

ID1te improvement 

sSlons by adding 

s (the 

wind speed and 

and correlation 

, there is a 

of the 

years and comof the 

scales are 

radiated coherionship to the 

scale length, 

ile or L 

cases 1n 

is clear why 

wi th the MOD-l 

iveness associated with 

between the L10 and Leq 

1983 MOD-2 results$ 

exceedence results 

run (ADS), which 

Ala) between 60 and 

than those of 

~-Je bel ieve 

structural 

three parampressure in the 

chord and flapmeasured on the 

7% span)" The 

time 
<D 

Q. 

o 

....... 

:!::: 

C 

::I 

.......... 

N 

I 

.......... 

II) 

"0 

<D 

Q. 

0 

u; 

+-' 

c 

<D 

() 

~ ..... 

<D 

0 

() 

10 

-- _--0 

-- o --7-1::r------:::JI.d_--..... 0-:-.... :: .. : .. : ... : ... :....:...: .: .... :::.:.;,.c..:....:." • : ..... : .. : •• - • - - ••••••••••••••••• 

-10 

-20 

-30 

Predictors 

Richardson number 0 

M-O z/L parameter 0 

Hub-ht. wind speed ~ 

Hub-ht. variance " 

1982 ---

1983 

-40 L-~ ________ ~ ____ ~ ____ ~ __ ~ __ ~~~~~ 

10 102 

Octave band pair center frequency (Hz) 

cOlmD.~r]SOn of inflow predictors on CBSL 

0.4 

0.0 . ..................... 

/' ,,-- /' 

,/'" /' ~- -0.4 ' /' 

,/'" / ,,/ / 

/ 

/ 

/ -0.8 / 

/ Predictors / 

b Richardson number 0 

-1.2 M-O z/L parameter 0 1982 

Hub-ht. wind speed ~ 1983 --- Hub-ht. variance " -1.6 

10 102 

Octave band pair center frequency (Hz) 

o 

C\J 

C') 

co 

o 

o 

N 

C') 

co 

0 

0 

TR-3036 

of inflow on the adjacent octave band Ccoef 

5-67 son of Inflow Predictor Sensitivities on Adjacent 

CBSLs and Ccoefs for 1982 and 1983 Turbine Configurations 

162 
Table 5-8 

Octave Band 

(Hz) 

6 

16/ 1 5 

es 

first zero 

mult 

Table 1 

each 

rotor 

1 /63 

MOD-2 

1982 

474 

7 

1 

TR-3036 

Correlated 

Set of 

MOD-l 

1983 23 RPM 

CBSL/Ccoef 

(dB/Hz) 

58 8/0 70 71.,2/0 .. 825 

50 

43 

7/0 355 

225 

66 .. 2/0.,773 

58,,1/0,,809 

sociated wi th the 

to space coordinates 

the 87% span station .. 

) correlations found for 

runs 

span station, the prochordwise moment ,. normal and 

of 65-75 ms, 

meters in space. Thus, 

characteristic spatial 

effec on the unsteady 
C 

::::l 

......... 

N 

I 

......... 

CO 

"'0 

(l) 

Q. 

o 

(f) 

-c 

3 

2 

o 

(l) -1 () 

--(l) 

