NASA/TM—2000-209916

October 2000

James H. Dittmar, David M. Elliott, Robert J. Jeracki, and Royce D. Moore

Glenn Research Center, Cleveland, Ohio

Tony L. Parrott

Langley Research Center, Hampton, Virginia

The Alternative Low Noise Fan
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The Alternative Low Noise Fan

NASA/TM—2000-209916

October 2000

National Aeronautics and

Space Administration

Glenn Research Center

James H. Dittmar, David M. Elliott, Robert J. Jeracki, and Royce D. Moore

Glenn Research Center, Cleveland, Ohio

Tony L. Parrott

Langley Research Center, Hampton, Virginia
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Hanover, MD 21076

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Aeronautics and Space Administration.
NASA/TM—2000-209916 1

THE ALTERNATIVE LOW NOISE FAN

James H. Dittmar, David M. Elliott, Robert J. Jeracki, and Royce D. Moore

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Tony L. Parrott

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23681

SUMMARY

A 106 bladed fan with a design takeoff tip speed of 1100 ft/sec was hypothesized as reducing perceived noise

because of the shift of the blade passing harmonics to frequencies beyond the perceived noise rating range. A 22 in.

model of this Alternative Low Noise Fan, ALNF, was tested in the NASA Glenn 9×15 Wind Tunnel. The fan was

tested with a 7 vane long chord stator assembly and a 70 vane conventional stator assembly in both hard and

acoustically treated configurations. In addition a partially treated 7 vane configuration was tested wherein the

acoustic material between the 7 long chord stators was made inactive.

The noise data from the 106 bladed fan with 7 long chord stators in a hard configuration was shown to be

around 4 EPNdB quieter than a low tip speed Allison fan at takeoff and around 5 EPNdB quieter at approach.

Although the tone noise behaved as hypothesized, the majority of this noise reduction was from reduced broadband

noise related to the large number of rotor blades. This 106 bladed ALNF is a research fan designed to push the technology limits and as such is probably not a practical device with present materials technology. However, a low tip

speed fan with around 50 blades would be a practical device and calculations indicate that it could be 2 to 3 EPNdB

quieter at takeoff and 3 to 4 EPNdB quieter at approach than the Allison fan.

7 vane data compared with 70 vane data indicated that the tone noise was controlled by rotor wake—stator

interaction but that the broadband noise is probably controlled by the interaction of the rotor with incoming flows.

A possible multiple pure tone noise reduction technique for a fan/acoustic treatment system was identified.

The data from the fully treated configuration showed significant noise reductions over a large frequency range

thereby providing a real tribute to this bulk absorber treatment design. The tone noise data with the partially treated

7 vane configuration indicated that acoustic material in the source noise generation region may be more effective

than similar material outside of the generation region.

INTRODUCTION

The conventional method for reducing fan noise has been to go to a low tip speed, high bypass ratio fan. This

type of fan would typically have a low number of rotor blades with the number of stator vanes selected to provide

for cutoff of the blade passing tone. In references 1 and 2 an alternative method was proposed to reduce the perceived noise of the fan tones. A paper study was conducted in these references on a 6 ft diameter fan. The baseline

fan had 53 rotor blades and 90 stator vanes. This fan had an 1100 ft/sec tip speed. A conventional low speed design

approach to the fan was undertaken. This resulted in a 20 bladed fan at 800 ft/sec tip speed with 44 stator vanes. An

alternative design approach which maintained the 1100 ft/sec tip speed was also investigated. This alternative

method used a high number of rotor blades to shift the harmonic tones to frequencies above the perceived noise rating range leaving only the blade passing tone to be rated. A set of low number, long chord stator vanes was then

used to reduce the level of the blade passing tone. In reference 1 the noise reduction possibilities of the two design

approaches, conventional and alternative, were evaluated using hypothetical spectra. The noise reduction possibilities were calculated as reductions from the baseline fan noise level.

Figure 1(a) shows a hypothetical noise spectra for the baseline fan. This is for a 6 ft fan with 53 rotor blades

turning at 1100 ft/sec tip speed. The fan has 90 stator vanes resulting in a cuton fan such that the blade passing tone
NASA/TM—2000-209916 2

propagates out the fan ducting to the far field. In this figure the blade passing tone is arbitrarily set at 100 dB and

each harmonic is reduced 3 dB from the previous one. As can be seen there are three harmonics in the perceived

noise rating range.

The conventional approach to noise reduction is represented in figure 1(b). Here the fan has 20 rotor blades

turning at 800 ft/sec tip speed. The fan has 44 stator vanes which results in a cutoff fan so no blade passing tone is

shown in the spectrum. The lower tip speed results in all of the tones being reduced by 6 dB from those of the

baseline fan. The harmonics again are each reduced 3 dB from the previous one. As seen in figure 1(b) there are

13 tones in the rated range. The large number of tones is the result of the lower tip speed and the lower number of

rotor blades combining to provide a lower blade passing frequency. Each of these tones is much lower in magnitude

than the baseline fan tones. The net result is that the conventional low noise fan is 9.1 PNdB quieter than the

baseline fan.

The spectrum for the alternative low noise fan is seen in figure 1(c). This fan has 106 rotor blades turning at

1100 ft/sec which shifts all but the blade passing tone to frequencies above the perceived noise rating ranges. The

fan has 14 long chord stator vanes which should result in a 6 dB reduction in the blade passing tone. The perceived

noise reduction of this alternative approach is 10.4 PNdB, slightly more reduction than the conventional fan’s noise

reduction.

To verify the predicted noise reduction potential of the alternative low noise fan, a 22 in. diameter model was

constructed and tested in the Glenn 9×15 Foot Low Speed Wind Tunnel. This paper presents the design of this

Alternative Low Noise Fan (ALNF) and the results from the acoustic testing in the wind tunnel.

APPARATUS AND PROCEDURE

A 0.305 scale model of the Alternative Low Noise Fan, 22 in. in diameter, was built and tested in the Glenn

9×15 Foot Low Speed Wind Tunnel. This fan was designed with 106 rotor blades and 14 long chord stator vanes to

be an alternative approach to a conventional low tip speed, low rotor blade number, cutoff fan such as the Allison

Engine Company model fan also tested in the 9×15 wind tunnel (ref. 3). The Alternative Low Noise Fan was also

tested using a more conventional 70 vane stator to separately examine the noise reduction achieved by the low number of long chord stators.

Aerodynamic Design

The aerodynamic design point of the Alternative Low Noise Fan was taken to be the takeoff condition. Here

the fan was designed to produce a stage pressure ratio of 1.3 at 1100 ft/sec tip speed. An existing flow path from a

20 in. fan tested previously in a test cell at Glenn Research Center (ref. 4) was scaled by a factor of 1.1 and used for

this fan. Cross section drawings of the ALNF with 14 and 70 stators are found in figures 2(a) and (b) respectively.

Both the 14 and 70 vane sets have the same solidity. Table I shows some of the key parameters for the ALNF fan

design. A complete aerodynamic design for the fan stage is found in appendix A and the blade coordinates are

shown in appendix B. The design in appendix A is for the 106 bladed rotor with the 14 vane stator. The 70 vane

design has the leading edge of the 70 vane stator at the same location as the 14 vane stator and has the same stator

inlet and exhaust conditions. The fan was designed to be tested with both hard wall and acoustically treated flow

paths.

