L. Danielle Koch

Glenn Research Center, Cleveland, Ohio

Validation of the Predicted Circumferential and 

Radial Mode Sound Power Levels in the Inlet and 

Exhaust Ducts of a Fan Ingesting Distorted Inflow

NASA/TM—2012-217253

February 2012

AIAA–2011–2808
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L. Danielle Koch

Glenn Research Center, Cleveland, Ohio

Validation of the Predicted Circumferential and 

Radial Mode Sound Power Levels in the Inlet and 

Exhaust Ducts of a Fan Ingesting Distorted Inflow

NASA/TM—2012-217253

February 2012

AIAA–2011–2808

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

17th Aeroacoustics Conference

cosponsored by the American Institute of Aeronautics and Astronautics and 

the Confederation of European Aerospace Societies

Portland, Oregon, June 5–8, 2011
Acknowledgments

I would like to thank the AAPL staff for their work obtaining the experimental data. I also appreciate the insight that my colleagues 

Ed Envia, Dan Sutliff, Brenda Henderson and Ray Hixon shared along the way. Funding for this research has been provided by 

the Subsonic Fixed Wing Project which is a part of NASA’s Fundamental Aeronautics Program.

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This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217253 1

Validation of the Predicted Circumferential and Radial 

Mode Sound Power Levels in the Inlet and Exhaust 

Ducts of a Fan Ingesting Distorted Inflow 

L. Danielle Koch 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Fan inflow distortion tone noise has been studied computationally and experimentally. Data from two 

experiments in the NASA Glenn Advanced Noise Control Fan rig have been used to validate acoustic 

predictions. The inflow to the fan was distorted by cylindrical rods inserted radially into the inlet duct one 

rotor chord length upstream of the fan. The rods were arranged in both symmetric and asymmetric 

circumferential patterns. In-duct and farfield sound pressure level measurements were recorded. It was 

discovered that for positive circumferential modes, measured circumferential mode sound power levels in 

the exhaust duct were greater than those in the inlet duct and for negative circumferential modes, 

measured total circumferential mode sound power levels in the exhaust were less than those in the inlet. 

Predicted trends in overall sound power level were proven to be useful in indentifying circumferentially 

asymmetric distortion patterns that reduce overall inlet distortion tone noise, as compared to symmetric 

arrangements of rods. Detailed comparisons between the measured and predicted radial mode sound 

power in the inlet and exhaust duct indicate limitations of the theory. 

Nomenclature 

a speed of sound 

Amnμ power-effective area for m, μ mode, n

th harmonic 

am,n direction cosine of n

th-harmonic, m-lobe mode, = 

B number of rotor blades 

cm,n amplitude of n

th harmonic, mth mode due to rotor interaction with single vane 

Cm,n amplitude of nth harmonic, m

th mode due to rotor interaction with stator assembly 

j index 

k index 

m circumferential mode index 

n harmonic index 

PWL sound power level 

q stator vane index 

t time coordinate 

V number of stator vanes 

W acoustic power 

Wref reference acoustic power, 10–12 watts 

βq (nB-m)Δθq

βj (nB-m)Δθj
NASA/TM—2012-217253 2

θ pressure field angular coordinate 

μ radial mode index 

ξmn cutoff ratio of m-lobe mode, n

th harmonic 

ρ density 

* complex conjugate 

1.0 Introduction 

Tone noise produced when a fan ingests distorted inflow is a problem for fans of all sizes—from the 

large fans featured in embedded aircraft engine concepts (Ref. 1) to the small fans in spacecraft 

ventilation systems (Ref. 2). In order to develop fan systems that minimize fan tone noise, design 

engineers need to be able to compute tone power levels for realistic models of the fan and duct. With 

acoustic prediction tools, different fan and duct geometries can be evaluated for acoustic performance 

early in the design cycle, which can ultimately lead to quieter final products. Several tone noise prediction 

methods are currently under development at NASA Glenn. Experiments in the NASA Glenn Advanced 

Noise Control Fan (ANCF) rig are currently being used to validate predicted results and gain a 

fundamental understanding of the physics of inlet distortion fan tone noise. 