8 -2 

~~~~d8==~~~~ ..................... . 

o Predictors 

Richardson number o 

Reference wind speed 0 

lux scale length t:". 

Iwx scale length \7 

"lwZ scale length • 

N 

N 

C') 

co 

o 

o 

TR-3036 

-3 L-__________ -L ______ J_ __ ~~ __ L__L __ ~J_~ 

0.10 

---

§ 0.05 ......... 

N 

I 

......... 

OJ 

"'0 

(l) 

Q. 

o 

-c 

(l) 

() 

~ 

-(l) 

o 

o 

0.00 

-0.05 

10 102 

Octave band pair center frequency (Hz) 

of inflow predictors on adjacent CBSL 

X:;;:;~;:::;:;:::;::::~~;:::::::;::::====~ ................... . 

Predictors 

Richardson number 0 

Reference wind speed 0 

lux scale length t:". 

Iwx scale length \7 

Iwz scale length • 

~ C') 

co 

o 

o 

-0.10 '--_____ --L-___ .L.-_--1_--1_--'---.JI....--....I.--...I--l 

5-68 

10 102 

Octave band pair center frequency (Hz) 

of inflow predictors on adjacent band Ccoef 

of Inflow Predictors on Adjacent CBSLs and 

Coefficients for 1983 MOD-2 Configurations 

164 
Table 

Octave Band Exceedence Levels 

{conditional on 8-Hz band level} 

Octave Band 

Criteria 

8-Hz BSL >60 dB/Hz 

>65 

>70 

l6-Hz SSL <, 

315Hz BSL 

Octave (Hz) 

16=Hz L10 

L20 

Leq 

3leS-Hz L10 

L 20 

Leq 

63-Hz 

1982 

(Percent 

81 

47 

21 

80 

1 

14 

81 

81 

) 

59 

7 

54 

49 

48 

47 

165 

of 

1983 

time 

88 

50 

13 

88 

85 

44 

7 

88 

88 

88 

58 

BSL 

TR-3036 

Band Exceedence 

Table 3-2 and the 

MOD-l 

BSL exceeded) 

100 

82 

60 

100 

100 

84 

66 

100 

100 

100 

72 

BSL 

(dB/Hz) (dB/Hz) 

68 81 

66 79 

63 70 

61 76 

60 74 

58 66 

56 67 

55 64 

54 57 

50 58 

49 57 

48 50 
TR-3036 

Table 5-10. Exceedence Level for 1983 LF-Range Octave BSL Values 

Run Percent of Time Exceeded 

Number 8-Hz Band 

BSL > 60 dB/Hz BSL > 65 dB/Hz BSL > 70 dB/Hz 

ADS 97 54 4 

A03 83 40 8 

A14-l 77 24 3 

A14-2 94 48 7 

Al8 99 84 46 

All 99 87 51 

l6-Hz Band 

BSL > 55 dB/Hz BSL > 60 dB/Hz BSL > 65 dB/Hz 

ADS 95 54 1 

A03 76 32 1 

Al4-l 76 36 3 

A14-2 87 24 1 

A18 98 75 28 

All 99 80 27 

3). 5-Hz Band 

BSL > 45 dB/Hz BSL > 50 dB/Hz BSL > 55 dB/Hz 

ADS 97 97 73 

A03 83 83 43 

A14-1 77 77 63 

A14-2 94 94 27 

Al8 99 99 77 

All 99 99 84 

166 
Table 5-11 .. 

Chord Moment Flap Moment 

Run 

No 1 1 tc 

(m) (ms) (m') (ms) 

A03~1 " 1 69 .. 5 5 .. 2 71,,0 

A05 3 72 3 5 .. 7 0 

A03 4 62 5 .. 73 ° 

1 8 65 5 74 .. 6 

.. 7 1 5 .. 72 7 

1 73@0 

8 7 1 7 

All 4 8 66 0 5 8 

8 4 

021 

Normal Force 

1 tc 

(m) (ms) 

4 .. 4 59 .. 6 

5 0 68,,0 

4 7 64 .. 8 

407 64,,5 

5 5 75 .. 0 

4 .. 7 

5,,1 70 .. 3 

5 8 80 .. 0 

5 4 73 .. 6 

° 74 10 2 

167 

TR-3036 

Scales for Four 

Acoustic 

Pressurea 

Ri UH 

1 No .. (mps) 

(m'1 

tc 

(ms) 

4 .. 4 60,,5 0 .. 26 6 .. 9 

4 .. 6 63 .. 3 12 7 .. 3 

5 ° 68 .. 4 -0,,12 7e9 

4,,6 62 .. 9 0 .. 13 8 .. 2 

4 .. 8 65 2 0 .. 26 9 .. 9 

4 .. 8 66 .. 0 0 .. 71 10 .. 3 

407 64,,5 1.26 12 .. 9 

4 .. 5 62 .. 5 6,,68 13 6 
TR-3036 

6®O MEASURING THE ANNOYANCE POTENTIAL OF A SINGLE MOD-2 TURBINE 

Given our experience with the MOD-l downwind turbine near Boone, N. Co, summarized in Ref .. [1], the desirability of assessing the potential of MOD-2 

emissions for interior, low-frequency annoyance problems in nearby homes is 

very important. Several approaches to assessing this annoyance potential are 

available to us and include 

(1) Comparing the known physical properties of the radiated acoustic pressure 

fields at a specified reference distance from the turbines 

(2) Assessing the interior annoyance potential from measurements of the MOD-2 

emissions and compar1ng them with documented low-frequency annoyance 

situations. 

The first of these was addressed to some extent in Section 5.3 and is expanded 

a bit more in this section. The real crux of the matter, however, lies with 

item (2), since the human element (ieee, the ultimate receptor of any annoyance) must somehow be brought into the equation. On this latter point, though 

considerable study has been done around the world, we have only a meager 

scientifi data base on exactly what constitutes interior low-frequency 

annoyance and what conditions must be present for it to occur. Human annoyance from low-frequency sound and associated stimuli is highly subjective by 

its very nature@ However, at some point the human element must be considered, 

and we have to do that in a companion study [7].. The results of 

that limited study have been applied to individual MOD-1 and MOD-2 turbine 

emlSS10n characteristics to estimate the potential for interior annoyance 

problems in homes near these machineso 

1 

In the previous section, we pointed out that the degree of impulsiveness or 