Acoustic Treatment Design

The acoustic treatment design goal was to reduce the noise at the blade passing frequency, ~18 000 Hz at takeoff for the scale model fan, and to reduce multiple pure tones that might be roughly centered at 9 000 Hz for the

22 in. model fan. To accomplish the reduction at blade passing frequency the design concentrated on reducing the

92, 78 and 64 spinning mode orders. A single degree of freedom liner was designed with a target design impedance

of (2.67+1.90i) ρc. A perforated sheet over bulk absorber design was chosen to achieve this impedance. The perforated plate had 30 percent open area and was designed to be 0.037 in. thick with 0.048 in. diameter holes. The bulk

absorber was Kevlar packed to a 4.5 lb/ft3 density in a 0.325 backing depth. A sketch of this acoustic absorber
NASA/TM—2000-209916 3

design is shown in figure 3(a). Because of strength requirements it was not possible to fabricate the flow path pieces

with perforated sheet as thin as the desired thickness. The actual liner as fabricated is sketched in figure 3(b). The

sheet thickness is 0.062 in. with the side facing the bulk absorber having been countersunk so that the straight portion of the hole is 0.037 in. long before the countersunk portion begins. It was hypothesized that this perforated plate

would act acoustically the same as the designed plate of 0.037 in.

The acoustic treatment was installed in the fan nacelle as shown in figure 2. The material was on the outer flow

path wall of the inlet and on the inner and outer flow path surfaces between the rotor and stator and downstream of

the stator. In the long chord stator version of figure 2(a), acoustic treatment was also placed on the inner and outer

flow path walls in the space between the stator vanes.

Fan Noise Testing

The Acoustic testing was done in the NASA Glenn 9×15 Anechoic Wind Tunnel. A photograph of the Alternative Low Noise Fan in this facility is shown in figure 4. The acoustic treatment on the tunnel walls is shown in the

photograph along with a traversing microphone used to acquire noise data. The acoustic data were obtained by both

the traversing microphone and fixed microphones installed in the wind tunnel. A top view sketch of the tunnel showing the traversing and fixed microphones is shown in figure 5.

A photograph of the 106 bladed fan is shown in figure 6(a). Here the complexity of fitting 106 blades into the

hub can be seen with two different types of blades, short and long stem, being needed to fit into the hub, figure 6(b).

A photograph of the 14 vane stator assembly is shown in figure 7 and the 70 vane stator is shown in figure 8. Acoustic data were taken both with a hard wall nacelle and with the acoustically treated nacelle. The acoustically treated

configurations were tested first. In going from the acoustically treated to the hard configurations most of the nacelle

acoustic surfaces were replaced with hard wall pieces. However for economy of manufacture, the pieces between the

stators in the long chord configuration and the pieces directly behind the stators in the short chord configuration

were made hard by filling the holes in the acoustic material with an epoxy material. This process then dictated that

the acoustically treated configurations be tested first before the holes were filled.

During the initial part of the testing schedule with the acoustically treated nacelle, aerodynamic data were

obtained before acoustic data were taken. This aerodynamic data showed that the fan flow was limited by internal

flow problems. This occurred with both the short and long chord stator versions. A plot of pressure ratio versus

weight flow is shown in figure 9. More complete aerodynamic information is found in reference 5. The first configuration, 14 vanes with acoustic treatment, showed higher pressure ratios and lower weight flows than design. Changes

in the fan nozzle area did not bring about the flow increases expected and high losses in the duct were suspected.

An attempt was made to improve the flow rate by removing every other one of the 14 long chord stators, resulting in

a 7 vane configuration. These vanes were permanently cut out of the stator assembly. A photograph of this 7 vane

configuration is found in figure 10. The removal of every other vane increased the flow but did not bring the weight

flow to the desired level. Configurations with 70 stator vanes and with only partial treatment showed additional improvements. However, the proper flows were only obtained for both the 7 and 70 vane configurations when the fan

was in the hard wall configuration. This indicates that in some way the acoustic treatment was the cause of the flow

problem. Some possible causes might include extra frictional losses from flow over the perforated plate or cross

flows inside the bulk material in the treatment.

Acoustic data were obtained for the 7 and 70 vane configurations with both hard wall and acoustically treated

nacelles. In addition a 7 vane configuration was tested where the nacelle was acoustically treated but the region

between the long chord stators was hard. This configuration was an attempt to determine if the acoustic treatment

between the stators was more effective because of its proximity to the source than other treatment in the nacelle.

Table II shows a list of the tested configurations.

The Alternative Low Noise Fan was designed to achieve a 1.3 pressure ratio at 1100 ft/sec tip speed which was

the takeoff condition. However, during the testing of the ALNF, the rotor blades encountered a high stress region at

speeds greater than 1000 ft/sec tip speed. This appeared to be a flutter instability and the stress level increased with

increasing tip speed. The coating material on the mid span dampers also showed wear after operating at tip speeds

above 1000 ft/sec. Some aerodynamic data were obtained at tip speeds above 1000 ft/sec because of the short time

needed to obtain the data but the acoustic data was limited to 1000 ft/sec because it was not deemed prudent to

remain at these high stress levels for the time needed to make an acoustic survey, ~20 min. Table III shows the tip

speeds, from 600 to 1000 ft/sec where the acoustic data were obtained for the 5 configurations listed in table II.
NASA/TM—2000-209916 4

The acoustic data were obtained by the traverse at 48 steps yielding emission angles from 24.6° to 135° at a

tunnel Mach number of 0.1 and by three fixed location microphones in the aft yielding emission angles of 136.4°,

147.2° and 158.1°. Two narrow band spectra were taken of the data at 5.9 and 59 Hz bandwidth. From these a 1/3rd

Octave spectra was also constructed. Sound power and PNdB values were calculated from the 1/3rd octave data. All

of the noise data presented in the report are in 1 ft loss less form at the emitted angles unless otherwise specifically

noted. Appendix C contains noise data taken at roughly 10° intervals for the 59 Hz bandwidth narrow band spectra

and the 1/3rd octave spectra.

RESULTS AND DISCUSSION

Hard Wall Configurations

Comparison of 7 Vane Data With Allison Fan.—In a previous report, reference 6, some comparisons were

made between the hard wall data for the ALNF and the Allison fan at the takeoff condition. This comparison was to

be at 1100 ft/sec tip speed for the ALNF and 840 ft/sec for the Allison fan. At these conditions they were both to

yield a pressure ratio of 1.3. As mentioned previously, the ALNF data were limited to 1000 ft/sec which yielded a

pressure ratio of 1.24. At the 840 ft/sec tip speed the Allison fan had a 1.26 pressure ratio. (This is a slight correction

from the 1.27 pressure ratio stated in reference 6.) Using a noise variation of 10 log thrust the pressure rise ratio of

0.26/0.24 would result in approximately a 0.4 dB correction that should be added to the 1000 ft/sec ALNF data for

comparison with the Allison data. For most of the direct spectrum to spectrum comparisons of the two fans, this

0.4 dB is not significant and is not included on the plots. It is however, included in the effective perceived noise

comparisons that will follow.

Some of the information from reference 6 is repeated herein for completeness. A comparison between the 7

vane stator ALNF at 1,000 ft/sec tip speed and the Allison fan at 840 ft/sec is shown in figure 11. Here narrow band

spectra from 0 to 50 000 Hz with a bandwidth of 59 Hz is shown in part A at 92.5°. This spectrum was presented in

Reference 6 as being at 90° but an error was made in the indexing process and the spectra are actually at 92.5°. For

consistency with the previous report, the 92.5° position will be used throughout this report. These data are shown as

taken on the 22 in. diameter model which has frequencies higher than they would be for the 6 ft diameter full scale

fan. For example, the ALNF blade passing tone is apparent around 18 000 Hz and the tone at twice blade passing

frequency is just visible near 36 000 Hz. On the 6 ft diameter fan these would be about 5 000 and 10 000 Hz. On this

figure the blade passing tone for the Allison fan would be around 2 750 Hz but since it is cut-off it is not visible in

the spectra. Tones at twice blade passing frequency, 5 500 Hz and four times blade passing frequency 11 000 Hz are

clearly visible and the tone at 3 times blade passing frequency, around 8 250, can be seen above the background.

These would be lower on the full scale fan with blade passing frequency being around 840 Hz, 2 bpf at 1680 Hz,

3bpf at 2520 and 4bpf at 336 0Hz.

