L. Danielle Koch, Tony D. Shook, Douglas T. Astler, and Samantha A. Bittinger

Glenn Research Center, Cleveland, Ohio

Tone Noise Predictions for a Spacecraft Cabin 

Ventilation Fan Ingesting Distorted Infl ow 

and the Challenges of Validation

NASA/TM—2012-217681

September 2012
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L. Danielle Koch, Tony D. Shook, Douglas T. Astler, and Samantha A. Bittinger

Glenn Research Center, Cleveland, Ohio

Tone Noise Predictions for a Spacecraft Cabin 

Ventilation Fan Ingesting Distorted Infl ow 

and the Challenges of Validation

NASA/TM—2012-217681

September 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217681 1 

Tone Noise Predictions for a Spacecraft Cabin Ventilation Fan 

Ingesting Distorted Inflow and the Challenges of Validation 

L. Danielle Koch, Tony D. Shook, Douglas T. Astler,1

 and Samantha A. Bittinger2

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

A fan tone noise prediction code has been developed at NASA Glenn Research Center that is capable 

of estimating duct mode sound power levels for a fan ingesting distorted inflow. This code was used to 

predict the circumferential and radial mode sound power levels in the inlet and exhaust duct of an axial 

spacecraft cabin ventilation fan. Noise predictions at fan design rotational speed were generated. Three fan 

inflow conditions were studied: an undistorted inflow, a circumferentially symmetric inflow distortion 

pattern (cylindrical rods inserted radially into the flowpath at 15, 135, and 255), and a circumferentially 

asymmetric inflow distortion pattern (rods located at 15, 52 and 173). Noise predictions indicate that 

tones are produced for the distorted inflow cases that are not present when the fan operates with an 

undistorted inflow. 

Experimental data are needed to validate these acoustic predictions, as well as the aerodynamic 

performance predictions. Given the aerodynamic design of the spacecraft cabin ventilation fan, a 

mechanical and electrical conceptual design study was conducted. Design features of a fan suitable for 

obtaining detailed acoustic and aerodynamic measurements needed to validate predictions are discussed. 

Introduction 

Stationary objects upstream of a fan, such as probes or struts, can distort the flow entering a fan. The 

inflow can also be distorted if the fan is attached to convoluted ducting, as is typical of a spacecraft 

ventilation system. Inflow distortion can reduce fan aerodynamic efficiency and increase fan tone noise. 

While inflow distortion is one of many possible sources of fan tone noise, it can be a dominant noise 

generation mechanism (Ref. 1). 

Fan system designers need computer programs to estimate the tone noise produced when a fan ingests 

distorted inflow in order to develop quiet systems. With such tools, engineers can compare different inlet 

configurations and select a final design that minimizes fan tone noise. When inlet distortion cannot be 

eliminated, detailed tone noise predictions can be used to develop passive and active noise control 

methods that, ideally, would reduce tone noise without further sacrificing aerodynamic performance. 

Fan inflow distortion tone noise is currently being studied computationally and experimentally at the 

NASA Glenn Research Center (NASA Glenn). SMK, a computer program that models fan inflow 

distortion tone noise based on the theory of Sofrin, Mathews and extended by Koch has been developed at 

NASA Glenn. Estimates from this tone noise model have been compared to data from prior experiments 

conducted in the NASA Glenn Advanced Noise Control Fan (ANCF) rig (Ref. 2). In the ANCF inlet 

distortion experiments, the inflow to the 1.2 m (48 in.) axial low-speed fan was distorted by cylindrical 

rods inserted radially into the inlet duct. The rods were arranged in both symmetric and asymmetric 

circumferential patterns. In-duct and farfield sound pressure level measurements were used to validate 

predictions. These studies have been used to identify strengths and weaknesses of the theory, and to 

identify trends in the acoustic performance of the ANCF fan. 

 

1

NASA Glenn Research Center, research performed as a cooperative education student. 

2

NASA Glenn Research Center, research performed as a cooperative education student. 
NASA/TM—2012-217681 2 

This theory has been used in the present report to predict the sound power levels in the ducting of a 

conceptual spacecraft ventilation fan 0.089 m (3.5 in.) in diameter. Predictions have been generated for 

three inflow conditions—an undistorted flow, a circumferentially symmetric distortion pattern, and a 

circumferentially asymmetric distortion pattern. Again, it is assumed that cylindrical rods inserted radially 

into the inlet duct upstream of the fan distort the flow. These acoustic predictions complement 

aerodynamic predictions already published for this fan (Ref. 3). 