coherence in the MOD-l emissions was a major factor in the degree of annoyance 

described by occupants within 1-3 km (0.6-108 miles) of the turbine. This impulsiveness is manifested by a strong phase coherency in the radiated acoustic 

pressure field and can be measured by the degree of cross-correlation in the 

8-, 16-, 31.5-, and 63-Hz octave band spectrum levels.. These four octave 

bands cover the structural resonance region of most home construction in the 

United States. Tables 5-13 and 5-14 showed that, with one exception, the 

MOD-2 emissions were much less correlated or impulsive than those of the MOD-1 

running at 23 or 35 RPM. 

The mean band pressure spectra of Figure 6-1 show an approximate annoyance 

envelope for the MOD-l at the reference distance of 1.5D. The upper curve 

(35 RPM) corresponds to the highest degree of interior annoyance at a farfield distance of 1 km and the lower one (23 RPM) to a moderate level.. The 

six available MOD-2 MBSL spectra are plotted over this envelope in Figure 6-2. 

As indicated, above the 5-Hz band all fall below the lower MOD-l curve. The 

fact that the two spectra associated with the highest winds do fall within the 

dotted area is insignificant, since this occurs below the lower structural 

resonance cut-off of 5 Hz. The 1982 MOD-2 case that we noted (Table 5-7) as 

having the potential to cause low-frequency annoyance is shown in Figure 6-3, 

overlaid on the MOD-l envelopee In this case, the MBSL spectrum falls 

168 
90 

N 80 

I 

"'- III 

-0 70 '-" 

Q) 

>-

~ 

60 

50 

-0 

c 

(\j 40 .!:l 

C 

(\j 

Q) 

:2: 30 ~ 

20 

1 

90 

80 

70 Q) 

>-

~ 

60 

50 

-0 

c 

(\j 40 .!:l 

C 

(\j 

Q) 30 :2: 

TR-3036 

'<t 

N 

(') 

00 

0 

0 

3S-rpm operation, 

maximum interior 

annoyance 

23-rpm 

moderate interior 

annoyance 

10 102 

1/3-octave center frequency (Hz) 

, 

"-

\ 

\ 

\ 

\ 

\ 

\ 

Run A11, -13.6 mps 0 

Run A18, 12.4 mps 0 

Run AOS, 7.3 mps 

1/3-octave center frequency (Hz) 

of MOD-1 Acoustic 

the 1983 

1 

169 

U") 

N 

(') 

00 

0 

0 

6-1 with 

A05) and 
N 

I 

........ 

CO 

"0 

........ 

Q) 

> 

.SI2 

E 

:J 

~ 

+=' 

<.) 

Q) 

a.. 

C/) 

"0 

C 

ro 

.0 

C 

ro 

Q) 

~ 

6-3" 

90 

80 

70 

60 

50 

40 

30 

20 1 

1982 MOD-2, severe off-design operation 

1/3-octave center frequency (Hz) 

<0 

N 

(') 

co 

o 

o 

COI~pclr]SOn of MOD-1 Acoustic of Figure 6-1 with 

from the 1982 MOD-2 Operating in a Severe OffCondition 25-1) 

TR-3036 

between the two MOD-l curves (between 5 and 100 Hz) and, under the right 

circumstances, we believe it could result in interior annoyance. This again 

demonstrates that even though the turbine's rotor s upwind of the support 

tower, there is the potential for acoustic emissions characteristics similar 

to a downwind turbine if the turbine is allowed to run severely off-design! 

.. 2 

We found that the interior annoyance problem associated with the MOD-1 turbine 

was a complex interaction between a room's structural and acoustic resonances 

excited by the external impulsive acoustical loads being propagated from the 

turbine [1]" It has never been totally clear whether the occupants heard, 

fel t, or experienced some combination of both stimul i as a resul t of thi s 

periodic loadingo Many experts believe both that tactile (feeling) and aural 

(hearing) responses to these stimuli are involved. As a result, we used 

volunteer subjects to conduct a limited evaluation of simulated, interior lowfrequency noise environments that could result from three wind turbine designs 

or installationso The details of the testing and the development of the proposed metric are given in Refe [7]. 

2 1 of Results of the Interior Low-Frequency Noise Evaluation 

A limited evaluation of the degree of annoyance in four simulated interior 

noise environments related to wind turbine installations was performed with 

volunteer subjectse The interior noise environments were associated with (1) 

a single, large upwind turbine (a random periodic source); (2) the same source 

as (1) except with a 40-dBA pink noise masking; (3) a single, downwind turbine 

170 
ing at 30 RPM (a 

turbines (a random impuls 

simulations, which were sys 

comparable to a small room 

of what they were 

6,,2 1 1 

The volunteers' responses 

terms of loudness 

of 

TR-3036 

impulsive source); and (4) multiple downwind 

source)~ The volunteers listened to these 

ly varied in intensity, in an environment 

cal home and provided written impressions 

ient Estimator of 

means of a five-level ranking in 

of annoyance and displeasure, any feelthe sensing of any pulsations" Table 6-1 

