Subcontract Report

NREL/SR-500-43472

February 2009

The Potential for Reducing 

Blade-Tip Acoustic Emissions 

For Small Wind Turbines

June 1, 2007—July 31, 2008

P. Migliore

Engineering Research and Project Management 

Arvada, Colorado
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-500-43472

February 2009

The Potential for Reducing 

Blade-Tip Acoustic Emissions 

For Small Wind Turbines

June 1, 2007—July 31, 2008

P. Migliore

Engineering Research and Project Management 

Arvada, Colorado

NREL Technical Monitor: Patrick Moriarty

Prepared under Subcontract No. LAM-7-77545-01
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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iii

Foreword

The U.S. Department of Energy (DOE), working through its National Renewable Energy Laboratory 

(NREL), is engaged in a comprehensive research effort to improve the understanding of wind turbine 

aeroacoustics. The motivation for this effort is the desire to exploit the large expanse of low wind speed 

sites that tend to be close to U.S. load centers. Quiet wind turbines are an inducement to widespread deployment, so the goal of NREL’s aeroacoustic research is to develop tools that the U.S. wind industry can 

use in developing and deploying highly efficient, quiet wind turbines at low wind speed sites. NREL’s 

National Wind Technology Center (NWTC) is implementing a multifaceted approach that includes wind 

tunnel tests, field tests, and theoretical analyses in direct support of low wind speed turbine development 

by its industry partners. NWTC researchers are working hand in hand with engineers in industry to ensure 

that research findings are available to support ongoing design decisions.

To that end, wind tunnel aerodynamic tests and aeroacoustic tests have been performed on six airfoils that 

are candidates for use on small wind turbines. Results are documented in two companion NREL reports:

Wind Tunnel Aeroacoustic Tests of Six Airfoils for Use on Small Wind Turbines, 

Stefan Oerlemans, Principal Investigator, the Netherlands National Aerospace Laboratory 

Wind Tunnel Aerodynamic Tests of Six Airfoils for Use on Small Wind Turbines,

Michael Selig, Principal Investigator, University of Illinois at Urbana-Champaign (UIUC)

A similar effort was undertaken for three airfoils that are candidates for use on large wind turbines. Results are reported in the following NREL report and in various conference papers.

Aeroacoustic Testing of Wind Turbine Airfoils, William Devenport and Ricardo Burdisso, Principal Investigators, Virginia Polytechnic Institute and State University

These reports provide valuable airfoil databases for designers who wish to consider the airfoils tested*

Possessing both theoretically predicted aerodynamic characteristics and wind tunnel test data for the same 

six airfoils provides an extraordinary opportunity to compare the performance, measured by energy cap-

. 

But inevitably, designers will want to evaluate other airfoils that have not been tested. And not only are 

wind tunnel tests expensive, it is often difficult to schedule the facilities required within the over-all time 

frame of a project development plan. This dilemma begs the question “Is it really necessary to conduct 

wind tunnel tests, or can we rely on theoretical predictions?” 

Predicting the aeroacoustic emission spectra of a particular airfoil shape is extremely difficult, but predicting the aerodynamic characteristics of a particular airfoil shape is routine practice. Nevertheless, there 

is always some uncertainty about the accuracy of the predictions in comparison to the results of wind tunnel tests or field performance, and there are questions about the efficacy of two principal airfoil analysis 

methods: the Eppler and XFOIL codes. To address these related issues, at least in part, a theoretical analysis was commissioned of the same airfoils tested in the wind tunnel. The results are documented in the 

following NREL report:

Theoretical Aerodynamic Analyses of Six Airfoils for Use on Small Wind Turbines Using Eppler 

and XFOIL Codes, D.M. Somers and M.D. Maughmer, Principal Investigators, Airfoils, Inc.

 * The extensive test data discussed in these reports can be provided in electronic format on compact disks (CDs) that 

may be obtained by calling the NWTC library at 303-384-6963.
iv

ture, of wind turbine rotors designed with the different data. This will provide the insight needed to assist 

designers in deciding whether to pursue wind tunnel tests. Although some differences in the resulting 

blade planforms (chord and twist distributions) can be expected, a more important question relates to the 

difference in energy capture and its significance in driving the choices that need to be made during the 

preliminary design stage. These issues are addressed in a report that compares the differences in Eppler 

and XFOIL predictions to the UIUC wind tunnel tests and examines the planform and energy capture differences in resulting blade designs:

Comparison of Optimized Aerodynamic Performance of Small Wind Turbine Rotors Designed 

with Theoretically Predicted versus Experimentally Measured Airfoil Characteristics, Michael 

Selig, Principal Investigator, University of Illinois at Urbana-Champaign (UIUC)

Another research effort undertaken in support of the U.S. wind turbine industry involves a series of aeroacoustic field tests conducted at the NWTC. Using well documented, consistently applied test procedures, 

noise spectra were measured for eight small wind turbine configurations. Test results provide valuable 

information to manufacturers as well as potential users of these turbines. To our knowledge, this is the 

first comprehensive database of noise data for small wind turbines. The results of this effort are documented in another NREL report:

Aeroacoustic Field Tests of Eight Small Wind Turbines, 

J. van Dam and A. Huskey, Principal Investigators, NREL’s National Wind Technology Center

Wind tunnel tests, field tests and theoretical analyses provided useful information for development and 

validation of a semi-empirical noise prediction code developed at NREL. This effort is described in the 

following reports:

Semi-Empirical Aeroacoustic Noise Prediction Code for Wind Turbines, 

P. Moriarty and P. Migliore, Principal Investigators, NREL’s National Wind Technology Center

Prediction of Turbulent Inflow and Trailing-Edge Noise for Wind Turbines,

P. Moriarty, G. Guidati and P. Migliore, Principal Investigators, NREL subcontracted research

The codes will be continuously improved, but may ultimately give way to more sophisticated, physicsbased computational aeroacoustic codes also being developed by NREL and its subcontractors. For example, researchers at Florida State University (FSU) and the National Aeronautics and Space Administration (NASA) applied modern computational methods to analyze wind turbine blade tip noise. This work 

was reported in the journal article:

Large-Eddy Simulation of Wing Tip Vortex on Overset Grids, Ali Uzun and Yousuff Hussaini of 

FSU and Craig Streett of the NASA Langley Research Center, Principal Investigators

In addition, a comprehensive research effort at the Pennsylvania State University was reported in a series 

of conference papers and other writings, including:

An Aeroacoustic Analysis of Wind Turbines,

Philip Morris, Lyle Long and Ken Brentner, Principal Investigators

A 3D Parabolic Equation Method for Wind Turbine Noise Propagation in Moving Inhomogeneous Atmosphere, R. Cheng, Philip Morris and Ken Brentner, Principal Investigators
v

Rotational Effects on the Aerodynamics and Aeroacoustics of Wind Turbine Airfoils, 

Steven Miller and Philip Morris, Principal Investigators

3-D Time-Accurate Inviscid and Viscous CFD Simulations of Wind Turbine Rotor Flow Fields, 

Nilay Sezer-Uzol, Ankur Gupta and Lyle Long, Principal Investigators

Many of the documents described above are published as NREL reports. Some results are presented in 

various journal articles or conference papers. All of the NREL reports will be available on NREL’s web 

site at http://www.nrel.gov/publications/. Collectively, these reports represent a significant compendium 

of information on the aerodynamics and aeroacoustics of contemporary wind turbines.

Clearly, this work represents a significant commitment of DOE resources as well as a significant commitment of personnel over an extended period. We are sure we express the sentiments of all the research 

participants in saying we sincerely hope the results of these efforts prove beneficial to the wind energy 

community.