Reference 3 presents a method for computing circumferential mode sound power levels of a fan 

ingesting distorted inflow. In this model, it was assumed that only the first radial mode contributes to each 

propagating circumferential mode. Sound power levels in the inlet and exhaust duct were assumed to be 

equal to the value calculated at the generation plane (the rotor leading edge), and the overall sound power 

level computed from their sum. Data from an experiment conducted in the ANCF in 2007 were used to 

validate those predictions. 

That prediction method and the associated set of assumptions (Ref. 3) were used to design a second 

inlet distortion test for the ANCF that was conducted in 2010. Like the 2007 experiment, the 2010 inlet 

distortion test used cylindrical rods inserted radially into the duct upstream of the fan to distort the inflow. 

The purpose of the 2010 inlet distortion test was to determine if predictions could be used to select 

circumferentially asymmetric distortion patterns that would reduce overall inlet distortion tone noise, as 

compared to symmetric arrangements of rods. The 2010 experimental results indicate that predictions 

assuming that only the first radial mode propagates, and that the inlet and exhaust sound power levels 

were equal to the value calculated at the generation plane (rotor leading edge), were in fact useful in 

identifying overall quieter configurations. 

Finally, the theory was applied to estimate the sound power levels at the inlet and exhaust planes. 

Instead of simply neglecting the contribution from all but the first radial mode, predictions for all 

propagating radial modes were made by assuming that the model for the unsteady pressure distribution at 

the interaction plane was constant for all radial modes. These estimations are compared to data from both 

the 2007 and the 2010 experiments, and strengths and weaknesses of the prediction method are discussed. 

Insight from this work is being used to develop a tone noise prediction code capable of estimating 

both circumferential and radial sound power levels based on the methodology used in the V072 

Rotor/Stator Interaction Code (Ref. 4) for more realistic models of the fan geometry. 

2.0 Experiments 

Two fan inlet distortion experiments were conducted in 2007 and 2010 using the NASA Glenn 

Advanced Noise Control Fan that is housed within AeroAcoustic Propulsion Laboratory (AAPL). The 

ANCF, shown in Figure 1, is 1.2 m (48 in.) in diameter and the centerline of the fan is 3.0 m (10 ft) above 

the floor. For both tests, no stator vanes were installed downstream of the rotor. The rotor had 16 blades 

set at a 28° pitch angle for all test conditions. 
NASA/TM—2012-217253 3

An Inflow Control Device (ICD) was used to condition the flow entering the fan, removing largescale turbulence and ground vortices. The inlet plane was located 0.920 m (36.2 in.) or approximately 

0.750 L/D ahead of the leading edge of the fan at the tip location. The exhaust plane was located 1.20 m 

(45.4 in.) or approximately 1.0 L/D downstream of the trailing edge of the fan at the tip location. 

Downstream of the test section, the centerbody diameter increases to 0.710 m (24.0 in.) to mimic a nozzle 

area contraction, yielding a hub-to-tip ratio of, σ = 0.500. The hub diameter at the rotor leading edge was 

0.460 m (18.0 in.), yielding a hub-to-tip ratio of, σ = 0.375. 

The inflow to the rotor was distorted by installing smooth cylindrical rods upstream of the rotor, as 

shown in Figure 2. The rods were 1.27 cm (0.500 in.) in diameter and were 31.8 cm (12.5 in.) long, 

resulting in a 6.35 cm (2.50 in.) gap between the bottom of the rods and the rotor hub. The centerline of 

the rods was 14.3 cm (5.63 in.) upstream of the rotor leading edge at the tip, or approximately one rotor 

chord length upstream. Data were recorded at three speed settings: 1400, 1800, and 2000 rpm. 

Four distortion patterns were tested in the 2007 experiment, as shown in Figure 3. A 30-rod mounting 

ring allowed rods to be placed at circumferential locations separated by increments of 12°. Angular 

locations of rods pictured in Configuration 4 (2007) can be found in Reference 5. 