These ranked responses were then 

low-frequency noise descriptors or 

suggested as measures of low-frequency 

the following spectral 

for Standards) propo ed weighting 

Standards Ins itute} "Ii'ileight [23] 

under 100 Hz for 

curves have been proposed 

c-range (less than 20 Hz) 

was or developed to 

for pure tones at high sound 

6-4, the ing" 

than the most common noise 

Toki ta et al" [22] proposed two 

environments, the LSPL (lowsound level)e The LSL 

ec three low-frequency noise 

structural cal, and psychological complaint 

The L8L has been proposed as an appropriate descriptor for 

interior environments that contain both and lowtic 

regression; the results for the louds are listed in Table 6-2" Immediateof the five metric that contain significant low 

with the A-weighted scales The highest correlation 

mean correlation coefficient of 0,,936 for 

We bel that the LSL weighting provides 

the annoyance ial to persons exposed to lowin their homes" 

171 
TR--3036 

Table 6-1" Environments Subjective Ranking Criteria 

Stimuli Response Rating 

Rank 0 1 2 3 4 5 

Perception 

Noise Can't Barely Weak, but Moderate High noise Very high 

Level Hear can hear definitely loudness level, loud noise level, 

(loudness) audible very loud 

Annoyance/ None Barely Definitely Moderate Very Extremely 

Displeasure aware of aware of distraction/ annoying, annoying, 

presence presence some irritating uncomfortable 

irritation 

Vibration/ None Feel Definitely Moderate Very Severe 

Pressure presence feel vibration/ noticeable vibration 

vibration/ pressure 

pressure feeling 

Pulsations None Barely Definite Moderate Heavy Very heavy 

feel pulses or booming booming pulses, 

pulses bumping or or booms, 

thumping thumps thumps 

-- ~ -- = = -- -- -- -- -- -- -- -- - -- - -- - -- -- -- -- -- - - - -- -- I- -- - - - -- - -- - -- -- - -- - -

Acceptable 77711717111111 Clearly unacceptable 

-- - -- -- -- - -- -- -- -- -- - -- - -- -----------

r--

20 N ~ 0 

0 

0 C 

co 

"0 A 

(i) -20 > 

~ LSL, LSPL Q) 

I- -40 C ::J 

U) 

U) 

Q) 

"-

Q. -60 

"0 

C 

::J 

0 -80 U) 

Q) 

> 

+-' -100 ~ Q) 

a: 

-120 

-140 

1/3-octave center frequency (Hz) 

6-4 Low-Frequency Noise Metrics Spectral Weightings 

172 
Ta.ble 6-2 .. 

Metric 

LSL 

C 

G1 

LSPL 

A 

Noise 

Level 

o 923 

0 .. 91 

o 891 

0 .. 

o 868 

0.,307 

TR-3036 

versus 

Annoyance/ Mean Displeasure 

0 .. 948 0,,936 

0,,938 0 .. 926 

0 .. 920 0 .. 906 

0 .. 913 0 .. 904 

0,,887 0 .. 878 

0 .. 270 o 289 

Scale 

The volunteer-con~ent four stimuli were summarized, 

and three final level for each" The ion threshold has 

been defined as the LSL value for an evaluation ranking of 1 .. 

The annoyance threshold s a ranking of 2,,5, and an unacceptable 

annoyance level ass of 4 or above The LSL values corresponding 

to the levels listed in Table 6-3 for the four stimuli evaluated.. As we 

can see in the upper ion Table 6-3, three of the four stimuli have similar threshold LSL values It is interest to note that even though many 

individually impulsive sources may be present, the net effect of a random sumtwo 

odic source 

of an 

6 2 .. 2 for 

invokes a response similar to that from a random 

threshold is cons lower for a source confew impulsive sources" This is reflected by the 

of Table 6-3 .. 

LSL Va.lues 

Often we must predict the annoyance potential to a surrounding 

community from a new turbine or turbines before installation as part of the 

siting approval process" Since we will rarely have the opportunity to have 

actual measurements within residences, we needed to find a way to estimate or LSL levels ( low-frequency sound level or 

PLSL), given the stic low-frequency acoustic emission statistics of 

a given wind turbine design .. 

Interior LSL Level 

In order LSL levels, gl ven a representative external 

acoustic we needed to a typical indoor/outdoor 

acoustic transfer function for housing construction in the United States We 

identified a cal transfer function by using available data collected from 

two homes under MOD~l impul ive excitation, from five homes in Oregon experienclng sive, low-frequency acoustic loading (the exhaust stack from a 

gas turbine s [24]), and from an experiment utilizing a 

tandard~construction test home operated by SERIo We found that a different 

173 
TR-3036 

Table 6-3e SERI Interior Low-Frequency Annoyance Criteria Employing 

the LSL Metric 

Stimuli 

Upwind turbines 

(random periodic) 

Single or few 

downwind turbines 

(periodic impulsive) 

Many downwind 

turbines 

(random impulsive) 

Upwind turbines 

with 40 dBA masking 