Paul G. Migliore

NREL/NWTC Project Manager, Retired

Patrick Moriarty

NREL/NWTC Project Manager
vi

Acknowledgements

The project reported herein began when the author was employed at NREL, with the Department of Energy providing the initial funding. The author developed the scope of work and designed the blade tip

shapes to be tested. Southwest Windpower employees Ben Polito, Jean Lonjaret and David Calley donated their time to computerize these shapes and construct stereo lithographic models of them. The tooling and wind tunnel models were then fabricated by Novakinetics, LLC, which also conducted and documented structural tests of the models. Although DOE budget constraints caused the project to be canceled, Southwest Windpower and the author decided to jointly and privately fund the wind tunnel tests at 

Georgia Tech Research Institute (GTRI). Dr. Krishan Ahuja enthusiastically guided the planning and Mr. 

Adam Churney diligently conducted the tests. Ultimately, DOE was able to resume the project and provide funding for reporting the results and making them available to the U.S. small wind turbine industry. 

Dr. Patrick Moriarty of NREL is the project manager for this subcontract.

The author commends Southwest Windpower for its commitment to this research and is grateful to NREL 

and DOE for supporting its completion.
vii

Executive Summary

Wind tunnel aeroacoustic tests of a typical small wind turbine blade were conducted in the open-jet test 

section of the Georgia Tech Research Institute (GTRI) Anechoic Flight Simulation Facility. The objectives of the tests were to determine the relative importance of tip shape, boundary layer tripping and trailing edge thickness on acoustic emissions. Six tip shapes, 3 boundary layer trip heights and 2 trailing edge 

thicknesses were investigated in a matrix of 72 velocity / angle of attack test points. No attempt was 

made to evaluate relative aerodynamic performance in comparison to aeroacoustic performance: nor was 

any attempt made to reconcile the potential impact of real-world atmospheric conditions compared to the 

idealized wind tunnel environment. Significant noise – including pure tones – was observed for certain 

test conditions. This was attributed to laminar boundary layer vortex shedding at low Reynolds numbers

(Re). Aggressive boundary layer tripping eliminated the tones and reduced broadband noise. The various 

tip shapes performed similarly, but at a low velocity (Reynolds number = 170,000) a difference in overall 

sound pressure level of 3.8 dBA was observed between the best and worst tip shapes. At a greater velocity (Reynolds number = 315,000), more typical of wind turbine operating conditions, a difference of 1.3 

dBA was observed. These differences are sufficient to justify the conclusion that tip shape can be important in further quieting an otherwise quiet wind turbine blade. Generally, only subtle differences were 

observed for the 2 trailing edge thicknesses of 1.1% and 0.8% chord that were tested. In 2 cases, however, increments of 1.2 dBA and 1.8 dBA were measured. Therefore, the author concludes that minimizing 

trailing edge thickness – subject to manufacturing limitations – is a worthwhile endeavor. 
viii
ix

Table of Contents

1.0 Introduction ................................................................................................... 1

1.1 Overview ............................................................................................. 1

1.2 Sources of Airfoil Noise ............................................................................ 1

1.3 Designing Quiet Wind Turbine Blades ........................................................... 2

2.0 Description of Tests .......................................................................................... 3

2.1 The GTRI Wind Tunnel ............................................................................ 3

2.2 Instrumentation ....................................................................................... 4

2.3 Measuring Angle of Attack ........................................................................ 5

2.4 Wind Tunnel Models ................................................................................ 5

2.5 Test Matrix ............................................................................................7

3.0 Test Results ................................................................................................... 8

3.1 Empty Tunnel Tests – Background Noise ........................................................ 8

3.2 Effect of Boundary Layer Tripping .............................................................. 10

3.3 Effect of Blade Tip Shape ........................................................................ 12

3.4 Effect of Trailing Edge Thickness ...............................................................16

4.0 Analysis of Test Results ................................................................................... 22

4.1 Effect of Boundary Layer Tripping ............................................................. 22

4.2 Effect of Blade Tip Shape ........................................................................ 25

4.3 Effect of Trailing Edge Thickness ...............................................................28

5.0 Summary and Conclusions ............................................................................... 30

6.0 References ................................................................................................... 31
x
1

1.0 Introduction

1.1 Overview

For the last several years the U.S. Department of Energy, working through its National Renewable Energy 

Laboratory (NREL), has engaged in a comprehensive research effort to improve the understanding of 

wind-turbine aeroacoustics. Motivation for this effort is the desire to exploit the large expanse of lowwind-speed sites that tend to be closer to load centers in the United States. Quiet wind turbines are an inducement to widespread deployment, and so the goal of NREL’s aeroacoustic research is to develop tools 

for use by U.S. industry in developing and deploying highly efficient, quiet wind turbines at these lowwind-speed sites.

NREL’s National Wind Technology Center (NWTC) has implemented a multifaceted approach that includes aerodynamic [1, 2] and aeroacoustic [3-5] wind-tunnel tests, field tests [6] and theoretical analyses

[7-15] in direct support of low-wind-speed turbine development by its industry partners. The present report describes wind-tunnel aeroacoustic tests of six tip shapes that are candidates for use on small wind 

turbine blades. The test article was a prototype blade for Southwest Windpower’s Skystream 3.7 turbine, 

which is a 1.8 kW, variable-speed, grid-connected machine. This blade incorporates NREL’s S822 airfoil

[16] for which there is extensive wind tunnel and field test information. 

The tests were conducted in the open-jet test section of the Georgia Tech Research Institute (GTRI) Anechoic Flight Simulation Facility. The acoustic measurements were done for a range of tunnel speeds and 

angles of attack with and without boundary layer tripping to assess the effect of that factor on sound production. A brief investigation of trailing edge thickness also was conducted to evaluate the merit of special efforts to manufacture blades with sharp trailing edges. The objective of the tests was to determine 

the potential impact of these conditions rather than to identify particular optima.

1.2 Sources of Airfoil Noise

There are six different sources [17] that independently generate airfoil acoustic emissions: Inflow turbulence, turbulent boundary layer trailing edge interaction, separating flow, laminar boundary layer vortex 

shedding, trailing edge bluntness (von Karman) vortex shedding and tip vortex formation. These sources 

are superimposed to form the total noise spectrum of a wind turbine blade. The spectra often are summed 

to calculate an overall sound power†

The other sources of noise are collectively called airfoil self noise, because they are caused by the airfoil 

interacting with its own boundary layer and near wake. If the trailing edge thickness of the airfoil is very 

thin relative to the boundary layer thickness, as was the case for the models tested, there is very little traillevel.

Inflow turbulence noise caused by the interaction of the leading edge of an airfoil with a turbulent inflow 

often is called leading edge noise. Researchers currently think that sharp leading edge geometries are 

more susceptible to inflow turbulence noise.

 † Whereas sound pressure level is described in Reference 17 as a property of the observer location, the total strength of a source 

of sound is characterized by the sound power emitted by the source. In general, the sound power P transmitted through a surface 

S is the integral of the sound intensity I (energy transmitted per unit time and unit area) over the surface. If the surface S encloses the source of the sound, then P is the total sound power emitted by the source. The definition of sound power level is:

Lw = 10 · log [ P ÷ Pref ] expressed in decibels, dB,

where Pref =10-12 watts is the standard reference sound power. The human eardrum can detect incoming sound power as weak as 

one picowatt, and exposure to incoming sound power of more than one watt will result in some hearing loss.
2

ing edge bluntness noise. Two-dimensional airfoil models tested between endplates do not have a tip vortex or any associated noise, although interaction between the endplate boundary layer and model-endplate 

juncture could cause extraneous noise.

Turbulent boundary layer trailing edge noise generally is considered to be the most important source of 

airfoil self noise for modern wind turbine blades. In this phenomenon, the unsteady pressure waves in the 

turbulent boundary layer are amplified and radiated by the sharp trailing edge. As the angle of attack increases, the thickness of the turbulent boundary layer increases and large-scale unsteady structures can 

dominate noise production from the trailing edge. For fully separated flow, noise can radiate from the 

entire chord.