As can be seen by looking at figure 11(a), not only has the ALNF achieved its goal of being as quiet as the

Allison fan but it appears even quieter. The tone levels of the ALNF would have a lower perceived noise rating than

the tones of the Allison fan not only because of the frequency shift to the lower rated regions of the spectra as indicated by reference 3 but also because the levels themselves are lower. As can be seen again in figure 11(a), the blade

passing tone sound pressure level of the ALNF is lower than that of the 2 times blade passing frequency tone of the

Allison fan and is about the same level as the 3 times blade passing frequency tone. Therefore the perceived noise of

the tones for the ALNF will be lower than the perceived noise of the tones for the Allison fan.

Possibly more interesting, however, is the broadband noise reduction apparent in the spectra. The ALNF

appears to have lower broadband noise over a frequency range from about 2 000 to 16 000 Hz. The broadband noise

reduction would be from about 600 to 5 000 Hz for the full scale 6 ft diameter fan. This broadband reduction would

have an even larger noise reduction effect on the perceived noise than would the lower tones.

The magnitude of the effect of the broadband noise reduction can be better observed in 1/3rd octave spectra as

used by the perceived noise rating method. Figure 11(b) shows the 1/3rd octave spectra for the 92.5° angle. When

viewed on the 1/3rd octave basis, the broadband noise reduction becomes even more evident. This broadband noise

reduction would result in perceived noise reductions at most reasonable fan sizes.

A 1/3rd octave power level plot is shown in figure 12. This power level result, integrated over the entire data

survey shows the significance of the broadband reduction.
NASA/TM—2000-209916 5

An indication of this perceived noise reduction can be seen in figure 13. Here the Effective Perceived Noise for

a single fan flyover at 1000 ft altitude on a standard day has been calculated for the Allison fan and the ALNF at

scale factors ranging from 2 to 6. Here the EPNdB of the ALNF has been increased by 0.4 dB to account for the

lower pressure ratio. The scale factor is the ratio of the full scale fan diameter to the 22 in. fan diameter. For

example, the 6 ft diameter fan would be at a scale factor of ~3.27.

As can be seen the ALNF is significantly quieter at all of the scale factors shown. As the scale factor increases

the two fans start to come closer together in level. This is because the frequency shift of the larger scale has

brought the higher frequencies of the model data, where the two fans have the same levels (20 000 Hz and above,

figure 11(b)), down into the most highly rated range of the EPNdB calculation.

The previously reported results from reference 6, and summarized above, were for comparisons at the takeoff

condition. A further comparison was made at the approach condition and also showed the ALNF to be quieter. Figure 14 shows spectral comparisons at the 92.5° angle for the ALNF and the Allison fan at the approach condition.

Here the Allison fan approach condition is taken as 50 percent speed, 500 ft/sec tip speed, with a pressure ratio of

1.083. The ALNF data were taken from the condition closest to the Allison fan conditions. Here the ALNF data

were taken at 6402 rpmc and a pressure ratio of 1.075. Using the same 10 log of thrust correction, the pressure rise

ratio of 0.083/0.075 yields a correction of 0.35 dB that should be added to the ALNF data. Again this correction is

not applied to the spectrum but it is applied to the perceived noise calculations.

Figure 14(a) shows the narrow band data, 0 to 50 000 Hz with a bandwidth of 59 Hz for the 22 in. model fan.

Again, as for the takeoff case, the blade passing tone of the ALNF (around 12,000 Hz) is lower in level than the

Allison 2 BPF tone (around 5,000 Hz). As with the takeoff data the ALNF tones at approach will have a lower perceived noise contribution than the Allison tones. Figure 14(a) also shows broadband reduction as did the takeoff

data in the 0 to 10 000 Hz range but some broadband noise increase is also shown at higher frequencies that was not

observed in the takeoff data.

Figure 14B shows the 1/3rd octave spectra at this 92.5° angle. The large region where the ALNF is quieter,

1 000 to 10 000 Hz, appears to more than overcome the areas where the ALNF is noisier. Figure 15 shows the overall power level comparison for the two fans at approach power. Again the large area of noise reduction for ALNF

overshadows the area of noise increase.

The approach flyover noise at 1000 ft, standard day conditions is shown in figure 16. Calculations are shown for

Allison fan and the ALNF at scale factors ranging from 2 to 6. (A 6 ft diameter fan would have a scale factor of

3.27.) In figure 16, 0.35 dB has been added to the ALNF data to correct for its slightly lower pressure ratio. At the

lower scale factors where the low frequency for the scaled ALNF reduction dominated (fig. 14), significant EPNdB

reductions are seen for the ALNF. At a scale factor of 2 the advantage is over 10 EPNdB. As the scale factor is

increased, the higher frequencies where the noise increases for ALNF are seen (fig. 14) become part of the EPNdB

calculation and the noise increases start to offset some of the noise reduction. At a scale factor of 3, the ALNF is

about 5 dB quieter and at a scale factor of 6 the reduction is only a little over 1 EPNdB. Thus a noise improvement

is seen for all of the represented scale factors with more reduction visible at the lower scale factors.

Comparisons of the ALNF and Allison data, shown here at takeoff and approach, show a significant EPNdB

noise advantage for the high blade number ALNF. Some of this noise reduction is the result of the larger blade number shifting the tones to higher frequencies where they would be weighted lower or not weighted at all by the perceived noise calculation. This was the original design intent of the fan. However, most of the EPNdB noise

reduction results from the lower broadband levels of the ALNF.

In reference 6, two possible explanations were advanced for the broadband noise reduction: (1) A reduced turbulence strength impacting the stator brought about by a reduction in the initially generated turbulence and an increased decay of this turbulence and (2) A shift in the turbulence to smaller length scales which results in reduced

levels of low frequency noise. Both of these explanations can be related to the increased number of rotor blades.

The reduced turbulence strength explanation centers on the fact that the pressure rise of the ALNF is spread

over 106 short chord rotor blades while the Allison fan has the pressure rise provided by 18 larger chord blades. This

provides lower loading per blade for the ALNF with less losses per blade. The result would then be smaller initial

wake defects and lower initial turbulence for the ALNF. In addition, since the wake/turbulence decays with the distance downstream measured in rotor chords, the ALNF with its smaller rotor chord would have more decay resulting

in even lower turbulence levels striking the stator.

The reduced length scale mechanism comes from the increased number of rotor blades giving a smaller gap

between blades. These smaller gaps do not permit as many of the large scale turbulence eddies to be generated. This

can result in the reduction in the low frequency noise that these large scale eddies generate. Looking at this in
NASA/TM—2000-209916 6

another manner for a Liepmann type of turbulence spectra, the integral length scale for the ALNF would be much

lower than that for the Allison fan. Hanson and Horan, reference 7, have indicated that at low frequencies these

small length scales generate less noise. In figure 10 of reference 7, done for a 12 ft diameter fan (scale factor of 6.5),

significant noise reductions were observed at the low frequencies with smaller length scales. At higher frequencies,

the smaller length scales generate extra noise. This behavior is similar to the ALNF-Allison data comparison at

approach (fig. 14). The similarity of the data and theory may indicate that the length scale mechanism is the explanation for the ALNF noise reduction at approach. In summary, the ALNF shows significant EPNdB noise advantages

primarily from the broadband noise reduction brought about by the increased number of rotor blades.

The 106 bladed ALNF is a research fan designed to push the technology level and as such it would probably not

be a practical device with present materials technology. However, a fan with about 50 blades could be a practical

device. In reference 6, estimates of the noise of a low tip speed fan with 50 blades, done both with the reduced turbulence level method and the reduced length scale method, were compared with that of an 18 bladed low tip speed

fan. A noise reduction of the order of 2 to 3 EPNdB was estimated at the takeoff condition.

Using the same noise ratios for the 50 bladed fan versus an 18 bladed fan as was done in reference 6, the

approach noise would be 3 to 4 PNdB quieter for the 50 bladed fan for a fan with a scale factor of 3.0. (More reduction would occur at lower scale factors and less at higher ones.) These results reinforce the conclusion of reference 6

that a significant noise reduction would be possible for a high blade number low tip speed fan.