Figure 4 shows the results of a parametric study used to design the 2010 experiment. For each rod 

count (6, 10, and 15 rods), overall tone power levels were calculated as a function of circumferential 

spacing angle. The computation included only the 1 BPF, 2 BPF, and 3 BPF tones and assumed that only 

the first radial mode propagated. Sound power levels were computed at the generation plane, assumed to 

be the leading edge of the rotor. The spacing angle was varied from 1° to the equally spaced angular 

position (60°, 36°, and 24°, respectively) for each rod count. Tone noise minima were predicted to occur 

near the 17° and 34° angular spacing for all rod counts studied. These predictions were carried out for the 

2000 rpm condition only. 

Ten distortion patterns were tested in the 2010 experiment, as shown in Figure 5. In the 2010 

experiment, both a 30-rod mounting ring and a 21-rod mounting ring were used. The 21-rod mounting 

ring allowed rods to be placed at circumferential locations separated by 17.1° increments, a number that 

was determined by the predictions plotted in Figure 4. Both experiments also included a configuration 

without any rods installed. 

Two 15-microphone arrays were used to record the farfield sound distribution from the ANCF. 

Farfield microphone measurements were acquired synchronously with shaft speed at a rate of 256 

samples per revolution, which allow for processing up to the 128th harmonic of the shaft frequency, or 

equivalently, up to the 8th harmonic of the blade passing frequency (BPF) for this test (Ref. 6). 

In-duct acoustic pressure measurements upstream and downstream of the rotor were acquired with the 

Rotating Rake system (Ref. 7). The Rotating Rake system is a continuously rotating radial microphone 

rake that is inserted into the duct, at either the inlet entrance or exhaust exit plane. The system utilizes 

spinning mode theory and Doppler-shift physics to separate circumferential modes and the cylindrical 

wave equation solution to reduce the radial modes. It provides a complete map of the acoustic duct modal 

magnitudes and phases present in the fan duct. For these experiments, the sampling rate for the Rotating 

Rake system was set to 128 samples per revolution, and the data were processed for the first two 

harmonics of blade passing frequency. 

3.0 Fan Tone Noise Predictions 

Sofrin and Mathews (Ref. 8) express sound power in a constant area annular duct with the following 

equation: 

 (1) 
NASA/TM—2012-217253 4

Reference 5 describes how the Sofrin and Mathews (Ref. 8) relations could be expressed in a form 

convenient for representing irregularly spaced vanes without a small deviation angle assumption. The 

asymmetry in the problem is captured in the term:

 

 (2) 

Note that this term was expressed without a dependence upon radial mode order. Power in each of the 

circumferential and radial modes can be calculated from: 

 (3) 

Power in the inlet and exhaust duct can be calculated if one assumed the appropriate values for duct 

cross sectional area (noting that the area term in Equation (1) does not vary with circumferential, radial, 

or harmonic index, but rather varies with axial location in the duct). 

What is computed here and validated against measurements is power levels for inlet and exhaust 

given by:

 (4)

 (5) 

PWL = 10 * log (W/Wref) (6) 

4.0 Comparison of Measurements and Predictions 

4.1 Determination of Low Noise Inlet Distortion Patterns 

Figures 6 through 8 summarize the results of the 2010 inlet distortion test and compare measurements 

to three sets of predictions all based on the theory of Sofrin, Mathews and Koch. Figure 6 compares the 

measured and predicted results for the 6-rod configurations; Figure 7 compares results for the 10-rod 

configurations; and Figure 8 shows the 15-rod configuration results. 

Only the 1 BPF and 2 BPF tones were included in the calculation of the overall tone power levels in 

Figures 6 through 8. All results shown are for the 2000 rpm fan speed. To obtain the overall tone sound 

power level from the farfield microphone measurements, the data from the inlet and exhaust arrays were 

combined. Likewise, to obtain the overall tone sound power level from the Rotating Rake measurements, 

the data from the inlet and exhaust duct rakes were combined. 

In general, overall tone sound power levels measured by the Rotating Rake were greater than those 

measured by the farfield microphone arrays. Better agreement between the Rotating Rake and farfield 

microphone arrays might be achievable with additional farfield microphones. The existing array does not 

detect variations in sound power levels in the circumferential direction. This may be of particular concern 

for those studying noise produced by asymmetric inflow distortions. Directivity plots from the 2007 inlet 

distortion test are presented in Reference 5. 