Laminar boundary layer vortex shedding noise is created by a feedback loop between vortices being shed 

at the trailing edge, and instability waves in the laminar boundary layer upstream. This source of noise 

can occur on either the suction or pressure side of the airfoil, and it can be particularly irritating because it 

often is manifested in pure tones that result from feedback amplification. It is not likely to be important 

for large turbines operating at a high Reynolds number, but there is ample evidence that it is significant 

for small wind turbines.

1.3 Designing Quiet Wind Turbine Blades

A strong desire for quiet wind turbines exists. This requires that mechanical and electrical noise sources 

associated with such elements as gearboxes, bearings, alternators, and power electronics must be eliminated. Engineers have been dealing with these issues for decades, however, and standard practices usually succeed in quieting these noise sources. Aeroacoustic emissions, however, which are not so well understood or controlled, often dominate wind turbine noise. Therefore, in recent years, considerable effort 

has been devoted to the elimination of wind turbine aeroacoustic noise, and tools are now available to 

design truly quiet wind turbines. The issue is whether designers and turbine manufacturers really are

committed to this result. Designing for noise elimination requires a thorough process including analyses, 

wind tunnel tests, and field verification. Most design teams do not have the patience or resources necessary for rigorously completing the process.

A suitable airfoil family first must be chosen and – rather than focusing exclusively on aerodynamic efficiency – aeroacoustic performance must be heavily weighted. This does not necessarily require sacrificing aerodynamic performance: literature reveals the availability of quiet airfoils with desirable aerodynamic characteristics, such as high lift-to-drag ratio and benign stall behavior. Airfoil choice should be 

informed by wind tunnel tests which – in the overall context of wind turbine development – are not as 

costly as some fear. After the airfoil family is chosen there are ample analytical tools available to optimize the blade planform. However, there are judicious choices to be made in this endeavor as well. Outboard on the blade, where most of the sound originates, the design angle of attack should not be so aggressive as to exacerbate trailing edge noise from large-scale unsteady structures emanating (near stall)

from the thickening boundary layer. Additionally, the spanwise diminution of lift should be gradual and 

smooth, to minimize noise associated with tip vortex shedding.

Assuming prudent airfoil choice and planform design, there still remain the issues of blade tip shape and 

trailing edge thickness. For small wind turbines operating at low Reynolds numbers (as they typically do) 

there also is the potential susceptibility to laminar boundary layer vortex shedding noise. Just how important are these phenomena and are they worth addressing during the design process? Those are the questions that the tests were designed to answer.
3

2.0 Description of Tests

2.1 The GTRI Wind Tunnel

The GTRI Anechoic Flight Simulation Facility illustrated in Figure 1 is an open-jet wind 

tunnel enclosed in an anechoic chamber lined 

with polyurethane foam wedges. The chamber, shown in Figure 2, is 4.3 m (14 ft) long, 

4.3 m (14 ft) wide and 6.1 m (20 ft) high between wedge tips. It is mounted on massive 

springs and completely isolated from the rest 

of the acoustics laboratory. A springtensioned cable floor, which is suspended 

from the walls, provides easy access to the 

interior of the chamber for instrumentation 

and hardware changes and for calibration. 

The enclosed test section is the potential-flow 

core of a 0.7 m (2.3 ft) diameter open jet nozzle capable of developing continuous free-jet 

velocities up to 95 m/s (312 ft/sec). Its intake 

is approximately 2.0 m (6.7 ft) by 2.8 m (9.3 

ft) and the length of the working section between the free-jet nozzle exit and the collector 

is 2.74 m (9.0 ft). Airflow is generated by a 

diesel-driven fan. Air is drawn into the intake 

and through the honeycomb, screens and contraction fairing; across the anechoic room to 

the collector; and then through a diffuser, two 

right angle corners (with acoustically treated 

turning vanes), duct silencers and the transition section to the powered exhaust section. 

The air supply duct in the center of the open 

jet normally is used to study jet plumes. This 

duct was removed for the present tests and the 

wind turbine blade tip models were mounted 

at the nozzle exit.

The facility has been operated by GTRI in 

support of NASA (National Aeronautics and 

Space Administration) and aviation industry 

research projects on a nearly continuous basis 

for the last decade and previously was used 

by Lockheed. Recently there have been several major upgrades, including the replacement of acoustic wedges within the five years

preceding these tests. Additional details of the 

facility are described by Ahuja et al [18].

Figure 1. Cross-section of the GTRI Anechoic 

Flight Simulation Facility

 

 

Figure 2. Anechoic chamber, exit nozzle and jet 

catcher; the small nozzle at the center of the open 

jet was removed and the test airfoil was mounted 

from a support hidden behind foam as shown in 

Figure 3.

Open Jet Nozzle
4

2.2 Instrumentation

Acoustic measurements were made with 6.4 mm (1⁄4-in) diameter B&K (Bruel & Kjaer) Type 4939 microphones. They can be placed anywhere in the anechoic chamber, but typically should be no closer than 

0.4 m (1.2 ft) from wedge tips so as to avoid any wedge near-field effects. In the present tests, the microphones were located 3 m (10 ft) away from the wind turbine blade tip, below the pressure side of the airfoil, and were set at 90° and 30° angles as shown in Figure 3. B&K 2669 pre-amplifiers were used in 

combination with a Nexus conditioner amplifier connected to an Agilent E1421B mainframe computer 

(that subsequently operated a Signal Calc 620 frequency analyzer). All acoustic data were analyzed as 

narrow band frequency spectra with 25 Hz increments in the range of 0 kHz to 20 kHz

10 ft

Collector Flow 28.5” Nozzle

10 ft

?

30° Mic

90° Mic

Figure 3. Microphone Locations

30°
5

2.3 Measuring Angle of Attack 

An electronic level accurate to 0.1 degree was 

used to measure angle of attack (α), as shown 

in Figure 4a. The zero angle was established 

at the inboard section of the blade where the 

removable tips were attached – approximately 

the 75% span location. A wooden clamping 

block was fabricated with its split line and upper surface machined parallel to the chord line 

of the model. The reference chord line A-B 

was established from the leading edge to the 

trailing edge of the exposed airfoil section. 

The electronic level then was placed on the 

top surface of the clamping block, which was 

used to mount the blade to the test facility. 

The angle of attack was set and measured for 

each test condition. This approach was sufficiently accurate because its objective was to 

make relative comparisons between blade tips.

2.4 Wind Tunnel Models

Tests were conducted on a realistic wind turbine blade provided by Southwest Windpower (SWWP) of 

Flagstaff, Arizona. The blade was taken from a prototype 1.8 kW fixed-pitch, variable-speed turbine 

called “Storm” which was developed under a DOE subcontract with technical assistance from the NWTC. 

The Storm, which had a 3.7 m (12.1 ft) rotor span, has been commercialized as the “Skystream” wind 

turbine. The outboard 330 mm (1.08 ft) of the 

blade – approximately 18 % of rotor span –

was placed in the test section as shown in Figure 4b. Six removable blade tips were tested, 

each 232 mm (0.76 ft) long with a 92 mm

(0.30 ft) root chord and comprising approximately 12.5 % of the rotor span. This was a 

deliberate choice, as it generally is accepted 

that the outboard 20% to 25% of the rotor is 

the primary source of aeroacoustic emissions.

The Storm and the Skystream employ NREL’s

S822 airfoil family, which was developed by 

Tangler and Somers [16] specifically for small 

wind turbines. The airfoil offered a good balance among several measures of merit, including aerodynamic performance, aeroacoustic 

performance and structural efficiency. The 

aeroacoustic performance of the S822 is well 

documented in reports by Oerlemans [3] and 

Migliore [4]. The aerodynamic performance 

is documented by Selig and McLanahan [1, 2].

This extensive documentation was another Figure 4b. Blade section immersed in open jet flow.