Acoustic and aerodynamic measurements are needed to validate the predictions for this spacecraft 

ventilation fan. Using the aerodynamic design of the spacecraft cabin ventilation fan as a starting point, a 

conceptual design study has been completed. The goal of the conceptual design study was to identify 

mechanical and electrical design features that would be needed in order to collect detailed aerodynamic 

and acoustic measurements of the fan in ground tests. This study will be summarized here, and challenges 

associated with experimentally measuring the aerodynamic and acoustic performance of a small fan 

ingesting distorted inflow will be discussed. 

Fan Tone Noise Predictions 

Details of the aerodynamic design of the spacecraft ventilation fan used in the acoustic study here 

have been published in Reference 3. As listed in Table 1, the axial fan has 9 rotor blades and 11 stator 

vanes. The blade-to-vane ratio was chosen so that the tones at the first three harmonics of blade passing 

frequency (BPF) were predicted not to propagate (cutoff) at design point speed of 12,000 rpm for the 

undistorted inflow condition. The fan aerodynamic design has been plotted in Figure 1. 

TABLE 1.—SPACECRAFT CABIN FAN DESIGN POINT 

GOALS AND PREDICTED/MODEL VALUES 

 Predicted/model value 

Flow rate 0.710 m3/s (150 cfm) 

Total pressure rise 925 Pa (3.72 in. of water) 

Pressure 101 kPa (14.7 psia) 

Temperature 21 C (70 F) 

Duct diameter 0.089 m (3.5 in.) 

Maximum axial length 0.22 m (9.0 in.) 

Rotor tip clearance gap 0.23 mm (0.0090 in.) 

Rotor speed 12,000 rpm 

Number of blades 9 

Number of vanes 11 

Figure 1.—Spacecraft cabin ventilation fan aerodynamic design. 
NASA/TM—2012-217681 3 

Figure 2.—Fan inflow distortion rod configurations 

used for tone noise predictions. 

Figure 3.—Predicted tone sound power levels for each blade passing 

frequency harmonic. The overall result is the summation of the first 

seven harmonics. 

Three fan inflow conditions were studied: an undistorted inflow, a circumferentially symmetric 

inflow distortion pattern, and a circumferentially asymmetric inflow distortion pattern. To create a 

symmetric inflow distortion, three cylindrical rods were assumed to be inserted radially into the flowpath 

at 15, 135, and 255 as shown in Figure 2(a). To create an asymmetric inflow distortion, three 

cylindrical rods were assumed to be inserted radially into the flowpath at 15, 52 and 173 as shown in 

Figure 2(b). The angular positions of the distortion rods were chosen so they would not be aligned with 

any of the stator vanes downstream of the rotor. 

Figure 3 summarizes the predictions for the sound power levels for the spacecraft ventilation fan 

generated by the interaction of the rotor, rods, and stator vanes using the SMK fan tone noise prediction 

code. When the fan rotates at design point speed, the first ten harmonics of blade passing frequency are 

potentially audible (1,800 to 18,000 Hz). The overall sound power level shown in Figure 3 is a 

summation over all duct modes of the sound power levels for each of the first seven BPF harmonics. 

Sound power levels evaluated at the inlet and far exhaust planes were predicted to contribute equally to 

the overall sound power level. When the flow entering the fan is not distorted, the first three harmonics of 

the blade passing frequency tone are predicted to be cutoff by design. When rods distort the fan inflow, 

though, sound power levels calculated at the inlet and exhaust planes indicate that tones at all BPF 

harmonics are produced. For nearly all BPF harmonics, the sound power levels produced by asymmetric 
NASA/TM—2012-217681 4 

inflow distortion are predicted to be greater than those of the symmetric distortion studied here. Previous 

tests in the large ANCF rig have indicated that the while the predicted overall sound power levels at the 

higher harmonics were significantly larger than measured values, examining trends in the first few BPF 

harmonics can be useful in designing low noise inlets. Experimental data is needed to validate the 

predictions for the smaller spacecraft cabin vent fan to determine if similar conclusions can be drawn. 