Three predictions are shown in Figures 6 through 8. The prediction labeled SMK 3 shown in black 

assumed only the first radial mode propagated. Sound power levels in the inlet and exhaust duct were 

assumed to be equal to the value calculated at the generation plane (the rotor leading edge), and the 
NASA/TM—2012-217253 5

overall sound power level was computed from their sum. These same assumptions were used to generate 

the predictions used to design the experiment. 

The prediction labeled SMK 2 shown in cyan in Figures 6 through 8, included the estimates for all 

propagating radial modes. SMK 2 predictions once again assumed that sound power levels in the inlet and 

exhaust duct were both equal to the value calculated at the generation plane. 

The prediction labeled SMK 1 shown in green in Figures 6 through 8 included estimates for all 

propagating radial modes. SMK 1 predictions calculated the inlet duct sound power levels at the inlet 

plane where the Rotating Rake is installed. SMK 1 estimates for the exhaust duct sound power levels 

were calculated at the exhaust plane where the exhaust Rotating Rake measurements were acquired. The 

overall tone sound power levels were computed from the sum of the inlet and exhaust duct estimates. 

These three sets of predictions are summarized in Table 1. 

The trends for all configurations tested were most accurately captured by the SMK 3 predictions. 

Deficiencies in the predictions of all propagating radial mode sound power levels in the inlet and exhaust 

duct are discussed in detail below. Even so, the comparisons in Figures 6 through 8 indicate that 

evaluating overall tone sound power level at the generation plane may be useful in parametric studies of 

fan inflow distortion tone noise. 

4.2 Validation of Radial Mode Sound Power Level Predictions 

Figures 9 through 14 compare predicted circumferential and radial sound power levels to 

measurements from the 2007 experiment in an effort to be consistent with previous reports (Refs. 5 and 

3). Results are shown for Configuration 1 (6 rods equally spaced) and Configuration 4 (8 rods, integer 

multiples of 12° apart). The red bars represent the total sound power levels in each circumferential mode, 

while the other colored bars indicate the sound power levels of the radial modes. Symbols here represent 

calculated values. Figures 9 and 10 demonstrate that the theory generally underpredicted the 

circumferential and radial mode power levels for the first harmonic of the blade passing frequency tone. 

Including estimates for all propagating radial modes slightly improved the accuracy of the total 1 BPF 

circumferential mode sound power level prediction for Configuration 4 where the second radial mode is a 

significant contributor. 

Figures 11 and 12 compare predictions to the measurements for the same configurations for the 2 

BPF harmonic. Figures 11 and 12 demonstrate that the theory generally did not accurately capture trends 

in measured radial mode sound power levels. While Figure 11 indicates that for Configuration 1 (6 rods 

equally spaced), the first radial mode power level was measured to be significantly greater than the other 

propagating radial modes Figure 12 indicates that this trend is not always observed when the inflow is 

distorted asymmetrically—for Configuration 4 (8 rods, integer multiples of 12° apart) both the first and 

third radial modes are significant contributors. While including estimates for all the radial modes did 

generally improve the accuracy of the total circumferential mode sound power level predictions in the 

exhaust duct for the 2 BPF tone, this was not true for the inlet duct. In the inlet duct, radial mode sound 

power level predictions were generally greater than the measured values. 

Significant contributions from higher order radial modes were most evident for the more asymmetric 

cases studied at design speed of 2000 rpm (Configurations 3 and 4, 2007). The first radial mode became 

more dominant for all cases studied as speed was decreased, as shown in Figures 10 and 11, showing the 

2 BPF results for Configuration 1 and 4 at 1800 rpm. These experiments demonstrate that asymmetry in 

the inlet distortion pattern can change both the circumferential and radial mode sound power level 

distributions. 

4.3 Validation of Inlet and Exhaust Duct Sound Power Level Predictions 

Figures 15 and 16 compare predicted sound power levels to measurements from the 2007 experiment. 