Figure 4a. Angle of attack was measured with an 

electronic level.

A

B
6

reason for choosing the S822 for the present study. The six removable and interchangeable blade tips are 

shown in Figure 5. The shapes were developed by the author, Paul Migliore, who also provided final lofting for the outboard 25% of the Storm blade in an effort to create a low-noise planform. A mathematical 

description of each blade tip was provided to SWWP engineers, who created electronic files suitable for 

computer-controlled machining. Three dimensional master shapes then were fabricated on a stereo lithographic printer. The actual fiberglass parts were manufactured and fitted to the blade root stub by Novakinetics, LLC, of Flagstaff, Arizona. The parts were fastened to the root section with countersunk machine screws – three on the suction surface and three on the pressure surface. To mitigate potential aerodynamic and aeroacoustic effects, the screws and the chordwise joint were covered with tape before testing. Blenderm® surgical tape, having a thickness of 0.10 mm (0.004 inches) (manufactured by 3M Company) was used. For safety reasons the structural integrity of the blade tips was verified by conducting 

load tests to safety factors of 1.5 and 2.0 on the anticipated normal and axial forces, respectively.

PM-1, PM-2 and PM-3 are similar to shapes that have been studied [17] in Europe. PM-4 is a radical 

shape that hypothesizes the formation of vortices to energize the flow and prevent separation-induced 

noise in the region of the blade tip. PM-5 is a simple winglet that is hypothesized to displace the blade tip 

vortex away from the trailing edge, thereby reducing the intensity of the scattered acoustic waves. For all 

of the tips, the S822 shape was maintained regardless of the local chord length.

It is important to note that these particular shapes are not presumed to be the optimum of their types, nor 

is any one of them presumed to be superior among the infinity of possibilities. They simply represent a 

broad range of options demonstrating the potential sound reduction that could be obtained by optimizing 

blade tip shape for an otherwise quiet blade. This presumes, of course, an aeroacoustically efficient design with respect to blade planform, airfoil shape, and trailing edge geometry. Furthermore, no matter 

what sound reduction might result from application of a particular tip shape, the aerodynamic impact certainly must be considered.

Figure 5. Wind turbine blade segment used for the test model (top photo) and the removable 

blade tips (bottom photo) – from left to right –Storm baseline blade tip and removable tips PM1 (leading edge sweep), PM-2 (trailing edge sweep), PM-3 (ogee shape), PM-4 (scalloped trailing 

edge) and PM-5 (winglet)
7

2.5 Test Matrix

Table 1 shows the tunnel speeds and angles of attack at which the models were tested. The tunnel speeds 

of 25 m/s (Re = 157,000), 50 m/s (Re = 315,000), and 75 m/s (Re = 472,000) were chosen to bracket the 

rotor speeds of an operating Storm turbine. The effective angle of attack of -1 degree is close to the angle 

of zero lift for the S822 airfoil and is typical of conditions where laminar boundary layer vortex shedding 

noise (pure tones) is typically exhibited [3, 4]. The maximum lift-to-drag ratio for the S822 airfoil occurs 

at the effective angle of 5 degrees, which would be the desired operating point for a variable speed turbine. The effective angle of 9 degrees is approximately where the S822 lift curve deviates from linear, 

thus representing insipient stall and, prospectively, separation noise. The angles of attack at which the 

models were set in the tunnel were different than the effective angles shown in Table 1. Corrections were 

made for the flow deflection associated with open jet wind tunnels. A heuristic approach, using the method of Brooks, Pope and Marcolini [19], resulted in the corrections shown in Table 2.

Number Description

E1* Empty Test Section 25 50 75 25 50 75 25 50 75

B1 Baseline Tip 25 50 75 25 50 75 25 50 75

1 Tip PM-1 25 50 75 25 50 75 25 50 75

2 Tip PM-2 25 50 75 25 50 75 25 50 75

3 Tip PM-3 25 50 75 25 50 75 25 50 75

4 Tip PM-4 25 50 75 25 50 75 25 50 75

5 Tip PM-5 25 50 75 25 50 75 25 50 75

B2 Basiline Tip Sharpened 25 50 75 25 50 75 25 50 75

E2* Empty Test Section 25 50 75 25 50 75 25 50 75

*Angle of attack variation does not apply to tests with an empty test section.

Effective Angle of Attack, a (degrees)

aeffective = 5

Tunnel Speed (m/s)

aeffective = 9

Tunnel Speed (m/s) Tunnel Speed (m/s)

Configuration aeffective = -1

Table 1. Summary of Configurations and Test Conditions

alpha 

setting

alpha 

effective

alpha 

correction

-3.00 -2.47 -0.53

-2.00 -1.64 -0.36

-1.00 -0.82 -0.18

0.00 0.00 0.00

1.00 0.82 0.18

2.00 1.64 0.36

3.00 2.47 0.53

4.00 3.29 0.71

5.00 4.11 0.89

Settings for GTRI Tests 6.00 4.93 1.07

7.00 5.75 1.25

8.00 6.57 1.43

9.00 7.40 1.60

10.00 8.22 1.78

11.00 9.04 1.96

12.00 9.86 2.14

13.00 10.68 2.32

14.00 11.50 2.50

Model Chord (inches) 3.62 15.00 12.33 2.67

Tunnel open-jet dimension normal to chord line (inches) 28.00 16.00 13.15 2.85

Tunnel Speed (fps) 246.06 17.00 13.97 3.03

Sigma constant (Brooks, Pope and Marcolini) 0.003 18.00 14.79 3.21

Zeta constant (Brooks, Pope and Marcolini) 1.217 19.00 15.61 3.39

Reynolds Number x 10-6 (calculated) 0.472 20.00 16.44 3.56

Table 2. Effective Angle of Attack Corrections for GTRI Open Jet Tunnel
8

3.0 Test Results

3.1 Background Noise – Empty Tunnel Tests

Before initiating tests of the wind turbine blade tips, it was necessary to determine the background noise 

level of the wind tunnel. Only by doing this is it possible to separate the effects of the models from

extraneous noise sources in the wind tunnel and surroundings. The background noise is given by the 

spectra shown in Figures 6 through Figure 9, which are the original GTRI graphs (including original 

numbering, located in the lower right corner of each figure). To distinguish the individual plots, however, 

the graphs must be viewed in color.

Low Speed Empty Tunnel

27 m/s, 90°, Δf = 25 Hz 

0

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

14

15

16

17

18

14 = Both Air units on

15 = CTRL Air off 

16 = Both Air units off

17 = Outer honeycomb in place

18 = Both honeycomb in place

Figure 6

3/2/06

Medium Speed Empty Tunnel

50 m/s, 90°, Δf = 25 Hz 

0

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

19

20

21

19 = Both Air units off

20 = Outer honeycomb in place

21 = Both honeycomb in place

Figure 7

3/2/06
9

High Speed Empty Tunnel

75 m/s, 90°, Δf = 25 Hz 

0

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

22

23

24

22 = Both Air units off

23 = Outer honeycomb in place

24 = Both honeycomb in place

Figure 8

3/2/06

Old New Speed Empty Tunnel

90° , Δf = 25 Hz 

0

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

Old Data 53 m/s

New Test 50 m/s

No flow

Figure 9

3/2/06

Note: To check the acoustic performance of the wind tunnel, background noise 

was compared to previous data from the spring of 2006, albeit at a slightly different tunnel speed. As Figure 9 illustrates, no anomalies were found.
10

3.2 Effect of Boundary Layer Tripping

Previous experience [3, 4] indicates that virtually all airfoils at low Reynolds number exhibit pure tones at 

some particular angles of attack. For other angles of attack, or at higher Reynolds numbers, the tones are 

likely to disappear. This phenomenon was attributed to laminar boundary layer vortex shedding. It therefore was expected that the wind turbine test blade would exhibit this same behavior unless turbulent flow 

was induced through effective boundary layer tripping. It is very difficult to predict the trip thickness 

required to accomplish this, so it was done empirically. We used a straight polyurethane protective tape 

(3M product number 8762) having a thickness of 0.2 mm (0.008 in) applied with its trailing edge at 5% of 

the chord on both the upper and lower blade surfaces. Several layers of tape were applied until the desired effect was observed through the acoustic measurements.