LSL 

Threshold 

Perception 

(dB) 

58 

53 

59 

59 

LSL 

Annoyance 

Threshold 

(dB) 

65 

57 

68 

65 

Considering Only General Source Characteristics 

Nonimpulsive source 58 65 

Impulsive source 53 57 

LSL 

Unacceptable 

Annoyance 

Level 

(dB) 

68 

60 

70 

67 

68 

60 

spectral response is typical of homes undergoing impulsive loading (from the 

MOD-1 or a few downwind turbines) compared with random excitation (sources 

such as a single upwind turbine or many downwind ones)" Figure 6-5 plots 

these two typical transfer or acoustic transmissibility functions. As we can 

see, impulsive loading is characterized by much higher acoustic transmissibilities above about 10 Hz in comparison to random, external acoustic loads. The 

details related to the physics responsible for these curves are discussed in 

[1]. Using each of the transfer functions plotted in Figure 6-5, we modified 

and replotted the original frequency weighting of the LSL parameter in 

Figure 6-6" An interior estimate of the PLSL level is calculated by applying 

the appropriate weighting curve of FigQre 6-6 (impulsive or nonimpulsive) to 

an externally measured 1/3-octave band pressure spectrum. 

6,,2.2,,2 Establishing a Reference External Acoustic Loading 

The method of estimating a representative PLSL value requires a suitable 

measure of ~he external acoustic loading spectrum. Since most homes are 

located at some distance from the nearest wind turbine(s), a method must be 

devised to provide a reference spectrum that takes into account situations in 

which atmospheric refraction and surface reflection combine to increase the 

observed levels above that expected from spatial divergence alone. In order 

to achieve a Itfigure-of-meritlt PLSL value for a single MOD-2 turbine, a 

174 
(1) 

en 

c 

o 

0. 

en 

(1) 

b 

15 

10 

5 

o 

-5 

-10 

-15 

1 

6-5 

20 

(1) 

> 

(1) 

(1) 

b 

:J 

en 

U) 

(1) 

l0. 

U 

C 

:J 

o 

en 

Q) 

> 

cr; 

C5 

a: 

o 

-20 

-40 

-60 

6-6 

10 

\ 

\ 

\ 

\ 

Impulsive source 

\ I", I L~_J \ A / 

\ "'", \/ 

Nonimpulsive source 

1/3-octave center frequency (Hz) 

Original LSL definition 

Impulsive PLSL modification 

Non impulsive modification 

1 /3-octave center frequency (Hz) 

PLSL Measurements 

175 

co 

N 

C'l 

co 

o 

o 

for 

for 

Ol 

N 

C'l 

co '0 

o 

TR-3036 
TR-3036 

reference distance of 1 km (approximately 10D) was chosen. Table 6-4 lists 

the 1983 MOD-2 PLSL and PLSL10 values at the reference distance of 1 km 

propagated by spati~1 divergence alone and with an additional 15 dB of 

terrain/atmospheric focusing as a worst-case estimate.. The worst-case 1982 

MOD-2 run (ST25-1) and the 35- and 23-RPM cases from the MOD-l are also listed 

for comparison in Table 6-4. 

6 .. 3 

community annoyance related to the acoustic emissions of a single MOD-2 

turbine has two sources: (1) emissions in the high-frequency range generally 

heard outdoors or through open windows, and (2) interior annoyance arising 

from structural loading by acoustic energy in the low-frequency range.. We 

discuss both sources below .. 

6 3 .. 1 from High-Frequency-Range Emissions 

From the resul ts of our measurements of the high-frequency characteristics 

discussed in Section 4.0, we have reached the following conclusions .. 

(1) The aural identification of a single turbine or a cluster of MOD-2 

turbines as the source of HF, broadband sounds can be expected., on the 

average, 1.2-1.5 km (4000-5000 ft) downwind of the nearest turbine. This 

recognition distance depends on the state of the acoustic environment at 

the observer's location and the distance to the nearest turbine. 

Table 

Run PLSLeq Number (dB) 

A03-l 37 

AD3 41 

AD5 39 

A14-l 42 

A14-2 40 

A15-l 44 

A18 43 

All 43 

Means 4T 

1982 MOD-2 severe off-des 

ST25-1 

MOD-1 worst cases 

35-RPM 

23-RPM 

56 

65 

54 

Interior LSL (PLSL) Values at 

(1983 Configuration) 

PLSLeq+15 

(dB) 

52 

56 

54 

57 

55 

59 

58 

58 

56 

71 

80 

69 

176 

PLSLlO 

(dB) 

39 

43 

44 

45 

43 

47 

46 

46 

44 

60 

68 

58 

1 kM from the 

PLSL10+15 

(dB) 

54 

58 

59 

60 

58 

62 

61 

61 

59 

75 

83 

73 
TR-3036 

(2) While the broadband sounds modulated at the blade passage frequency can 

often be distinguished above local acoustical background at distances 

exceeding 1 .. 6 km (1 mile) under the right atmospheric conditions (i"e .. , 

low wind speeds in the vicinity of the observer), there appears to be a 

very low probability of community annoyance from a single MOD-2 turbine at 

distances beyond 1 km (0 .. 6 mile).. Typical 1 levels measured at the 