Both figures compare the total sound power level in each circumferential mode to the contributions from 

the inlet and exhaust duct individually. The colored bars represent the rotating rake measurements, while 
NASA/TM—2012-217253 6

the symbols indicate the predictions. Results are again shown for Configuration 1 (6 rods equally spaced) 

and Configuration 4 (8 rods, integer multiples of 12° apart) from the 2007 experiment at 2000 rpm. 

It was discovered that for positive circumferential modes (m>0), measured circumferential mode 

sound power levels in the exhaust duct were greater than those in the inlet duct (PWLm,ex > PWLm,in), and 

vice versa—for negative circumferential modes (m<0), measured total circumferential mode sound power 

levels in the exhaust duct were less than those in the inlet duct (PWLm,ex < PWLm, in). These trends were 

present in both the 2010 and 2007 data sets, most evident in the 1 BPF results though they are also seen in 

many of the 2 BPF results. None of the predictions captured these trends. 

The overall sound power levels for all four configurations tested in 2007 are shown in Figure 17. The 

overall sound power levels shown in Figure 17 only included contributions from the first two BPF 

harmonics. Again differences between measured trends may be attributed to limitations in the farfield 

data set. While the farfield microphone array did capture the directivity of sound power level at a fixed 

circumferential position, the array did not capture any directivity in the circumferential direction. The 

farfield microphone array is illustrated in Figure 1, and angular positions of the microphones have been 

tabulated in Reference 5. 

Two sets of predictions are shown in Figure 17. SMK Prediction 1 included all propagating radial 

modes. Sound power levels at the inlet plane and the exhaust plane were calculated and combined to find 

the overall sound power levels for the 1 BPF and 2 BPF tones. SMK Prediction 3 included the first radial 

mode only. Sound power levels in the inlet and exhaust duct were assumed to be equal to the value 

calculated at the generation plane (rotor leading edge). The reader may again refer to Table 1 for a 

summary of the assumptions associated with the predictions. Measured trends are more similar to SMK 

Prediction 3, again indicating that calculating overall sound power levels at the generation plane may be 

useful in studying fan tone noise produced by asymmetric inflow distortions. 

Finally, if one does not limit the overall sound power level calculation to the first two harmonics of 

blade passing frequency, but rather include contributions from the first seven harmonics, shortcomings in 

the theory become very evident. Figure 18 compares the overall sound power levels for each harmonic 

measured by the farfield microphone array and from predictions for Configuration 4 of the 2007 test. 

Clearly, the theory does not accurately represent the amplitude of the higher order radial mode sound 

power levels and improvements in this area are needed. 

5.0 Conclusion 

Fan inflow distortion tone noise has been studied computationally and experimentally at NASA 

Glenn Research Center. Data from two experiments in the NASA Glenn Advanced Noise Control Fan rig 

have been used to validate acoustic predictions. It was found that trends in overall sound power level 

computed using the theory of Sofrin, Mathews, and Koch agreed well with measured trends if sound 

power levels in the inlet and exhaust duct were assumed to be equal to the value calculated at the 

generation plane (the rotor leading edge) and limited to include the first two harmonics. Predicted trends 

in the overall sound power level were proven to be useful in indentifying circumferentially asymmetric 

distortion patterns that would reduce overall inlet distortion tone noise, as compared to symmetric 

arrangements of rods. 

While the theory could also be used to calculate the tone sound power levels at the inlet and exhaust 

planes, measured trends in sound power levels were not accurately predicted. Likewise, while the theory 

can be applied to estimate sound power levels for all propagating radial modes, predicted trends did not 

agree well with measurements. Deficiencies in the model can clearly be seen for this fan for higher blade 

passing frequency harmonics that typically have many propagating modes. 

Finally, examination of the data from both experiments uncovered a trend in the data--for positive 

circumferential modes, measured circumferential mode sound power levels in the exhaust duct were 

greater than those in the inlet duct, and for negative circumferential mode, measured total circumferential 

mode sound power levels in the exhaust duct were less than those in the inlet duct. 
NASA/TM—2012-217253 7

These investigations are useful in further defining the physics of fan inflow distortion tone noise. 