Each blade passing frequency tone is examined more closely in Figures 4 to 6. Figure 4 shows 

acoustic predictions for the undistorted fan; Figure 5 shows acoustic estimates for the symmetric inflow 

distortion case; and Figure 6 shows acoustic predictions for the asymmetric inflow distortion 

configuration. All results are for the 12,000 rpm fan speed. Each figure contains seven plots--one for each 

blade passing frequency harmonic (1 through 7 BPF). Each plot displays the estimated sound power level 

in the inlet duct for each propagating circumferential and radial mode. The rods upstream of the rotor are 

predicted to generate acoustic energy in more (but not all) circumferential duct modes (as compared with 

the undistorted case) that are allowed to propagate. Previous tests in the large ANCF rig have indicated 

that the theory did not accurately predict trends in radial mode sound power distribution, but that by 

examining trends in the first radial mode for the first few BPF harmonics, it can be useful in designing 

low noise inlets. Again, experimental data is needed for the much smaller spacecraft cabin vent fan to 

validate the predictions presented here. 

The development of computer programs that can estimate duct mode sound power levels when an 

axial fan ingests distorted inflow is the subject of continuing research at NASA Glenn. Modeling more 

distributed inflow distortions, such as those that would occur if a fan were installed downstream of a 

convoluted duct, is a near term goal. When it is not possible to avoid fan inflow distortion, experiments 

on the larger Advanced Noise Control Fan at NASA Glenn have suggested that the SMK code might be 

able to be applied to aid in designing low noise inlets for some fans. Obtaining acoustic data for this 

spacecraft cabin ventilation fan is needed to determine whether the theory can be useful for parametric 

design studies for this class of fans. 

Challenges of Validation 

Experimental data is required to validate the overall sound power levels for the spacecraft cabin vent 

fan. The results shown in Figure 3 could be validated either by farfield or in-duct sound pressure level 

measurements. Farfield sound pressure level measurements are required to determine if the motor is a 

significant source of noise for this spacecraft cabin vent fan. A test technique is needed to accurately 

measure the farfield sound pressure levels for the fan as it is throttled through its expected operating 

range. The pressure rise at design point for this fan (925 Pa) exceeds the maximum recommended 

(750 Pa) in ISO 10302—Method for the measurement of airborne noise emitted by small air moving 

devices. Reference 4 describes a modification to the ISO 10302 techniques that could be considered for 

testing this spacecraft cabin vent fan. 

Previous tests in the ANCF rig have indicated that the SMK code predicted higher than measured 

values for radial mode sound power levels. In order to validate the predictions shown in Figures 4 to 6 

in-duct sound pressure levels would need to be measured. While in-duct sound pressure measurements 

could also be used to validate the results shown in Figure 3, in-duct sound pressure measurements could 

not be used to quantify motor noise. Test techniques are also needed for these in-duct measurements. The 

duct diameter for this fan (89 mm) falls below the minimum diameter required (150 mm) by ASHRAE 

Standard 68—Laboratory method of testing to determine the sound power in a duct. 

Experimental data is needed to validate the aerodynamic predictions reported in Reference 3. The 

aerodynamic predictions indicate that the rotor blade Reynolds number at design point is 75,000 and 

stator vane Reynolds number is approximately 60,000. Aerodynamic performance predictions were 

generated using both a low-Reynolds number turbulence model and a high-Reynolds number turbulence 

model. The fan performance at design point conditions was also estimated for three values of the rotor tip 

clearance gap. Aerodynamic data is needed to validate these Computational Fluid Dynamics code 

predictions in this low Reynolds number regime. 
NASA/TM—2012-217681 5 

Figure 4.—Predicted inlet duct mode sound power levels for the undistorted inflow case. 