To avoid extraneous acoustic sources, the joint between the root section and the blade also was also taped 

during the tests. Blenderm® surgical tape (3M Company) having a thickness of 0.10 mm (0.004 inches)

was used. The procedure for applying the tape was to begin at the 90% chord location on the pressure 

(concave) side of the airfoil; wrap toward and then around the trailing edge; wrap toward the leading 

edge; wrap toward the trailing edge; and terminate at the 95% chord location thereby overlapping the 

starting point. The tape was carefully adhered so as to leave no creases or bubbles and making sure to 

completely cover the joint and all of the screw heads.

Figure 10 through Figure 12 illustrate the impact of leading edge tape thickness and the effect of tripping 

the boundary layer to induce turbulent flow. At 27 m/s (Re = 170,000) and 11o angle of attack the flow is

predominantly laminar, perhaps accompanied by some flow separation. Laminar boundary layer vortex 

shedding and / or flow separation produce strong tones in the frequency range of 1.5 kHz to 2.5 kHz. 

Three layers of tape (0.61 mm = 0.024 inches) are required to suppress these tones. At lower frequencies 

where tones are not present, heavy tripping reduces sound pressure level by 2 dB to 3 dB. At 50 m/s (Re 

= 315,000) the strong tones remain, but shift to a frequency range of 3.5 kHz 6.0 kHz. At 63 m/s (Re = 

397,000) the tones are eliminated, and broadband noise is reduced over the entire frequency range by application of 3 layers of tape. This leading edge treatment was used for subsequent tests.

Wind turbine Leading Edge Tape on B1

27 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

3 Layers of Tape - 143

2 Layers of Tape - 146

1 Layer of Tape - 150

3/20/06

Figure 10
11

Wind turbine Leading Edge Tape on B1

50 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

3 Layers of Tape - 144

2 Layers of Tape - 147

1 Layer of Tape - 151

3/20/06

Figure 11

Wind turbine Leading Edge Tape on B1

63 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2u-5 Pa)

3 Layers of Tape - 145

2 Layers of Tape - 148

1 Layer of Tape - 152

3/20/06

Figure 12
12

3.3 Effect of Blade Tip Shape

Acoustic spectra for the six blade tip shapes are given in the Figures 13-21. The important frequency 

range of 1 kHz to 5 kHz is more carefully examined in Section 4, Analysis of Test Results. Caution is 

advised in drawing conclusions without simultaneous consideration of turbine operating characteristics.

Wind turbine tips

27 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

 Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 213

pm1 - 174

pm2 - 177

pm3 - 192

pm4 - 195

pm5 - 210

Empty - 222

3/22/06

Figure 13

Wind turbine tips

50 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

b1 - 214

pm1 - 175

pm2 - 178

pm3 - 193

pm4 - 196

pm5 - 211

Empty - 223

3/22/06

Figure 14
13

Wind turbine tips

63 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 215

pm1 - 176

pm2 - 179

pm3 - 194

pm4 - 197

pm5 - 212

Empty - 224

Figure 15

Wind turbine tips

27 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 216

pm1 - 171

pm2 - 180

pm3 - 189

pm4 - 198

pm5 - 207

Empty - 222

3/22/06

Figure 16
14

Wind turbine tips

50 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 217

pm1 - 172

pm2 - 181

pm3 - 190

pm4 - 199

pm5 - 208

Empty - 223

3/22/06

Figure 17

Wind turbine tips

63 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 218

pm1 - 173

pm2 - 182

pm3 - 191

pm4 - 200

pm5 - 209

Empty - 224

3/22/06

Figure 18
15

Wind turbine tips

50 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 220

pm1 - 169

pm2 - 184

pm3 - 187

pm4 - 202

pm5 - 205

Empty - 223

3/22/06

Figure 20

Wind turbine tips

27 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 219

pm1 - 168

pm2 - 183

pm3 - 186

pm4 - 201

pm5 - 204

Empty - 222

3/22/06

Figure 19
16

3.4 Effect of Trailing Edge Thickness

It is well known [20] that blunt trailing edges can be sources of noise related to vortex shedding. Acoustic modeling of this phenomenon typically relates sound pressure level to the geometry of the body, including trailing edge thickness and boundary layer characteristics, such as the displacement thickness. 

Regardless of the desire for thin trailing edges, constraints exist due to manufacturing limitations and 

damage considerations. Various “rules of thumb” often are used to establish targets for trailing edge 

thickness. Analytical and empirical studies both suggest that a trailing edge thickness of approximately 

0.25% of chord length is not likely to result in additional noise. A thickness of approximately 0.50% of 

chord, which is difficult to achieve in fabrication, is thought to produce some additional – but acceptable 

– source of noise. Consequently, there was some interest in learning whether there is anything to be 

gained from a trailing edge thickness sharper than the baseline Storm blade.

Measured at a position that was 0.64 mm (0.025 in) forward of the trailing edge, the wind tunnel model 

and baseline blade tip B1 had a uniform trailing edge thickness of 1 mm (0.04 in) over the span of 0.33 m 

(13 in) immersed in the flow. This trailing edge thickness is approximately 1.1% of the model chord. 

Tests of this section were repeated after sharpening the trailing edge to a uniform thickness of 0.76 mm 

(0.03 in) or approximately 0.8% chord. Results are presented as sound pressure level spectra in Figure 22 

through Figure 30. Details of the blade trailing edge sharpening are provided in Figure 31.

Wind turbine tips

63 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

Leading edge tape = 0.024" 

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

b1 - 221

pm1 - 170

pm2 - 185

pm3 - 188

pm4 - 203

pm5 - 206

Empty - 224

3/22/06

Figure 21
17

Wind turbine tips

27 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 213

Sharpened - 231

Empty - 222

3/23/06

Figure 28

Wind turbine tips

50 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 214

Sharpened - 232

Empty - 223

3/23/06

Figure 29
18

Wind turbine tips

27 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

Unsharpened - 216

Sharpened - 228

Empty - 222

3/23/06

Figure 25

Wind turbine tips

63 m/s, α = -1°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 215

Sharpened - 233

Empty - 224

3/23/06

Figure 30
19

Wind turbine tips

50 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

Unsharpened - 217

Sharpened - 229

Empty - 223

3/23/06

Figure 26

Wind turbine tips

63 m/s, α = 6°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10 -5 Pa)

Unsharpened - 218

Sharpened - 230

Empty - 224

3/23/06

Figure 27
20

Wind turbine tips

27 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 219

Sharpened - 225

Empty - 222

3/23/06

Figure 22

Wind turbine tips

50 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 220

Sharpened - 226

Empty - 223

3/23/06

Figure 23
21

Distance 

From Tip 

(inches)

Orignal 

Thickness 

(inches)

Sharpened 

Thickness 

(inches)

1.00 0.040 0.030

2.00 0.040 0.030

3.00 0.040 0.030

4.00 0.040 0.030

5.00 0.040 0.030

6.00 0.040 0.030

7.00 0.040 0.030

8.00 0.040 0.030

9.00 0.040 0.030

10.00 0.040 0.030

11.00 0.040 0.030

12.00 0.040 0.030

13.00 0.040 0.030

14.00 0.040 0.030

15.00 0.040 0.035

15"

14"

13"

12"

11"

Trailing Edge

Tip

Figure 31. The blade trailing edge was sharpened as shown in the table. Only

the outboard 330 mm (13 inches) of the blade was immersed in the air flow.