Insight will be used to guide the design of future experiments and the development of new tone noise 

prediction codes. 

References 

1. Hall, C. A. and Crichton, D., “Engine and Installation Configurations for a Silent Aircraft,” ISABE2005-1164, 2005. 

2. Goodman, J. R., “Noise Control in Space Shuttle Orbiter," nc10_218, 2010. 

3. Koch, L. D., “Predicted and Measured Modal Sound Power Levels for a Fan Ingesting Distorted 

Inflow,” AIAA 2010-3715, 2010. 

4. Meyer, H. D. and Envia, E., “Aeroacoustic Analysis of Turbofan Noise Generation,” NASA CR4715, 1996. 

5. Koch, L. D., “An Experimental Study of Fan Inflow Distortion Tone Noise,” NASA/TM—2010-

215844, 2010. 

6. Loew, R. A., Lauer, J. T., McAllister, J. and Sutliff, D. L., “The Advanced Noise Control Fan,” 

NASA/TM—2006-214368, 2006. 

7. Sutliff, D. L., “Rotating Rake Turbofan Duct Mode Measurement System,” NASA/TM—2005-

213828, 2005. 

8. Sofrin, T. G. and Mathews, D. C., “Asymmetric Stator Interaction Noise,” Journal of Aircraft, 

Vol. 17, Issue 8, 1979, 554-560. 

TABLE 1.—DESCRIPTION OF PREDICTION ASSUMPTIONS 

Prediction label Prediction assumptions 

SMK 1 all propagating radial modes predicted at inlet and exhaust plane locations 

SMK 2 all propagating radial modes predicted at generation plane (rotor leading edge); 

inlet and exhaust duct sound power levels assumed to be equal 

SMK 3 only the first radial mode predicted at the generation plane (rotor leading edge); 

inlet and exhaust duct sound power levels assumed to be equal 
NASA/TM—2012-217253 8

 Figure 1.—The Advanced Noise Control Fan in the NASA Glenn AeroAcoustic Propulsion Lab. 

Figure 2.—Cross-sectional diagram of Advanced Noise Control Fan for the inlet distortion tests. 
NASA/TM—2012-217253 9

Figure 3.—Circumferential locations of inlet distortion rods tested in 2007 in the ANCF at NASA Glenn. 

Figure 4.—Total Tone Sound Power Level predictions as a function of rod circumferential spacing angle measured 

in degrees used to design 2010 ANCF inlet distortion test. Vertical lines indicate angular spacing possible with the 

30-rod and 21-rod mounting rings. 
NASA/TM—2012-217253 10

Figure 5.—Circumferential locations of inlet distortion rods tested in 2010 in the ANCF at NASA Glenn. 
NASA/TM—2012-217253 11

Figure 6.—Comparison of measured and predicted overall tone sound power levels for 2010 6-rod configurations 

at 2000 rpm. Calculation of overall sound power levels includes contributions from the first two harmonics of the 

blade passing frequency tone only. 

Figure 7.—Comparison of measured and predicted overall tone sound power levels for 2010 10-rod 

configurations at 2000 rpm. Calculation of overall sound power levels includes contributions from the first two 

harmonics of the blade passing frequency tone only. 
NASA/TM—2012-217253 12

Figure 8.—Comparison of measured and predicted overall tone sound power levels for 2010 15-rod 

configurations at 2000 rpm. Calculation of overall sound power levels includes contributions from the 

first two harmonics of the blade passing frequency tone only. 
NASA/TM—2012-217253 13

(a)

(b)

Figure 9.—Predicted and measured in-duct circumferential and radial sound power levels for Configuration 1 

(2007), 1BPF, 2000 rpm. Horizontal dashed line indicates noise floor for the data set. (a) Inlet duct and 

(b) exhaust duct. 
NASA/TM—2012-217253 14

(a)

 (b)

Figure 10.—Predicted and measured in-duct circumferential and radial sound power levels for Configuration 4 

(2007), 1 BPF, 2000 rpm. Horizontal dashed line indicates noise floor for the data set. (a) Inlet duct and (b) 

exhaust duct. 
NASA/TM—2012-217253 15

 (a) (b) 

Figure 11.—Predicted and measured in-duct circumferential and radial mode sound power levels for 

Configuration 1 (2007), 2 BPF, 2000 rpm. Horizontal dotted line indicates the noise floor for the data set. 