Total circumferential mode sound power level is shown in red adjacent to the sound 

power level estimates for each propagating radial mode. 
NASA/TM—2012-217681 6 

Figure 5.—Predicted inlet duct mode sound power levels for the symmetric inlet distortion 

case. Total circumferential mode sound power level is shown in red adjacent to the 

sound power level estimates for each propagating radial mode.
NASA/TM—2012-217681 7 

Figure 6.—Predicted inlet duct mode sound power levels for the asymmetric inlet 

distortion case. Total circumferential mode sound power level is shown in red adjacent 

to the sound power level estimates for each propagating radial mode.
NASA/TM—2012-217681 8 

Using the aerodynamic design as a starting point, a mechanical and electrical conceptual design 

study has recently been conducted at NASA Glenn. A cross section of a fan thought to include features 

desirable for a series of acoustics and aerodynamic ground tests is shown in Figure 7. A modular design 

was chosen to facilitate the development of active and passive noise control methods. Struts downstream 

of the stator vanes were added in this conceptual design because the stator vanes are very thin and would 

be difficult to route the motor wiring through. A honeycomb section was added upstream of the fan to 

straighten the airflow and decrease the turbulence entering the fan. A tongue and groove method was 

chosen to join the modular components over flanged connections in an effort to accommodate 

instrumentation anticipated for possible in-duct acoustic and aerodynamic measurements. 

Rapid prototyping (RP) technology is an effective way to quickly fabricate relatively inexpensive 

models of this small spacecraft ventilation fan that are suitable for research ground testing. Based on the 

conceptual design study conducted at NASA Glenn, a Selective Laser Sintering (SLS) or Direct Metal 

Laser Sintering (DMLS) rapid prototyping process is currently recommended. Using traditional 

manufacturing techniques to reproduce the contoured flow paths, fan blades, and stator blades would take 

weeks instead of days with a SLS rapid prototype process. 

These RP methods are additive manufacturing process that uses a laser to fuse very small particles 

(20 to 50 μm) of plastic or metal powder into a solid part. The laser selectively fuses powdered material 

on cross-sections of a solid model part that is generated by a computer aided design (CAD) program. As 

each cross section is fused, the powder bed is lowered by one cross section thickness and a new layer is 

sintered. This process is repeated until the part is completed. 

A preliminary rotor stress analysis was conducted for the conceptual design of the fan shown in 

Figure 7. The analysis determined stresses and displacements of the model components when specified 

loads were applied. Stress induced by loading, multiplied by a factor of safety, shall not exceed the 

material's ultimate strength properties. Additionally, displacements at the fan blade tips shall not exceed 

the blade/casing gap distance of 0.23 mm (0.0090 in.), as prolonged rub may cause premature fan blade 

failure. MSC Nastran was used to perform the rotor finite element stress analysis. Aerodynamic loading 

was considered negligible in this preliminary analysis, so the only loading applied was the result of 

Figure 7.—Fan conceptual design cross section. 

9.0 in 
NASA/TM—2012-217681 9 

rotational speed of 14,000 rpm about the rotor's central axis. The results from this stress analysis indicated 

that three metals (17-4 PH Stainless Steel, EOS Titanium Ti64, Aluminum 6061-T6) and two plastics (3D 

Duraform GF, 3D Duraform HST) are candidate materials for the ground test fan. 

A preliminary rotor modal analysis was also conducted at NASA Glenn. Modal analysis extracts 

frequencies of excitation from a model. If the motor were to vibrate at these frequencies, it could excite 

the rotor, causing resonance. During testing, it was assumed that the rotor would be run at various speeds 

in the test range of 6,000 to 14,000 rpm. Resonant frequencies may be passed through, but should not be 

dwelled at during testing. MSC Nastran was used to carry out finite element modal analysis. Resonant 

frequencies of the rotor were found for the five materials listed above. The analysis showed that the 

targeted test speed of 12,000 rpm is within resonance range for all five material candidates and rotor hub 

redesign is needed in order to mature this concept to test-worthy hardware. 

Several commercially available motors were identified that could potentially be used to drive the fan. 

The controllable speed of the fan was assumed to range from 6,000 to 14,0001.0 rpm as the fan flow is 

throttled through is expected operating range. While the prototypes may be useful for proof-of-concept 

tests, there may be differences in performance between a rapid prototype of the fan and a metal fan 

produced by traditional manufacturing techniques that is designed to meet spaceflight requirements. For 

example, a motor that is able to drive a plastic fan rotor may not be adequate for driving a heavier metal 

rotor, and the two motors may have different acoustic performance. So while rapid prototypes may prove 

to be useful in some research and development tests, acoustic tests of the fans produced for spaceflight 

should also be conducted. Comparisons should be made between acoustics tests of the rapid prototype 

fans and the spaceflight fans in an effort to identify any differences in performance and work towards 

future improvements. 