Wind turbine tips

63 m/s, α = 11°, Δf = 25Hz, Mic @ 90° and 10ft

10

20

30

40

50

60

70

80

90

100

10 100 1000 10000

Frequency (Hz)

SPL dB (rel. 2x10-5 Pa)

Unsharpened - 221

Sharpened - 227

Empty - 224

3/23/06

Figure 24
22

4.0 Analysis of Test Results

The acoustic spectra of Figure 10 through Figure 30 provide some interesting information about the test 

configurations. Relative amplitudes in various frequency ranges are evident and pure tones are readily 

apparent. It also is useful, however, to characterize the perceived sound as a single quantity representing 

the total sound energy throughout the frequency range. This so-called overall sound pressure level 

(OASPL) is given by Equation 1, in which SPLFA is the A-weighted sound pressure level at frequency F. 

For these analyses the summation was arbitrarily taken over the frequency range of 25 Hz to 10,000 Hz. 

SPLFA is related to the un-weighted sound pressure level (SPLF) by Equation 2 – a technique designed to 

reflect the response of the human ear, which does not respond equally to all frequencies. To describe 

sound in a manner representative of the human ear’s response it is necessary to reduce the effects of the 

low and high frequencies with respect to the medium frequencies. The resultant sound level is said to be 

A-weighted, and the units are dBA. The A-weighted sound level is also called the noise level. [21]

OASPL (dBA) = 10 log ∑ 10 (SPLFA/10) Equation 1 (left) and Equation 2 (below)

SPLFA = SPLF + 2 + 20 log {[122002 · F4

] ÷ [(F2 + 20.62) (F2 + 122002) (F2 + 107.72)

0.5 (F2 + 737.92

)

0.5]}

The data that follow are presented first in tabular form and then in bar charts. Reiterating an important 

premise of this study, the absolute values of OASPL are of lesser interest than the differences between 

configurations, which are the measures of merit for deducing the effects of the test variables.

4.1 Effect of Boundary Layer Tripping

The graphs below are excerpts from Figure 10 through Figure 12 at 9 degrees effective angle of attack 

and tunnel speeds of 27 m/s, 50 m/s and 63 m/s (from left to right). One and 2 layers of trip tape clearly 

are not sufficient to suppress the aggressive noise mechanism, probably laminar boundary layer vortex 

shedding. As the tunnel speed (and Reynolds number) increases, the tones move to higher frequencies 

and it is likely that trailing edge noise becomes the dominant mechanism. Table 3 and Figure 32 indicate 

that at 27 m/s (Re = 170,000) with the boundary layer untripped, the blade has a sound pressure level 5.3 

dBA greater than when it is heavily tripped. At 50 m/s (Re = 315,000) the effect still is pronounced and 

the increase is 4.2 dBA. Test data were not obtained at -1 degree and 5o degrees effective angles of attack, therefore all the details of this behavior cannot be deduced with confidence. However, it is not difficult to extrapolate from the wind tunnel data to the likely result when the wind turbine is in operation. 

Small wind turbines operating at low Reynolds numbers (as they do) are likely to exhibit considerable 

extraneous noise if aggressive boundary layer tripping is not affected. Indeed, this report’s author has 

first-hand corroboration of this hypothesis from field tests of the Windward Engineering Endurance turbine and the Southwest Windpower Skystream turbine observed during developmental tests to optimize 

aerodynamic and aeroacoustic performance. In both cases, appropriate treatment of the blades resulted in 

substantial quieting of the wind turbines.

 

1000 10000

 z)

 

 

 

Figure 10

 

 

1000 10000

 z)

 

 

 

Figure 11

 

 

1000 10000

 z)

 

 

 

Figure 12
23

Alpha 27 50 63 -1 6 11

-1 One Layer 53.3

6 Two Layers 49.8

11 53.3 67.1 70.0 Three Layers 48.0

Alpha 27 50 63 -1 6 11

-1 One Layer 67.1

6 Two Layers 64.4

11 49.8 64.4 69.3 Three Layers 62.9

Alpha 27 50 63 -1 6 11

-1 One Layer 70.0

6 Two Layers 69.3

11 48.0 62.9 68.9 Three Layers 68.9

V = 27 m/s

Tunnel Speed Alpha

V = 50 m/s

Alpha

Three Layers 

(0.024")

One Layer 

(0.008")

Tunnel Speed Alpha

Two Layers 

(0.016")

V = 63 m/s

Tunnel Speed

Table 3. Overall Sound Pressure Level (dBA) for Three Boundary Layer Trip Thicknesses
24

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 27 m/s; 11 degrees alpha

20

25

30

35

40

45

50

55

One Layer Two Layers Three Layers

Boundary Layer Trip Tape

OASPL (dBA)

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 50 m/s; 11 degrees alpha

20

30

40

50

60

70

One Layer Two Layers Three Layers

Boundary Layer Trip Tape

OASPL (dBA)

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 63 m/s; 11 degrees alpha

20

30

40

50

60

70

80

One Layer Two Layers Three Layers

Boundary Layer Trip Tape

OASPL (dBA)

Figure 32. Overall sound pressure level (dBA) for three boundary layer trip thicknesses
25

4.2 Effect of Blade Tip Shape

Although it is important to understand the implications of boundary layer tripping (section 3.2, section

4.1) and trailing edge thickness (section 3.4, section 4.3), the greatest interest probably is regarding the 

effect of blade tip shape. Table 4 and Figure 33 summarize the test results.

Noticeable differences exist between blade and background (empty tunnel) noise. At 27 m/s the smallest 

and largest differences in OASPL are 1.9 dBA and 10.4 dBA, respectively. At 50 m/s, the smallest and 

largest differences in OASPL are 3.3 dBA and 6.2 dBA, respectively. At 63 m/s, the smallest and largest 

differences in OASPL are 3.1 dBA and 4.9 dBA, respectively. Thus, it seems reasonable to draw conclusions from the test data regarding the relative acoustic performance of the different tip shapes. It is interesting to note that at each tunnel speed the largest difference in OASPL occurred at an effective angle of 

attack of -1 degree. In this situation the blade is lightly loaded and acoustic sources other than trailing 

edge noise, such as laminar boundary layer vortex shedding and a blunt trailing edge, have an opportunity 

to dominate.

To draw useful conclusions regarding the efficacy of various blade tip shapes, it is necessary to consider 

the typical operating conditions for small wind turbines. Variable speed turbines attempt to operate at an 

angle of attack near the maximum lift-to-drag ratio. The angle of attack is determined by the tip speed 

ratio – the ratio of the blade tip speed to the wind speed. Logically, given a choice the designers would 

choose a blade tip shape that performed well (was quiet) at this operating point. For the test blade, that 

point is the effective angle of attack of 5 degrees (alpha = 6) and a tunnel speed of 50 m/s. Table 4 shows

little difference among the 6 blade-tip shapes at that point, although PM-5 might as well be avoided. If 

the turbine is stall regulated, even if it is variable speed, then it might be worth looking at a representative 

operating point for that condition, which is the effective angle of attack of 9 degrees (alpha = 11) and a 

tunnel speed of 63 m/s. Again, there is nothing in the data to recommend a particular blade tip shape. If 

designers were to consider a turbine blade operating at low angles of attack and wind speeds, which might 

be the case for a fixed speed turbine near startup, then PM-2, PM-3 and PM-5 might as well be avoided.

It is possible to become mired in the small differences between several blade tip shapes that perform relatively well. The larger issue might be to avoid making a poor choice. At 50 m/s and an effective angle of 

attack of 5 degrees (alpha = 6) – a key operating point – there is a difference of 1.3 dBA between the bestand worst-performing blade tip shapes‡

 ‡ The wind tunnel data are for only one blade. Most turbines employ three blades, and although a wind turbine rotor 

is not really a compact acoustic source, one could approximate the effect of three blades using Equation 3, in which 

OASPL3 and OASPL1 are the overall sound pressure level for three blades and one blade, respectively.