Goodnoe Hills Site and 0 9 km downwind of Turbin~qNo" 2 were 43-45 dBA for 

representative recording periods of 6 hours, which included a wide range 

of inflow conditions.. The corresponding 1 10 and 190 levels were 53 and 

43 dBA, respectively" 

(3) A of narrowband (12 .. 5-Hz resolution) analyses of the high-frequency 

portion of the ed spectrum taken covering a range of inflow conditions showed no persistent tonal (discrete frequency) noise. This indicates that the noise sources are well controlled, making the 

rotor's aerodynamically sounds the dominant HF source 

" .,2 

Using the P1SL discussed in Section 6 .. 2, we concluded that the likelihood of interior annoyance result from acoustical loading produced by 1983 

MOD-2 LF emissions on homes located a kilometer or more away is very remote. 

Even under the worst and propagation conditions (a home located 

where a 10n reflection and atmospheric refraction is 

15 dB of enhancement), it is unlikely that the turbine would be 

detectable within a home with the windows closed a kilometer or more away .. 

177 
TR-3036 

7.0 CONCLUSIONS 

7 .. 1 

At the Goodnoe Hills site, the average audible range of a single MOD-2 turbine 

was found to be about 1.2-105 km (4000-5000 ft). A polynomial which describes 

the average fall-off with distance for the Goodnoe Hills site is 

Leq(A) = -3.89454 x4 + 46.6729 x3 - 191.884 x2 + 287.151 x - 28.4 , 

where L (A) is the equivalent sound pressure level in dB and x is the log10 

of the ~gwnwind distance in feet. When multiple turbines were operating, the 

turbine noise level experienced by an observer is dominated by the nearest 

turbine" 

The Leq(A) at a distance of 1.5 rotor diameters (1.5D, 137 m or 450 ft) from 

the roror plane was found to vary, primarily with the hub-height wind speed. 

This variation in dB can be expressed to within ±0.5 dB over a wind-speed 

range of 6-15 mls by 

L (A) = 1/2 UH + 57 , eq 

where UH is the hub wind speed in mise 

A comparison of the 1982 and 1983 turbine configurations revealed an apparent 

upper-band-limiting of the 1983 emlSSlons as a result of the vortex generator 

installation and pitch schedule modifications. 

No significant, steady tone noise components were found in the high-frequencyrange narrowband (25-Hz) spectra. This indicates that the mechanical noise 

sources are well controlled, and there appear to be no discrete aeroacoustic 

sources of consequence. 

7 .. 2 

The varlances observed of the the ensemble mean and the first three statistical moments of randomly sampled, low-frequency acoustic emission spectra in 

1/3-octave band resolution were found to be adequately explained via a linear, 

multivariate regression model, with the following inflow scales as the most 

efficient predictors: 

• A mean wind speed at a height within the rotor disk layer 

The gradient Richardson number stability parameter (Ri) measured across the 

rotor disk layer 

• The Monin-Obukov length scale, L 

The vert ical or upwash turbulence integra}:~ scale, Iw Z 0 

178 
TR-3036 

Comparisons made between the 1982 and 1983 emissions showed that the 1983 turbine was far more acoustically sensitive to inflow stability_ 

The physics of the rotor-turbulence interaction are such that unsteady blade 

loads are the most common source of radiated low-frequency acoustic emissions 

and much of the rotor aeroelastic response. We can achieve a high correlation 

with the former using bulk scaling measures of the inflow, so this technique 

might be successfully extended to the latter. This would allow a given rotor 

aeroelastic response to be expressed directly in terms of surface-layer 

tur.bulence bulk propert 

The common origin of the rotor low-frequency acoustic and aeroelastic 

responses was demonstrated by space-time correlation measurements. We found 

that observed flap and chordwise bending moments, blade normal forces, and the 

8-Hz band, acoustic pressures were similarly correlated over time 

periods of 65-75 mSe These periods suggest that the unsteady processes 

responsible for the fluctuations take place over a rotor travel distance of 

about 5 m for the MOD-2 blade 

1 3 

Measurements of the MOD-2 showed that the 1983 turbine configuration produced 

les coherent (impulsive) low-frequency acoustic radiation than the 1982 version and much less than the MOD-l turbine operating at ei ther 35 or 23 RPM 

under the worst impulsive conditions The degree of coherency (impUlsiveness) 

in the turbine emissions was found to be strongly related to the vertical 

atmospheric stabili (Richardson number), the vertical or upwash turbulence 