Variation With Speed

The noise variation with speed of the ALNF in the hard wall configuration with 7 stators is shown in figures 17

to 21. Figure 17 shows a series of narrow band spectra taken at the 92.5° angle. The narrow band spectra for the

different speed are shown two at a time so they can be more easily compared. Figure 17(a) shows the 6402 rpmc

(600ft/sec tip speed) and the 7736 rpmc (725 ft/sec) data points. Increases in both the tone and broadband noise are

seen here as the speed is increased. Figure 17(b) repeats the 7736 rpm (725 ft/sec) spectrum and adds the 8537 rpmc

(800 ft/sec) spectra. In this figure the presence of some multiple pure tones (MPTs) is observed. This is very low in

tip speed for shock induced multiple pure tones to be appearing and these MPTs may be the result of blade to blade

spacing differences. Mismatch between blade spacing is entirely possible and indeed likely with this many rotor

blades in this scale model fan. In figure 17(b), the broadband noise is seen to increase with speed but the blade passing tone and its harmonics are lower at the 8537 rpm point than at the 7736 point. This may be do in part to the

transfer of energy from these tones to the MPTs. Figure 17(c), 9604 rpmc and 8537 rpmc follows the same trend

On figure 17(d) a large increase in multiple pure tone noise is seen in going from 9604 rpmc (900 ft/sec) to

10137 rpmc (950 ft/sec). This is most likely the result of the shock induced multiple pure tone mechanism being

seen in the 10137 rpmc data. At the highest speed tested, 10671 rpmc (100ft/sec), figure 17(e), the blade passing

tone is seen to increase again.

Figure 18 shows the 1/3rd octave data for the 92.5° angle. Here the trends are the same as seen in the

narrow band data. In particular, the increase in MPT activity from 9604 to 10147 rpmc is shown most clearly in

figure 18(d).

Figure 19 shows the 1/3rd octave variation of the sound power at the different speeds. Here again in

figure 19(d) the significant increase in multiple pure tone noise is observed in going from 9604 to 10137 rpmc. As

mentioned previously, this is probably the region where the shock induced multiple pure tones are being observed.

Effective perceived noise levels were calculated for the various speed conditions and are plotted in figure 20.

These EPNdB values are for the 1000 ft flyover of one fan on a standard day. As can be seen the EPNdB level

increases as the tip speed is increased for all of the scale factors.

A typical variation of the EPNdB with speed is seen in figure 21 for the scale factor of 3.0. Here the EPNdB is

plotted versus the corrected fan rpm (rpmc). The noise is seen to increase slowly as the speed is increased from 6402

to 7736 rpmc and then to 8537 rpmc. As the speed is increased further to 9604, 10137, and 10671 rpmc, the noise

increase with speed becomes larger, thus creating a parabolic shaped curve. This corresponds to the increase in the

multiple pure tone noise which is in the rated frequency range.
NASA/TM—2000-209916 7

7 Vane Data Comparison With 70 Vane Data

As mentioned previously, a 70 vane stator was also tested to determine the noise reduction effect of the low

number of long chord stator vanes (7 vane case). Figure 22 shows the data comparison between the 7 and 70 vane

data at two angles for two fan speeds.

Figures 22(a) and (b) show the data comparisons at a subsonic tip speed, 8537 rpmc (800 ft/sec tip speed) where

no shock induced MPTs should be present and the rotor alone blade passing tone should not propagate. Part A is for

the 24.5° angle and part B is for the 130.5° angle. These are the same two data plots shown in reference 6. In reference 6, the 130.5° angle data was, because of an indexing error mentioned previously, mislabeled as 128°. Significant reductions in the blade passing tone are observed in going from 70 to 7 vanes.

Previous work has been done on the effect of stator vane number on the broadband noise generated by rotor

wake turbulence—stator interaction noise (ref. 8). This work indicated that the noise was reduced with lower stator

vane numbers, varying approximately as 10 times the log of the ratio of the stator number. Therefore it would be

expected that in going from 70 to 7 vanes that the broadband noise would be reduced about 10 dB. In the front,

24.5°, there is no effect of the vane number on the broadband noise and there is only a small 3 dB or so effect seen

in the rear, 130.5°. In both cases much less than the expected 10 dB. A probable explanation for this lack of broadband noise reduction is that the rotor wake turbulence-stator noise mechanism is no longer dominant for the broadband noise of the ALNF as a result of the large number of rotor blades. As indicated previously the rotor wake of

the 106 bladed fan is lower in initial magnitude, decays more rapidly in a fixed axial distance and has a smaller turbulence length scale. As a result the strength of the rotor wake turbulence-stator noise mechanism is reduced and

another noise mechanism becomes dominant.

In the 70 blade data at the 130.5° angle some MPT noise is seen with a significant spike at one half blade passing frequency. This is probably the result of the rotor blades having more blade to blade variations in the 70 vane

data because of mid span damper wear. This situation will be discussed more fully in the following high speed data

discussion.

Figures 22(c) and (d) show the data at 10671 rpmc, 1000 ft/sec tip speed, for the 24.5 and 130.5° angles respectively. As can be seen here again there is little broadband noise reduction indicating that some source other than

rotor wake turbulence—stator interaction is the dominant broadband mechanism. Of particular interest in this data is

the strong tone at one half blade passing frequency for the 70 vane data. At the 130.5° position this tone is significantly louder than the blade passing tone. In figure 22(d), 130.5°, the 7 vane data exhibits multiple pure tone activity

mostly in the 3000 to 4000 Hz range. The 70 vane data has most of its MPT activity at one half blade passing frequency. An explanation for this difference may be found in the construction of the rotor. Figure 6(b) shows the construction of the rotor hub. In order to fit the 106 blades into the hub, every other blade has a longer stem. In other

works, two types of blades were installed in the hub, ones with short stems and ones with long stems. As can be seen

in figure 6(a), the blades are “locked” together with mid span dampers (part span shroud). At the beginning of the

program, during the 7 vane testing, these mid span dampers fit very tightly and effectively locked the blades together even with the fan at rest. As the testing program continued some wear was observed on these part span dampers, particularly after the high speed , high stress points taken at speeds above 1000 ft/sec tip speed. During the

70 vane testing the wheel was much looser with the ability for more blade to blade movement because of the worn

dampers. A probable explanation for the tone at one half blade passing frequency is that the long and short stem

blades are twisting differently resulting in blade to blade angle changes. This would result in a wheel with a 53 cell

pattern. This then could explain the 70 vane data having its MPT activity centered around one half blade passing

frequency.

This change in the structure of the MPT noise may point to a possible noise reduction technique. It may be possible to design a fan to generate its multiple pure tone noise at one specific frequency, like one half blade passing

frequency, which could then be more effectively removed by acoustic treatment. The acoustic treatment could be

tuned to be most effective in removing the tone at this frequency and the net result would be a quieter fan/ acoustic

treatment system.

The 1/3rd octave data for two speeds, 800 and 1000 ft/sec, at two angles 24.5° and 130.5° are shown in

figure 23. Parts A and B are 24.5° and 130.5° for 800 ft/sec tip speed and parts C and D are 24.5° and 130.5° for

1000 ft/sec tip speed. These figures support the same general conclusions as drawn from the narrow band data, specifically that little change in broadband noise was observed in going from 70 to 7 vanes. This further indicates that

some source other than rotor wake turbulence—stator interaction is the primary broadband noise source for the

ALNF.
NASA/TM—2000-209916 8

The sound power level for the 7 and 70 vane configurations at 800 ft/sec tip speed and 1000 ft/sec are shown in

figure 24. Figure 24(a) shows the low speed 800 ft/sec case and on an overall power basis does show a few decibel

broadband noise reduction when going from 70 to 7 vanes at 1 000 Hz and above but much less than the expected

10 dB. At the higher speed, 1000 ft/sec for figure 24(b), some broadband reduction is observed but again much less

than expected. The difference in multiple pure tone activity is very evident in figure 24(b). The 7 vane case has its

MPTs in a broad range from 3 000 to 4 000 Hz while the 70 vane configuration has a strong spike at one half blade

passing frequency, in the 10 000 Hz band.