OASPL3 = 10 log [3 ∙10 (OSPL1/10)] Equation 3

It can be shown from Equation 3 that regardless of the magnitude OASPL1, tripling the number of sources increases 

the overall sound pressure level by 4.8 dBA. 

. It could be argued that this difference is too small to distinguish 

between PM-4 and PM-5, but it is also arguable that such a difference actually might exist between two 

competing shapes. At 27 m/s the difference is 3.8 dBA: this is a significant increment in the acoustic signature of a small wind turbine. The author interprets this result as implying that it is a worthwhile effort to 

conduct tests for the purpose of selecting a blade tip shape.

It is important to recognize that these observations regarding blade tip shape and its influence on acoustic 

emissions do not take aerodynamic performance into consideration. Both aerodynamic and aeroacoustic 

performance are of considerable importance in the design of wind turbines and it is extremely advantageous for designers to have reliable information on both topics before finalizing a rotor design.
26

Alpha 27 50 63

-1 6 11

44.9 59.3 65.3 Empty 44.9 44.9 44.9

Baseline 53.8 50.4 50.1

PM-1 53.8 48.1 48.5

PM-2 55.0 49.9 48.7

Alpha 27 50 63 PM-3 54.5 49.3 46.8

-1 53.8 64.6 69.1 PM-4 53.2 48.9 49.9

6 50.4 62.8 68.7 PM-5 55.3 51.9 49.5

11 50.1 63.7 69.4

Alpha 27 50 63

-1 53.8 63.6 69.0 -1 6 11

6 48.1 62.7 68.8 Empty 59.3 59.3 59.3

11 48.5 63.3 69.6 Baseline 64.6 62.8 63.7

PM-1 63.6 62.7 63.3

PM-2 65.5 62.7 62.7

Alpha 27 50 63 PM-3 63.8 62.8 63.0

-1 55.0 65.5 68.9 PM-4 63.3 62.6 63.8

6 49.9 62.7 68.5 PM-5 65.1 63.9 64.3

11 48.7 62.7 69.1

Alpha 27 50 63

-1 54.5 63.8 68.5 -1 6 11

6 49.3 62.8 68.5 Empty 65.3 65.3 65.3

11 46.8 63.0 68.9 Baseline 69.1 68.7 69.4

PM-1 69.0 68.8 69.6

PM-2 68.9 68.5 69.1

Alpha 27 50 63 PM-3 68.5 68.5 68.9

-1 53.2 63.3 68.5 PM-4 68.5 68.4 69.3

6 48.9 62.6 68.4 PM-5 70.2 69.4 69.7

11 49.9 63.8 69.3

Alpha 27 50 63

-1 55.3 65.1 70.2

6 51.9 63.9 69.4

11 49.5 64.3 69.7

V = 63 m/s

Alpha

V = 27 m/s

Alpha

V = 50 m/s

Alpha

Tunnel Speed

Empty 

Tunnel

Tunnel Speed

PM-4

Tunnel Speed

Baseline

Tunnel Speed

PM-1

PM-5

Tunnel Speed

PM-2

Tunnel Speed

PM-3

Tunnel Speed

Table 4. Overall Sound Pressure Level (dBA) for Six Blade Tip Shapes
27

Overall Sound Pressure Level for Six Blade Tips

Tunnel Speed = 27 m/s

30

40

50

60

70

80

Empty Baseline PM-1 PM-2 PM-3 PM-4 PM-5

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees

Overall Sound Pressure Level for Six Blade Tips

Tunnel Speed = 50 m/s

0

10

20

30

40

50

60

70

80

Empty Baseline PM-1 PM-2 PM-3 PM-4 PM-5

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees

Overall Sound Pressure Level for Six Blade Tips

Tunnel Speed = 63 m/s

30

40

50

60

70

80

Empty Baseline PM-1 PM-2 PM-3 PM-4 PM-5

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees

Figure 33. Overall sound pressure level (dBA) for six blade tip shapes
28

4.3 Effect of Trailing Edge Thickness

The spectra of Figures 22-30 show very little to distinguish between sharpened and unsharpened trailing 

edges. One exception seems to be the tunnel speed of 50 m/s and effective angle of attack of -1 degree

shown in Figure 29, excerpted below (left). Table 5 indicates that the unsharpened blade has an OASPL 

1.2 dBA greater than the sharpened blade. This is an unlikely operating condition for a wind turbine, 

perhaps corresponding to a sudden lull in wind speed. Considering the continually varying operating 

conditions, an increase in sound pressure level such as this is not likely to persist or be noticed against 

background noise. Another exception is at 27 m/s and an effective angle of attack of 9 degrees (alpha = 

11 degrees) shown in Figure 22, excerpted below (right). Here the unsharpened blade has an OASPL 1.8 

dBA greater than the sharpened blade. In this condition – hypothesizing substantially laminar flow on the 

forward portion of the airfoil, but with incipient separation – there could be some feedback interaction 

between the trailing edge and the vortices shed upstream. While these occasional, off-design conditions 

are not likely to cause noise issues, it seems prudent to sharpen blade trailing edges to the extent practical.

Table 5. Overall Sound Pressure Level (dBA) for Sharpened and Unsharpened Blades

Alpha 27 50 63 -1 6 11

Empty Empty 44.9 44.9 44.9

Tunnel 44.9 59.3 65.3 Unsharpened 53.8 50.4 50.1

Sharpened 53.9 50.3 48.3

Alpha 27 50 63 -1 6 11

-1 53.8 64.6 69.1 Empty 59.3 59.3 59.3

6 50.4 62.8 68.7 Unsharpened 64.6 62.8 63.7

11 50.1 63.7 69.4 Sharpened 63.4 62.9 63.9

Alpha 27 50 63 -1 6 11

-1 53.9 63.4 68.7 Empty 65.3 65.3 65.3

6 50.3 62.9 68.7 Unsharpened 69.1 68.7 69.4

11 48.3 63.9 69.5 Sharpened 68.7 68.7 69.5

Baseline 

Sharpened

Tunnel Speed

Baseline 

Unsharpened

Tunnel Speed

Tunnel Speed

V = 27 m/s

Alpha

V = 50 m/s

Alpha

V = 63 m/s

Alpha

1000 10000

 

 

 

 

 

1000 10000

 

 

 

50 m/s, Alpha = -1 deg 27 m/s, Alpha = 11 deg
29

Figure 34. Overall sound pressure level (dBA) for sharpened and unsharpened blades

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 27 m/s

20

25

30

35

40

45

50

55

60

Empty Unsharpened Sharpened

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 50 m/s

20

30

40

50

60

70

Empty Unsharpened Sharpened

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees

Overall Sound Pressure Level for Baseline Blade Tip

Tunnel Speed = 63 m/s

20

30

40

50

60

70

80

Empty Unsharpened Sharpened

Blade Tip Shape Designation

OASPL (dBA)

-1 degree 6 degrees 11 degrees
30

5.0 Summary and Conclusions

Wind tunnel aeroacoustic tests were conducted to investigate the effect of boundary layer tripping, tip 

shape and trailing edge thickness on the acoustic emissions of a small wind turbine blade. An actual blade 

from a prototype of Southwest Windpower’s Skystream turbine was used, and the test matrix covered 

realistic operating conditions, including a Reynolds number range of 170,000 to 397,000. Preliminary 

measurements in the empty test section indicated that the background noise was sufficiently low to permit 

reasonable conclusions about relative differences between test configurations. 

Measured results confirmed observations from previous wind tunnel and field tests regarding potential 

noise problems for small wind turbines operating at low Reynolds numbers. At certain tunnel speeds and 

blade angles of attack that are representative of actual operating conditions, significant pure tones and 

elevated broadband emissions were noticed. These are attributed to laminar boundary layer vortex shedding, which can be mitigated by aggressive boundary layer tripping. Extrapolating this result to realworld wind turbine designs, it can be concluded that blade leading edge trips are likely to be required to 

prevent annoying aeroacoustic noise. Field tests should be conducted to investigate this possibility.