scale length, I z, and the blade loading, as indicated by the mean axial ve- w locityo 

It is clear in 1982, the MOD-2 rotor instabilities associated with unstable, 

high-veloci ty inflows were responsi ble for producing low-frequency acoustic 

emissions that resembled those of the downwind MOD-l turbine under the worst 

impulsive conditions A definite improvement was made in the levels and impulsiveness of the LF emissions by the addition of the vortex generators and 

pitch schedule changes 

1 .. 4 

While the broadband sounds modulated at the blade passage frequency can often 

be distinguished above the local acoustical background at distances exceeding 

106 km (1 mile) under the right atmospheric conditions, there appears to be a 

very low probability that community annoyance will result from a single MOD-2 

turbine in 1 ion beyond a distance of 1 kmo Even under the 

propagat conditions (a home located where a combination terrain reflection and atmospheric refraction produces 15 dB of 

enhancement), it is unlikely that the turbine noise would be detectable within 

a home with closed windows a kilometer or more away. 

179 
TR-3036 

8.0 RECOMMENDATIONS 

We strongly recommend that the techniques developed for the acoustic characterization of the MOD-2 Unit No .. 2 at Goodnoe Hills be applied to the one 

remaining MOD-2 turbine currently operating in California (Unit No.5).. This 

will permit the following to be accomplished: 

(1) A verification of the atmospheric surface-layer bulk properties as 

accurate predictors of ensemble statistical distributions of low-frequency 

acoustic emlSSlons 

(2) Comparisons of the statistically derived acoustic response functions for 

the Goodnoe and Solano Hills sites 

(3) Detailed comparisons of the turbulent inflow properties for the two 

regimes. 

We further recommend that the techniques developed for the MOD-2 turbine be 

applied to a range of wind turbine designs and sizes in order to isolate 

critical scaling parameters to be used in predicting acoustic noise and in 

developing control strategies. 

180 
9 .. 0 REFERENCES 

1. Kelley, N.D., H.E. McKenna, ReR. Hemphill, CeL. Etter, R.L. 

N.C. Linn, Acoustic Noise Associated with the MOD-I Wind 

Source, Impact and Control, SERI/TR-635-1156, Golden, co: 

Research Institute (February 1985), 262 pp. 

TR-3036 

Garrelts, and 

Turbine: Its 

Solar Energy 

2. Hubbard, H.H., KeP. Shepherd, and FeW. Grosveld, Sound Measurements of 

the MOD-2 Wind Turbine Generator, NASA CR-165752, Hampton, VA: NASA 

Langley Research Center (July 1981), 29 pp. 

3. Stephens, D.G.; K.P. Shepherd, H.H. Hubbard, and FeW. Grosveld, Guide to 

the Evaluation of Human Exposure to Noise from Large Wind Turbines, NASA 

TM-83288, Hampton, VA: NASA Langley Research Center (March 1982), 70 pp. 

4. Shepherd, KeP .. , Detection of Low Frequency Impulsive Noise from Large 

Wind Turbines, NASA CR-172511, Hampton, VA: NASA Langley Research Center 

(January 1985), 23 pp. 

5. Hemphill, R.R., An Acoustic Ranging Technique with Application to 

Assessment of Low-Frequency Acoustic Noise of Wind Turbines, SERI/TP-215-

1954, Golden, co: Solar Energy Research Institute (May 1983), 10 pp. 

6. Kelley, N.D., H.E. McKenna, and E.W. Jacobs, Observed Acoustic and 

Aeroelastic Spectral Responses of a MOD-2 Turbine Blade to Turbulence 

Excitation, SERI/TP-214-2374, Golden, co: Solar Energy Research 

Institute (June 1984), 7 pp. 

7. Kelley, N.D. A Proposed Metric for Assessing the Potential of Community 

Annoyance from Wind Turbine Low-Frequency Noise, SERI/TP-217-3261, 

Golden, co: Solar Energy Research Institute (October 1987), 9 pp. 

8. Simms, D .. A., N.D. Kelley, HeE. McKenna, and EeW. Jacobs, A Comparison of 

Micrometeorological Measurements Made from a Tethered Balloon and a 

Meteorological Tower, SERI Technical Report, in preparation. 

9. Panofsky, H.A., and J.A. Dutton, _A_t_m_o~ _______ T_u_r_b_u_l_e_n __ c_e New York: John 

Wiley & Sons, Inc. (1984) 397 pp. 

10. Homicz, G.F., Broadband and Discrete Frequency Noise Radiation from 

Subsonic Rotors, Ph.D. Dissertation, Ithaca, NY: Cornell University -::-( 1:-:9---7-::'""3 --) .----

11 .. Homicz, G"F .. , and AeR. George, 

Radiation from Subsonic Rotors," 

(1974), pp. 151-177. 

"Broadband and Discrete Frequency 

J.. of Sound and Vibration, Vol.. 36 

12 .. George, A.R .. , and S .. T .. Chou, "Comparison of Broadband Noise Mechanisms, 

Analyses, and Experiments on Helicopters, Propellers, and Wind 

Turbines," presented at AIM 8th Aeroacoustics Conference, Paper No. 

AIAA-83-0690, Atlanta, GA, 11-13 April 1983. 

181 
TR-3036 