Sound power levels were obtained for the front and aft quadrants to further investigate the small amount of

broadband reductions observed. Ninety degrees was arbitrarily chosen for the split between front and aft power.

Figures 25(a) and (b) are for the front and aft powers at 800 ft/sec and figures 25(c) and (d) are for the front and aft

powers for the 1000 ft/sec tip speed case. For both of the front power comparisons figures 25(a) and (c), very little

broadband noise difference exists between the 7 and 70 vane cases. This indicates that in the front some source other

than the rotor wake turbulence-stator mechanism is controlling the broadband noise for both the 70 and 7 vane cases.

In the aft, figures 25(b) and (d), some broadband noise reduction is seen but much less than the 10 dB expected. This

indicates that although some reduction in rotor wake turbulence-stator noise is observed, a noise floor is reached

such that the 7 vane data broadband noise is not controlled by rotor wake turbulence-stator interaction. This floor

level is then caused by some other source. The fact that almost no reduction occurs in the front indicates that the

controlling broadband noise source is probably related to an interaction involving the rotor. The interaction of the

wall boundary layer with the rotor or the interaction of incoming turbulence with the rotor are two possibilities for

the controlling broadband noise mechanism for the ALNF.

Acoustic Treatment Results

7 Vane-Hard Versus Fully Treated.—The ALNF was tested in both a hard configuration and an acoustically

treated configuration. As shown in figure 2, acoustic treatment was present on the outer flow path surface for the fan

inlet and on both inner and outer flow path surfaces for areas downstream of the rotor. This section compares the

noise data for the 7 vane hard configuration and the 7 vane fully treated configuration.

As indicated in figure 9, the fully treated configuration had lower flow rates and higher pressure ratio than the

hard configuration at the same tip speed. The higher losses, which give the higher pressure ratios and lower flows of

the treated case, would tend to result in the fan generating more noise. So, while the acoustic treatment would be

removing noise, the fan could be generating more noise to begin with. Therefore comparisons between hard and

treated cases are subject to question because the fan source noise has changed between the two configurations. The

exact amount of treatment attenuation is then not available from these comparisons but the comparisons are performed because they give qualitative results for the treatment performance.

Spectra comparing the hard and fully treated 7 vane configurations are found in figures 26 to 31. Figures 26 and

27 are for the 6402 rpmc, 600 ft/sec tip speed data point, figures 28 and 29 for the 8537 rpmc, 800 ft/sec, and figures 30 and 31 for the 10671 rpmc, 1000 ft /sec, data point. Each of the figures has part A for the 24.5° angle, B for

the 92.5° angle and C for the 130.5° angle. Narrow band spectra are found if figures 26, 28 and 30 with the corresponding 1/3rd octave data in figures 27, 29 and 31.

In general, the spectral data show little attenuation at the 24.5° angle. In fact, at the lower speeds 6402 rpmc

(fig. 26(a)) and 8537 rpmc (fig. 28(a)), small noise increases are seen with the treated configurations. It should be

noted that not much attenuation would be expected at this far forward an inlet angle and therefore the noise increase

may be indicative of the additional fan noise present at the treated configuration operating conditions. As more aft

angles are investigated, 92.5 and 130.5°, the treatment does show attenuation at all speeds. As the speed is increased,

more attenuation is observed until sizable attenuations are observed at the aft angles of the 10671 rpmc, 1000 ft/sec

tip speed case (figs. 30(c) and 31(c)). Of particular note is the large frequency range over which attenuation is

observed. Again looking at the 1/3rd octave data of figure 31(c), noise attenuations are observed all the way from

1 000 to 100 000 Hz. This range of attenuation is a real tribute to the broadband nature of this bulk absorber design.

Sound power levels for the six speeds tested are found in figure 32 starting with the lowest speed, 6402 rpmc

(600 ft/sec tip speed) in part A and going up to 10671 rpmc (1000 ft/sec) in part F. As can be seen in this figure,

only small amounts of attenuation are seen at low speed with ever increasing attenuations as the speed is increased.

The most attenuation occurs at 10137 rpmc (950 ft/sec tip speed) and at 10671 rpmc (1000 ft/sec) where large

attenuations of the multiple pure tones are observed. As has been noted in the past, multiple pure tone noise is particularly attenuable by treatment and this data continues to show that trend. (see, for example, ref. 9)
NASA/TM—2000-209916 9

Front and aft PWLs are shown in figures 33 and 34 respectively. For the front power (90° and forward) at the

lower speeds, 6402 and 7736 rpmc, the noise increases with the fully treated configuration. Again this is possibly the

result of a change in the fan noise generation since the fan had a different operating point. As the speed is increased,

the treatment starts to show attenuation particularly in the MPT frequency range below blade passing tone until sizable attenuations are observed at 10671 rpmc. The aft PWLs, figure 34, show attenuations at all speeds with the

amount of the attenuation increasing as the speed is increased.

The difference between the hard and fully treated power levels is shown in figure 35 with the lowest speed,

6402 rpmc, being part A and proceeding till part F which is the highest speed, 10671 rpmc. The level of attenuation

is of the order of 3 dB at the lower speeds and increases with increasing speed until as much as 17 dB is seen around

4000 Hz at the 10671 rpmc (1000 ft/sec tip speed) condition. Again, looking at figure 35(f), the large frequency

range of attenuation, from 2 000 to 100 000 Hz is a real tribute to the bulk absorber design.

Figures 36 and 37 show the delta PWLs for the front and rear power levels respectively. In the front, figure 36,

at low speeds, the extra noise from the fully treated configuration is seen as negative attenuations. As the speed is

increased, the inlet attenuations become larger so that at the higher speeds the front power deltas show a positive

noise reduction for the treated configuration. In the aft, figure 37, attenuations are seen for all of the speeds with

more attenuation at the higher speeds than at the lower speeds. By comparing the inlet and aft power level deltas it is

seen that more noise is removed from the aft quadrant than from the front quadrant. This is particularly true at the

higher frequencies, 10 000 Hz and above. The additional attenuation in the aft is as expected because of the additional treatment in the exhaust duct as compared with that in the inlet.

Effect Of Treatment Between Stators

A partially treated configuration was tested with the long chord stators. Here the acoustic treatment between the

long chord stators was made acoustically hard by filling with an epoxy resin. This experiment was done to determine

if acoustic material within the noise source region would be more effective at removing noise.

Figures 38 to 43 show spectral comparisons between the fully treated and partially treated configurations at the

three speeds of 6402 rpmc (600 ft/sec tip speed), 8537 rpmc (800 ft/sec) and 10671 rpmc (1000 ft/sec). These comparisons suffer from the same problem as did the hard versus fully treated comparisons in that the fan operating line

is not the same for the two cases. Figure 9 shows the operating line for the fully treated and partially treated cases.

These two operating lines are closer together than the hard and fully treated line but some differences still exist that

can obscure the acoustic results.

The spectral comparisons of figures 38 to 43 show that the acoustic material between the stators is removing a

significant amount of noise toward the aft angles. Noise reductions are seen at the tones and over a broad bandwidth.

More attenuation is seen in the aft and more attenuation is present at the higher speeds as was observed for the fully

treated cases.

Figure 44 shows the power level comparisons at the six speeds of 6402, 7736, 8537, 9604, 10137 and

10671 rpmc. Again the power level comparisons show increased differences between the fully treated and the partially treated configurations as the speed is increased.

Figures 45 and 46 show the front and aft powers. In the front only small differences are seen between the fully

treated and partially treated cases while in the aft significant amounts of attenuation are observed for the treatment

between the stators.