(a) Inlet duct and (b) exhaust duct. 
NASA/TM—2012-217253 16

 

 (a) (b) 

Figure 12.—Predicted and measured in-duct circumferential and radial mode sound power levels for Configuration 4 

(2007), 2 BPF, 2000 rpm. Horizontal dotted line indicates the noise floor for the data set. (a) Inlet duct and 

(b) exhaust duct. 
NASA/TM—2012-217253 17

 (a) (b) 

Figure 13.—Predicted and measured in-duct total, inlet-total, exhaust-total circumferential mode sound power levels for 

Configuration 1 (2007), 2 BPF, 1800 rpm. Horizontal dotted line indicates the noise floor for the data set. (a) Inlet duct 

and (b) exhaust duct.
NASA/TM—2012-217253 18

 (a) (b) 

Figure 14.—Predicted and measured in-duct circumferential and radial sound power levels for Configuration 4 (2007), 

1BPF, 1800 rpm. Horizontal dashed line indicates noise floor for the data set. (a) Inlet duct and (b) exhaust duct. 
NASA/TM—2012-217253 19

(a)

(b)

Figure 15.—Predicted and measured in-duct total, inlet-total, exhaust-total circumferential mode sound power levels for 

1 BPF, 2000 rpm. Horizontal dotted line indicates the noise floor for the data set. Uncertainty in measured mode sound 

power levels is ± 2dB. (a) Configuration 1 (2007) and (b) Configuration 2 (2007). 
NASA/TM—2012-217253 20

 (a) (b) 

Figure 16.—Predicted and measured in-duct total, inlet-total, exhaust-total circumferential mode sound power levels 

for 2 BPF, 2000 rpm. Horizontal dotted line indicates the noise floor for the data set. Uncertainty in measured mode 

sound power levels is ± 2dB. (a) Configuration 1 (2007) and (b) Configuration 2 (2007). 
NASA/TM—2012-217253 21

Figure 18.—Predicted overall sound power levels for Configuration 4 (2007) at 2000 rpm compared to farfield 

microphone array measurements. Calculation of overall sound power levels includes contributions from the first 

seven harmonics of the blade passing frequency tone only. 

Figure 17.—Comparison of measured and predicted overall sound power levels for all 2007 configurations at 

2000 rpm. Calculation of overall sound power levels includes contributions from the first two harmonics of the 

blade passing frequency tone only. Bars represent the total sound power levels, while the symbols indicate the 

sound power levels in the inlet and exhaust ducts. 
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Validation of the Predicted Circumferential and Radial Mode Sound Power Levels in the Inlet 

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Unclassified-Unlimited 

Subject Category: 71 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Fan inflow distortion tone noise has been studied computationally and experimentally. Data from two experiments in the NASA Glenn 

Advanced Noise Control Fan rig have been used to validate acoustic predictions. The inflow to the fan was distorted by cylindrical rods 

inserted radially into the inlet duct one rotor chord length upstream of the fan. The rods were arranged in both symmetric and asymmetric 

circumferential patterns. In-duct and farfield sound pressure level measurements were recorded. It was discovered that for positive 

circumferential modes, measured circumferential mode sound power levels in the exhaust duct were greater than those in the inlet duct and 

for negative circumferential modes, measured total circumferential mode sound power levels in the exhaust were less than those in the inlet. 

Predicted trends in overall sound power level were proven to be useful in indentifying circumferentially asymmetric distortion patterns that 

reduce overall inlet distortion tone noise, as compared to symmetric arrangements of rods. Detailed comparisons between the measured and 

predicted radial mode sound power in the inlet and exhaust duct indicate limitations of the theory.

15. SUBJECT TERMS

Predicted circumferential; Radial; Mode sound; Fan 

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