Six blade tip shapes were tested, not to identify an optimum, but to establish the approximate difference 

that might exist between “good” and “bad” design choices. At low velocity (Reynolds number) and angle 

of attack, where subtle differences in geometry and flow conditions can be important, a 3.8 dBA difference in sound pressure level was measured between the best-and worst-performing tip shapes. At conditions representing more realistic operating conditions – near the airfoil’s maximum lift-to-drag ratio – a 

1.3 dBA difference was observed. Although conventional practice teaches not to ascribe excessive credence to differences of this magnitude, it is arguable, based on test results, that such differences very well 

could exist. Therefore, in the quest for very quiet wind turbine blades, the designer should carefully consider alternative tip shapes for an otherwise quiet blade.

The test blade with baseline tip shape was evaluated with two trailing edge thicknesses of 1.1% chord and 

0.8% chord. The former is representative of deliberate attempts, constrained by manufacturing limitations, 

to achieve a thin trailing edge for the Storm and Skystream blades. At 1 of the 9 test conditions, the sharpened trailing edge resulted in a 1.2 dBA reduction in sound pressure level. At a second test condition, a 

difference of 1.8 dBA was observed. There were no noteworthy differences at the other test conditions. 

It is difficult to draw sweeping conclusions based on these limited test results; however, within the perspective of other data in the literature, it seems reasonable to conclude that minimizing trailing edge thickness is beneficial.

There are subtle but important differences between aeroacoustic testing in steady, low-turbulence rectilinear flow as opposed to the unsteady, turbulent, curvilinear flow of operating wind turbines. In the latter 

case, for example, the local velocity and angle of attack vary along the blade span and there can be important three-dimensional effects. Therefore, it is best to draw only general conclusions from the GTRI test 

results. Nevertheless, the data seem to support the conclusion that boundary layer tripping, tip shape and 

trailing edge thickness are worthy of consideration in attempting to design quiet blades for small wind 

turbines. Of similar import, but not considered here, is the tradeoff between aerodynamic noise and aerodynamic performance (energy capture).
31

6.0 References

1. Selig, M.S. and McGranahan, B.D., Wind Tunnel Aerodynamic Tests of Six Airfoils for Use on 

Small Wind Turbines, NREL/SR-500-34515, National Renewable Energy Laboratory, Golden, CO, 

May 2004.

2. Selig, M.S. and McGranahan, B.D., Wind Tunnel Aerodynamic Tests of Six Airfoils for Use on Small 

Wind Turbines, Journal of Solar Energy Engineering, Vol. 126, No.4, p. 986-1001, The American Society of Mechanical Engineers, New York, November 2004.

3. Oerlemans, S., Wind Tunnel Aeroacoustic Tests of Six Airfoils for Use on Small Wind Turbines, 

NREL/SR-500-35339, National Renewable Energy Laboratory, Golden, CO, May 2004.

4. Migliore, P. and Oerlemans, S., Wind Tunnel Aeroacoustic Tests of Six Airfoils for Use on Small 

Wind Turbines, Journal of Solar Energy Engineering, Vol. 126, No.4, p. 974-985, The American Society of Mechanical Engineers, New York, November 2004.

5. Devenport, W., Burdisso, R., et al, Aeroacoustic Testing of Wind Turbine Airfoils, NREL/SR-500-

43471, National Renewable Energy Laboratory, Golden, CO, July 2008.

6. Migliore, P., van Dam, J. and Huskey, A. (October 2003). Acoustic Tests of Small Wind Turbines, 

NREL/CP-500-34622, National Renewable Energy Laboratory Golden, Colorado.

7. Moriarty, P. and Migliore, P, 2003, Semi-empirical Aeroacoustic Noise Prediction Code for Wind 

Turbines, NREL TP-500-34478.

8. Moriarty, P. J.; Guidati, G.; Migliore, P. (2005). Prediction of Turbulent Inflow and Trailing-Edge 

Noise for Wind Turbines. AIAA 2005-2881. Collection of Technical Papers from the 11th 

AIAA/CEAS Aeroacoustics Conference, 23-25 May 2005, Monterey, California. New York: American Institute of Aeronautics and Astronautics, Inc. 16 pp.; NREL Report No. CP-500-39690.

9. Moriarty, P.; Migliore, P. (2003). Semi-Empirical Aeroacoustic Noise Prediction Code for Wind Turbines. 39 pp.; NREL Report No. TP-500-34478.

10. Uzun, A., Hussaini, Y. and Streett, C.L., Large-Eddy Simulation of Wing Tip Vortex on Overset Grids, AIAA Journal, 2006, 44: 1229-1242. 

11. Morris, P.J., Long, L.N., and Brentner, K.S., An Aeroacoustic Analysis of Wind Turbines, 23rd 

ASME Wind Energy Symposium, AIAA Paper No. 2004-1184, Reno, NV, January 5-8, 2004.

12. Sezer-Uzol, N. and Long, L. N., 3-D Time-Accurate CFD Simulations of Wind Turbine Rotor Flow 

Fields, AIAA Paper 2006-394, 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 

9-12 January, 2006.

13. Cheng, R., Morris, P.J. and Brentner, K.S., A 3D Parabolic Equation Method for Wind Turbine Noise 

Propagation in Moving Inhomogeneous Atmosphere, 12th AIAA/CEAS Aeroacoustics Conf., 2006.

14. Miller, Steven A.E. and Morris, Philip J, Rotational Effects on the Aerodynamics and Aeroacoustics 

of Wind Turbine Airfoils, 12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics 

Conference); Cambridge, MA; USA; 8-10 May 2006. 2006

15. Sezer-Uzol, N., Gupta, A. and Long, L.N., 3-D Time-Accurate Inviscid and Viscous CFD Simulations 

of Wind Turbine Rotor Flow Fields, Proceedings of PARALLEL CFD 2007, International Conference 

on Parallel Computational Fluid Dynamics, May 2007, Antalya TURKEY.

16. Tangler, J.L. and Somers, D.M., NREL Airfoil Families for HAWTS, NREL TP-442-7109, National 

Renewable Energy Laboratory, Golden, CO, 1995.

17. Wagner, S., Bareiß, R. and Guidati, G., 1996, Wind Turbine Noise

18. Ahuja, K.K., Tester, B.J. and Tanna, H.K., The Free Jet as a Simulator of Forward Velocity Effects 

on Jet Noise, Final Report, NASA CR-3056, 1978.

, Springer-Verlag, Berlin, pp. 14-

21; 172-178.

19. Brooks, T., Pope, D. and Marcolini, M. (1989), Airfoil Self-Noise and Prediction, NASA Reference 

Publication 1218.

20. Moriarty, P. and Migliore, P., Semi-Empirical Aeroacoustic Noise Prediction Code for Wind Turbines, NREL/TP-500-34478, National Renewable Energy Laboratory, Golden, CO, Dec. 2003.

21. http://www.acousticalsolutions.com/education/glossary.asp (accessed 06/17/08).
32
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4. TITLE AND SUBTITLE

The Potential for Reducing Blade-Tip Acoustic Emissions for Small 

Wind Turbines: June 1, 2007 – July 31, 2008

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DE-AC36-08-GO28308

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P. Migliore

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Paul Migliore, Engineering Research and Project Management

Arvada, Colorado

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14. ABSTRACT (Maximum 200 Words)

This report describes the results of wind tunnel aeroacoustic tests conducted on a typical small wind turbine blade in 

the open-jet test section of the Georgia Tech Research Institute (GTRI) Anechoic Flight Simulation Facility. 

15. SUBJECT TERMS

wind turbine blade aeroacoustic tests; Georgia Tech Research Institute; wind energy; wind turbine blade design

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