The delta PWL values are seen in figure 47. Positive differences in PWL correspond to increased attenuation

with full acoustic treatment.As mentioned before the total attenuation and the difference between the fully and partially treated cases increases with speed. The front and aft delta PWLs are shown if figures 48 and 49. Differences of

less than 2 dB exist in the front PWL between the fully treated and partially treated configurations. In the aft, however, the differences are larger with the treatment between the stators showing 4 to 5 dB of increased attenuation

around the blade passing tone.

When this experiment was conducted, it was believed that rotor wake turbulence-stator interaction would be the

primary source of broadband noise. In that case, the material between the stators might be more effective at broadband noise suppression because it was within the noise source region. However, as mentioned earlier in the report as

a result of reduced rotor wake strength and smaller turbulence length scale, there is strong indication that the rotor

wake turbulence-stator interaction mechanism is not the primary broadband noise source for the 7 vane stator configuration and that a rotor related source is probably dominant. In this case, the material between the stators would
NASA/TM—2000-209916 10

be expected to be no more effective than any other treatment downstream of the rotor in reducing the aft broadband

noise.

Table IV shows the approximate surface areas of the treatment in the 7 vane configuration. In the aft, the treatment between the stators, is ~37 percent of the total treatment downstream of the rotor. Since the passage height is

approximately the same for all of the treatment downstream of the rotor, the attenuation of each section should be

roughly proportional to the treated area. Then 37 percent of the downstream attenuation should come from the treatment between the stators. To check this a number of aft PWL points were checked at a frequency that should be

controlled by broadband noise. The total aft PWL attenuations were taken from figure 37 and the attenuations from

the treatment between the stators from figure 49. Table V contains attenuation comparisons, one at each speed for

the 25 000 Hz 1/3rd octave band. The percentage attenuation from the treatment between the stators varies from

40 percent at 6402 rpmc to 56 percent at 10671 rpmc with an average around 50 percent. This is only slightly more

than the 37 percent that this material should be removing. This indicates for the broadband noise that the material

between the stators is behaving a little better but not much different than the other lining material downstream of the

rotor.

As mentioned previously in the report, that although the broadband noise was not controlled by rotor wakestator interaction, the blade passing tone did appear to be controlled by rotor wake-stator interaction. Therefore the

material between the stators, being within the noise generation region, may have a proportionally greater effect on

the blade passing tone. If the leading edge of the stator is taken as the reference point, the acoustic treatment

between the stators represents 44 percent of the material downstream of this point. Table VI compares the 1/3rd

octave band aft powers for the bands that contain the blade passing tone at the various speeds tested. Here it is seen

that the percentage of the total noise that is removed by the material between the stators is much higher for the

blade passing frequency than for the broadband noise. The percentage goes as high as 75 percent at the 10671 rpmc

case with the average being 65 percent. These high values indicate that the material between the stators may be

more effective than the other treatment material. This material may be more effective because its proximity to the

source may allow more noise to be incident onto the acoustic treatment than would occur for downstream treatment

sections.

Although the data does not prove the point conclusively, it does indicate that acoustic treatment within the

noise source region, the BPF case, does act more effectively than treatment away from the source. It is also seen that

this same treatment material between the stators acts approximately the same as the other materials in the duct when

it is not within the noise source region, the broadband case.

70 Vane-Hard Versus Fully Treated

The ALNF was tested with the 70 vane stator set in both a hard and acoustically treated configuration. As

shown in figure 2, acoustic treatment was present on the outer flow path surface for the fan inlet and on both the

inner and outer flow path surfaces for areas downstream of the rotor. This section compares the noise data for the

70 vane hard configuration and the 70 vane fully treated configuration.

As indicated in figure 9, the fully treated 70 vane configuration had lower flow rates and higher pressure ratios

than the hard configuration at the same tip speed. The difference between the hard and treated operating lines is not

as great as that which existed for the 7 vane configurations but the difference is still significant. The higher losses

which give the higher pressure ratios and lower weight flows of the treated case would tend to result in the fan generating more noise. So, while the acoustic treatment would be removing noise, the fan could be generating more

noise to begin with. Therefore comparisons between hard and treated cases are subject to question because the fan

source has changed between the two configurations. The exact amount of treatment attenuation is then not available

from these comparisons but the comparisons are performed here because they give qualitative results for the treatment performance.

Spectra comparing the hard and fully treated 70 vane configurations are found in figures 50 to 55. Figures 50

and 51 are for the 6402 rpmc, 600ft/sec tip speed data point, figures 52 and 53 for the 8537 rpmc, 800 ft/sec, and

figures 54 and 55 for the 10671 rpmc, 1000 ft/sec, data point. Each of the figures has part A for the 24.5° angle, B

for the 92.5° angle and part C for the 130.5° angle. Narrow band spectra are found in figures 50, 52 and 54 with the

corresponding 1/3rd octave data in figures 51, 53 and 55.

In general, the spectral data show little attenuation at the 24.5° angle. In fact some regions of noise increase are

seen at this far forward angle (figs. 50(a), 52(a) and 54(a)). It should be noted that not much attenuation would be
NASA/TM—2000-209916 11

expected at this far forward an inlet angle but the noise increase may be indicative of the additional fan noise present

at the treated operating condition. As the more aft angles are investigated, 92.5 and 130.5°, the treatment does show

attenuation at all speeds. Significant attenuation is observed at the high speed, 10671 rpmc, condition and the MPT

tone at 1/2 blade passing frequency is particularly attenuated (figs. 54(c) and 55(c)). As mentioned previously, tailoring the MPTs to occur at one particular frequency, here at one half blade passing frequency, could be used as a

noise reduction technique. The high attenuation possible at for a specific frequency could be combined with the

MPTs at one frequency to provide a very effective fan/acoustic treatment system.

Again as with the 7 vane data, the large frequency range over which attenuation is observed is of particular

note. Looking at the 1/3rd octave data of figure 55(c), noise attenuations are observed all the way from 1 000 to

100 000 Hz. This large frequency range of attenuation is a real tribute to the broadband nature of this bulk absorber

design.

Sound power levels for the six speeds are found in figure 56 starting with the lowest speed, 6402 rpmc

(600 ft/sec tip speed) in part A and going up to 10671 rpmc (1000 ft/sec tip speed) in part F. Significant attenuations

are seen at all speeds with the most attenuations being seen at the 10137 and 10671 rpmc data points. These large

attenuations at these higher speeds seem centered around the removal of the multiple pure tone noise. As has been

noted in the past, multiple pure tone noise is particularly attenuable by treatment and this data continues the trend.

Front and aft PWLs are shown in figures 57 and 58 respectively. At the lower speeds, 6402, 7736, and

8537 rpmc, the attenuations in the front power are small. At the higher speeds some significant attenuation is seen

particularly in the MPT region of the spectra. The aft PWLs, figure 58, show attenuations at all speeds. Large attenuations are seen at the higher speeds in the MPT regions of the spectra.

The difference between the hard and fully treated power levels is seen in figure 59 with the lowest speed,

6402 rpmc, being part A and the highest speed, 10671 rpmc, being part F. Significant attenuations are seen at all

speeds with as much as 16 dB being seen at the blade passing frequency at 10137 rpmc (fig. 59(e)). Again the frequency range of the attenuation is of particular note.

Figures 60 and 61 show the delta PWLs for the front and rear power levels respectively. At the lower speeds in

the front, 6402 to 8537 rpmc, figures 60(a) to (c), the peak attenuation is of the order of 5 dB at the lower frequencies with some regions of noise increase at the higher frequencies. At the higher speeds, 10137 and 10671 rpmc,

figures 60(e) and (f), the peak attenuation is over 10 dB in the region of the blade passing tone with little or no regions of noise addition. In the aft, figure 61, the noise attenuations are greater than in the front with peak attenuations at all angles being over 10 dB and with no regions of noise addition. This additional attenuation in the aft is as

expected because of the additional treatment in the aft as opposed to the inlet.