13. Monin, A.S .. , and A"Me Obukhov, "Basic Regularity in Turbulent Mixing in 

the Surface Layer of the Atmosphere," Trudy Geophys.. Inst. ANSSSR, 

No. 24, as referenced by J .. L. Lumley and H.A. Panofsky in The Structure 

of Atmospheric Turbulence, New York: John Wiley & Sons, Inc CD (1964), 

239 pp. 

140 Monin, A.S., and A.M. Yaglom, 

J.L. Lumley, ed", Cambridge, MA: 

Statistical Fluid Mechanics, Vol. 

The MIT Press (1971), 769 pp. 

I, 

15. Businger, JoA .. , JoC. Wyngaard, Y. Izumi, and E.F .. Bradley, "Flux-Profile 

Relationships in the Atmospheric Surface Layer," J. Atmospheric Science, 

Volo 28 (1971), pp" 181-189. 

16. Kaimal, J.C., "Turbulence Spectra, Length Scales and Structure Parameters 

in the Stable Surface Layer," Boundary-Layer Meteorology, Volo 4 (1973), 

ppe 289-309. 

17. George, A .. Re, and Y .. N" Kim, "High-Frequency Broadband Rotor Noise," AIM 

Journal, Vol. 15, No.4 (April 1977), pp .. 538-545. 

18" Hojstrup, J .. , "A Simple Model for the Adjustment of Velocity Spectra in 

Unstable Conditions Downstream of an Abrupt Change in Roughness and Heat 

Flux," Boundary-Layer Meteorology, Vol. 21 (1981), pp. 341-356 .. 

19. Kelley, NoD .. , H .. E" McKenna, and R .. R .. Hemphill, itA Methodology for 

Assessment of Wind Turbine Noise Generation," J. Solar Engineering, Vol. 

104 (May 1982), ppe 112-120. 

20. Wyngaard, J" C" , "On Surface-Layer Turbulence," in Workshop on 

Micrometeorology, DoA. Haugen, ed., Boston, MA: American Meteorological 

Society (1973), pp. 101-149" 

21. International Organization for Standardization (ISO), Draft Proposal for 

"Acoustics Methods for Describing Infrasound," ISO/DIS 7196, Geneva, 

Switzerland: ISO. 

22. Tokita, Yo, A. Oda, and K. Shimizu, "On the Frequency Weighting 

Characteristics for Evaluation of Infra and Low Frequency Noise," Proc .. 

1984 Confo on Noise Control Engineering, G.Co Maling, Jr., ~ 

Poughkeepsie, NY: Institute of Noise Control Engineering (1984), 

pp" 917-9200 

23. American National Standards Institute (ANSI), "American National Standard 

Specification for Sound Level Meters," ANSI Sl.4-1983, New York, NY: 

ANSI (1983)" 

24 Robin Towne, As soc., Envi ronmental Study of Low Frequency Noi se and 

Vibration, A Report to the Portland General Electric Co"' Portland, OR: 

Robin Towne Assoc (1974), 144 pp. 

25. Kelley, N.D., A Detailed Examination of the Goodnoe Hills Microscale 

Turbulence Structure and Its Impact on Aeroacoustic Noise Generation, 

SERI Technical Report, (a proposed report). 

182 
TR-3036 

26. Ffowcs Williams, JoE .. , and DaL .. Hawkings, "Theory Relating to the Noise 

of Rotating Machinery," J. Sound and Vibration, Vol.. 10 (1969), 

pp .. 10-21 .. 

183 
Document Control 11. SERI Report No. 

Page TR-217-3036 

1 2. NTIS Accession No. 

4. Title and Subtitle 

The MOD-2 Wind Turbine: Aerocoustical Noise Sources, 

Emi ss ions, and Potent i a 1 Impact 

7. Author(s) 

N.D. Kelley, H. f

v

1cKenna, E. Jacobs, R. Hemphill, 

9. Performing Organization Name and Address and N. 8i rkenheuer 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

12. Sponsoring Organization Name and Address 

15. Supplementary Notes 

16. A0stract (Limit: 200 words) 

3. Recipient's Accession No. 

5. Publication Date 

January 1988 

6. 

8. Performing Organization Rept. No. 

10. Project/Task/Work Unit No. 

t~E721201 

11. Contract (C) or Grant (G) No. 

(C) 

(G) 

13. Type of Report & Period Covered 

Technical Report 

14. 

This report summarizes research into characteristics of acoustic noise emissions 

of the DOE/NASA MOD-2 wind turbine. The results of this study showed that the MOD~2 noise levels are well below annoyance thresholds within residential 

structures a kilometer or more from the turbine rotor. It was also found that 

the inflow turbulent structure has a major influence on the level and characteristics of the low-frequency-range (2-150 Hz) acoustic emissions which, in turn, 

have implications for the associated structural response of the rotor assembly. 

The high-freq~ency-range (A-weighted) levels were found to vary primarily with 

the mean hub-height wind speed. The rotor inflow turbulence characteristics 

at the Goodnoe Hills site were found to be controlled almost entirely by the 

diurnal variation in the vertical stability of the first 100 m of the 

atmospheric boundary layer. 

17. Document Analysis 

a. Descriptors 

Wind power, wind turbines , acoustic measurements , turbulence 

b. Identifiers/Open-Ended Terms 

c. UC Categories 

60 

13. A.vailability Statement 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, Virginia 22161 

Form No. 0069 (3~25-82) 

19. No. of Pages 

211 

20. Price 

Ala