Selecting and accommodating instrumentation for in-duct acoustic and aerodynamic measurements is 

expected to be a challenge. Annulus height at the stator leading edge is approximately 0.011 m (0.45 in.) 

and the duct diameter is 0.089 m (3.5 in.). If a hotwire probe were to be chosen for rotor wake velocity 

measurements, a hole through the duct wall would be needed to insert the probe. A radial traverse 

mechanism would also be needed to sweep the probe from hub to tip. If a hotwire probe were to be 

chosen for stator wake velocity measurements, a circumferential slot and seal in the duct may be needed 

in order to move the probe through the stator vane wake region. Measuring the inflow velocities upstream 

of the fan is a challenge, as well. The use of Particle Image Velocimetry to map the inflow velocities 

should also be examined more closely. 

Conclusion 

Tone noise predictions have been presented for a spacecraft cabin ventilation fan ingesting distorted 

inflow. Three inflow distortion patterns were examined—an undistorted inflow, a circumferentially 

symmetric inflow and a circumferentially asymmetric inflow. The SMK fan tone noise prediction code 

developed at NASA Glenn was used to estimate duct mode sound power levels. These acoustic 

predictions complement aerodynamic predictions previously reported. 

Acoustic and aerodynamic data is needed to validate all predictions for this fan. Towards that goal, a 

conceptual design study has been conducted. Using the aerodynamic design of the spacecraft cabin 

ventilation fan as a starting point, a fan design suitable for a series of ground tests has been generated. 

Preliminary rotor stress analyses indicate that rapid-prototyping may be a cost-effective way to fabricate 

fan models for research. Preliminary rotor modal analyses indicated that several metal and plastic 

materials could be used, but a rotor redesign is needed to mature this concept to test hardware. 

Test techniques still need to be identified for this fan, and the instrumentation needed for these tests 

must still be selected. Accommodating any instrumentation for in-duct acoustic or aerodynamic 

measurements is expected to be a challenge given the size of the fan. 
NASA/TM—2012-217681 10 

References 

1. Fitzgerald, J.M., Lauchle, G.C., Reduction of discrete frequency noise in small, subsonic axial-flow 

fans, Journal of the Acoustical Society of America, 76(1), 158–166, 1984. 

2. Koch, L.D., “Validation of the Predicted Circumferential and Radial Mode Sound Power Levels in the 

Inlet and Exhaust Ducts of a Fan Ingesting Distorted Inflow,” To be presented at the 17th 

AIAA/CEAS Aeroacoustics Conference, Portland, OR, June 5–7, 2011, publication as a NASA 

Technical Memorandum (TM) pending. 

3. Tweedt, D.L., “Aerodynamic Design and Computational Analysis of a Spaceflight Vehicle Cabin 

Ventilation Fan,” NASA/TM—2010-216329 2009. 

4. Bard, S.E. and Nobile, M.A., “Acoustical implications of airflow impedance on both the inlet and 

outlet of an air-moving device,” NC08-179, 2008. 

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Tone Noise Predictions for a Spacecraft Cabin Ventilation Fan Ingesting Distorted Inflow and 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

A fan tone noise prediction code has been developed at NASA Glenn Research Center that is capable of estimating duct mode sound power 

levels for a fan ingesting distorted inflow. This code was used to predict the circumferential and radial mode sound power levels in the inlet 

and exhaust duct of an axial spacecraft cabin ventilation fan. Noise predictions at fan design rotational speed were generated. Three fan 

inflow conditions were studied: an undistorted inflow, a circumferentially symmetric inflow distortion pattern (cylindrical rods inserted 

radially into the flowpath at 15°, 135°, and 255°), and a circumferentially asymmetric inflow distortion pattern (rods located at 15°, 52° and 

173°). Noise predictions indicate that tones are produced for the distorted inflow cases that are not present when the fan operates with an 

undistorted inflow. Experimental data are needed to validate these acoustic predictions, as well as the aerodynamic performance predictions. 

Given the aerodynamic design of the spacecraft cabin ventilation fan, a mechanical and electrical conceptual design study was conducted. 

Design features of a fan suitable for obtaining detailed acoustic and aerodynamic measurements needed to validate predictions are 

discussed. 

15. SUBJECT TERMS

Fans; Noise 

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