CONCLUDING REMARKS

A 106 bladed fan with a design takeoff tip speed of 1100 ft/sec was hypothesized as reducing perceived noise

because of the shift of the blade passing tone harmonics to frequencies beyond the perceived noise rating range. A

22 in. model of this alternative low noise fan, ALNF, was tested in the NASA Glenn acoustically treated 9×15 tunnel to confirm this hypothesis. The ALNF was tested with both a 7 vane long chord stator assembly to reduce interaction noise and a 70 vane conventional stator assembly. Both the 7 and 70 vane configurations were tested in hard

and acoustically treated configurations. The 7 vane configuration was also tested in a partially treated configuration

where the acoustic treatment between the 7 stator vanes was made hard to evaluate the relative effectiveness of this

treatment that was presumably in the noise source generation region.

The noise data from the 106 bladed fan with the 7 vane long stator in an acoustically hard version, no acoustic

treatment, was compared with the noise of a low tip speed fan previously designed by Allison and tested in the same

9×15 wind tunnel. The 106 bladed fan at 1000 ft/sec tip speed was shown to be around 4 EPNdB quieter than the

Allison fan at 840 ft/sec for the takeoff condition and 5 EPNdB quieter at approach (600 ft/sec ALNF, 500 ft/sec

Allison) for a 1000 ft flyover of an ~6 ft diameter fan on a standard day. EPNdb reductions were obtained from the

tone noise reduction and frequency shift as expected from the design but the majority of the noise reduction came

from reduced broadband noise of the ALNF. This broadband reduction was related to the large number of ALNF

rotor blades. The large number of rotor blades resulted in smaller initial wakes which decayed more rapidly in a

fixed distance and had smaller turbulence length scales.

The 106 bladed ALNF is a research fan designed to push the technology and as such it probably is not a practical device with present materials technology. However, a low tip speed fan with 50 blades could be a practical
NASA/TM—2000-209916 12

device. Estimates of the noise of a low tip speed fan indicate that it would have a 2 to 3 EPNdB advantage at takeoff

and 3 to 4 EPNdB advantage at approach speed over the Allison fan in a low tip speed device about 6 ft in diameter.

Tone noise reductions were observed in the hard wall data when the 70 vane stator was changed to the 7 vane

stator. These reductions were expected with the lower number of long chord stators reducing the rotor wake stator

interaction noise. However, the expected level of reduction in broadband noise was not observed, indicating that

some source other than rotor wake turbulence–stator interaction is the primary broadband noise source. The low

level of rotor wake turbulence–stator interaction noise is attributed to the large number of rotor blades reducing both

the magnitude and scale of the wake turbulence.

The structure of the MPTs at higher speeds changed considerably as the fan dampers started to wear. The MPTs

became strongly centered at one half blade passing frequency because the design of the blade stems allowed every

other blade to find a different operating position when the dampers were no longer keeping the blades in fixed positions. With every other blade looking differently, the MPTs came out primarily at one half blade passing frequency.

This occurrence points to a possible noise reduction technique. A net noise reduction could be achieved for a high

tip speed fan by designing blade to blade differences so that the MPTs would be generated at a specific frequency,

like the one half blade passing frequency observed herein, and designing acoustic treatment to remove that frequency. Acoustic treatment is specifically effective when it can be designed for a small frequency range. Therefore a

design combining the MPTs at one frequency and acoustic treatment tailored specifically to remove that frequency

could result in higher noise reductions from the fan/acoustic treatment system.

The Alternative Low Noise Fan results with acoustic treatment were somewhat obscured because the fan performed on a different operating line with the acoustic treatment. This different line was expected to give more fan

noise with the treatment operating line than without the treatment possibly yielding less measured attenuation than

the treatment actually provided. Even with these different operating lines, the fully treated cases, 7 and 70 vane comparisons, showed significant noise reduction over a very large frequency range. These large amounts of attenuation

over such a large frequency range are a real tribute to this bulk absorber treatment design.

Data from the partially treated configuration, acoustic treatment material between the stator vanes made hard,

was compared with the full treatment data and the effect of the material between the stators was measured. The

intent here was to see if acoustic material within the region where noise was generated would be more effective than

material outside this region. The broadband data showed the lining material between the stators to be about as effective as the other acoustic material downstream of the rotor. Since the previous 7 versus 70 vane data indicated that

the broadband noise was probably controlled by an interaction on the rotor, all of the treatment material, including

that between the stators was outside of the broadband noise generation region and would be expected to behave

similarly. The blade passing tone noise, however, was believed to be controlled by rotor wake–stator interaction

noise and the treatment material between the stators would be in the tone source generation region. The blade passing tone data appeared to show the acoustic material between the stators as being more effective than material outside of the noise generation area. This is possibly because more of the noise is incident on the acoustic material

close to the source than on material downstream. This result would indicate that acoustic treatment is more effective

if placed within the source generation region.

REFERENCES

1. Dittmar, James H., and Woodward, Richard P., An Evaluation of Some Alternative Approaches for Reducing

Fan Tone Noise, NASA TM–105356, 1992.

2. Dittmar, James H., A concept for a Counterrotating Fan with Reduced Tone Noise. NASA TM–105736,1992.

3. Woodward Richard P., et.al., Benefits of Swept and Leaned Stators foe Fan Noise Reduction, NASA TM TBD,

AST Paper AST018,1997.

4. Gelder, T.F., and Soltis, R.F., Inlet Noise of 0.5 Meter-Diameter NASA QF-1 Fan as Measured in an Unmodified Compressor Aerodynamic Test Facility and in an Anechoic Chamber, NASA TN D–8121,1975.

5. Jeracki, Robert J., The Aerodynamic Performance of the Alternative Low Noise Fan, to be published.

6. Dittmar, James H., A Method to Further Reduce the Perceived Noise of Low Tip Speed Fans, NASA AST

Report, AST 027, 1998.
NASA/TM—2000-209916 13

7. Hanson, Donald B., and Horan, Kelly, Turbulence/Cascade Interaction: Spectra of Inflow, Cascade Response,

and Noise. AIAA Paper 98-2319. Presented at the 4th AIAA/CEAS Aeroacoustics Conference, 1998.

8. Ganz, Ulrich W., et. al., Boeing 18-Inch Fan Rig Broadband Noise Test, NASA CR–1998-208704, 1998.

9. Sofrin, T.G., and Pickett, G.F., Multiple Pure tone Noise Generated by Fans at Supersonic Tip Speeds, in

NASA SP 304, 1970.

TABLE I.—ALTERNATIVE LOW NOISE FAN MODEL

Fan diameter ..................................................................22 in.

Takeoff pressure ratio ........................................................ 1.3

Takeoff tip speed .................................................. 1100 ft/sec

Rotor blade number .......................................................... 106

Stator vane number ...........................................14 long chord

70 short chord

Nacelle configurations ..............................................hardwall

acoustic treatment

TABLE II.—TEST CONFIGURATIONS

7 long chord stator vanes, acoustically treated nacelle

7 long chord stator vanes, acoustically treated nacelle except

 the treatment between the stators has been made hard

7 long chord stator vanes, hard wall nacelle

70 short chord stator vanes, acoustically treated nacelle

70 short chord stator vanes, hard wall nacelle

TABLE III.—TEST CONDITIONS

Corrected,

rpm

Tip speed,

ft/sec

6402

7736

8537

9604

10137

10671

600

725

800

900

950

1000
NASA/TM—2000-209916 14

TABLE IVa.—TREATED AREAS LONG CORD STATOR

Treatment section Approximate surface area in.2

A

B

C

D

E

F

G

Total between stators

Total downstream of rotor

Total downstream of stator L.E.

924

188

113

446

268

565

399

714

1919

1618

TABLE IVb.—TREATED AREAS SHORT CORD STATOR

Treatment section Approximate surface area in.2

A

B

C

D

E

F

G

924

188

113

440

264

565

399
NASA/TM—2000-209916 15

TABLE V.—BROADBAND TREATMENT ATTENUATIONS

Corrected

revolutions/minute

Frequency band

containing

broadband noise

Full treatment

attenuation

Treatment between

stators attenuation

Percentage of attenuation

from treatment between

stators

10671

10137

9604

8537

7736

6402

25 000

25 000

25 000

25 000

25 000

25 000

8.0

7.0

9.0

7.5

7.5

7.5

4.5

3.75

4.75

3.75

3.5

3.0

56

54

53

50

47

40

TABLE VI.—BLADE PASSING TONE ATTENUATIONS

Corrected

revolutions/minute

Frequency band

containing

broadband noise

Full treatment

attenuation

Treatment between

stators attenuation

Percentage of

attenuation from

treatment between

stators

10671

10137

9604

8537

7736

6402

20 000

20 000

16 000

16 000

12 500

12 500

7.0

6.0

6.5

7.0

5.25

7.0

5.25

4.25

4.75

4.75

3.0

3.0

75

71

73

68

57

43
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Appendix A

 The following pages are the output from a compressor design program that was used to design the Alternative

Low Noise Fan. The program uses 11 streamlines with number 1 being at the tip and number 11 at the hub. The

program uses 12 axial stations. Stations 1 to 3 are upstream of the rotor. Station 4 is the inlet to the rotor and station

5 is the rotor outlet. Station 6 is between the rotor and stator. Station 7 is the stator inlet and station 8 is the stator

outlet. Stations 9 to 12 are downstream of the stator. The column headings for each station are for the most part self

explanatory and the values are presented in English units.
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Appendix B

BLADE COORDINATES
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Appendix C

 The noise data from the Alternative Low Noise Fan is presented in this appendix. The data is arranged in 5 sections corresponding to the five tested configurations: 7 vane hard, 7 vane fully treated, 7 vane partially treated,

70 vane hard and 70 vane fully treated. In each of these sections the data from the six fan speeds is presented. These

are 10671, 10137, 9604, 8537, 7736, and 6402 rpmc. The data are presented at roughly 10° angular positions from

158° in the aft to 24.5° in the front. Even this mass of data is only a sampling of the data since the data is taken at

roughly every 2.5°. At each of the speeds both narrow band data from 0 to 50 000 Hz and 1/3rd octave data are presented with the narrow band data being presented first. The narrow band data plots are sound pressure level in decibels plotted versus frequency in kilohertz. The title of the plot gives both the speed and the angle of the plot. The

1/3rd octave plots are sound pressure level in decibels plotted versus frequency in hertz. The frequency axis is a log

scale with the lowest frequency shown being 10 Hz and the highest frequency being 100 000 Hz. Again the title of

the plot gives both the speed and the angle of the plot.

Appendix C Section 1—7 Vane hard configuration

Appendix C Section 2—7 Vane fully treated configuration

Appendix C Section 3—7 Vane partially treated configuration

Appendix C Section 4—70 Vane hard configuration

Appendix C Section 5—70 Vane fully treated configuration

Appendix C is contained on the enclosed CD.
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SECTION 1

7 VANE HARD CONFIGURATION

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SECTION 2

7 VANE FULLY

TREATED CONFIGURATION

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NASA/TM—2000-209916 605
NASA/TM—2000-209916 606
NASA/TM—2000-209916 607
NASA/TM—2000-209916 608
NASA/TM—2000-209916 609
NASA/TM—2000-209916 610
NASA/TM—2000-209916 611
NASA/TM—2000-209916 612
NASA/TM—2000-209916 613
NASA/TM—2000-209916 614
NASA/TM—2000-209916 615
NASA/TM—2000-209916 616
NASA/TM—2000-209916 617
NASA/TM—2000-209916 618
NASA/TM—2000-209916 619
NASA/TM—2000-209916 620
NASA/TM—2000-209916 621
NASA/TM—2000-209916 622
NASA/TM—2000-209916 623
NASA/TM—2000-209916 624
NASA/TM—2000-209916 625
NASA/TM—2000-209916 626
NASA/TM—2000-209916 627
NASA/TM—2000-209916 628
NASA/TM—2000-209916 629
NASA/TM—2000-209916 630
NASA/TM—2000-209916 631
NASA/TM—2000-209916 632
NASA/TM—2000-209916 633
NASA/TM—2000-209916 634
NASA/TM—2000-209916 635
NASA/TM—2000-209916 636
NASA/TM—2000-209916 637
NASA/TM—2000-209916 638
NASA/TM—2000-209916 639
NASA/TM—2000-209916 640
NASA/TM—2000-209916 641
NASA/TM—2000-209916 642
NASA/TM—2000-209916 643
NASA/TM—2000-209916 644
NASA/TM—2000-209916 645
NASA/TM—2000-209916 646
NASA/TM—2000-209916 647
NASA/TM—2000-209916 648
NASA/TM—2000-209916 649
NASA/TM—2000-209916 650
NASA/TM—2000-209916 651
NASA/TM—2000-209916 652
NASA/TM—2000-209916 653
NASA/TM—2000-209916 654
NASA/TM—2000-209916 655
NASA/TM—2000-209916 656
NASA/TM—2000-209916 657
NASA/TM—2000-209916 658
NASA/TM—2000-209916 659
NASA/TM—2000-209916 660
NASA/TM—2000-209916 661
NASA/TM—2000-209916 662
NASA/TM—2000-209916 663
NASA/TM—2000-209916 664
NASA/TM—2000-209916 665
NASA/TM—2000-209916 666
NASA/TM—2000-209916 667
NASA/TM—2000-209916 668
NASA/TM—2000-209916 669
NASA/TM—2000-209916 670
NASA/TM—2000-209916 671
NASA/TM—2000-209916 672
NASA/TM—2000-209916 673
NASA/TM—2000-209916 674
NASA/TM—2000-209916 675
NASA/TM—2000-209916 676
NASA/TM—2000-209916 677
NASA/TM—2000-209916 678
NASA/TM—2000-209916 679
NASA/TM—2000-209916 680
NASA/TM—2000-209916 681
NASA/TM—2000-209916 682
NASA/TM—2000-209916 683
NASA/TM—2000-209916 684
NASA/TM—2000-209916 685
NASA/TM—2000-209916 686
NASA/TM—2000-209916 687
NASA/TM—2000-209916 688
NASA/TM—2000-209916 689
NASA/TM—2000-209916 690
NASA/TM—2000-209916 691
NASA/TM—2000-209916 692
NASA/TM—2000-209916 693
NASA/TM—2000-209916 694
NASA/TM—2000-209916 695
NASA/TM—2000-209916 696
NASA/TM—2000-209916 697
NASA/TM—2000-209916 698
NASA/TM—2000-209916 699
NASA/TM—2000-209916 700
NASA/TM—2000-209916 701
NASA/TM—2000-209916 702
NASA/TM—2000-209916 703
NASA/TM—2000-209916 704
NASA/TM—2000-209916 705
NASA/TM—2000-209916 706
NASA/TM—2000-209916 707
NASA/TM—2000-209916 708
NASA/TM—2000-209916 709
NASA/TM—2000-209916 710
NASA/TM—2000-209916 711
NASA/TM—2000-209916 712
NASA/TM—2000-209916 713
NASA/TM—2000-209916 714
SECTION 3

7 VANE PARTIALLY

TREATED CONFIGURATION

NASA/TM—2000-209916
NASA/TM—2000-209916 716
NASA/TM—2000-209916 717
NASA/TM—2000-209916 718
NASA/TM—2000-209916 719
NASA/TM—2000-209916 720
NASA/TM—2000-209916 721
NASA/TM—2000-209916 722
NASA/TM—2000-209916 723
NASA/TM—2000-209916 724
NASA/TM—2000-209916 725
NASA/TM—2000-209916 726
NASA/TM—2000-209916 727
NASA/TM—2000-209916 728
NASA/TM—2000-209916 729
NASA/TM—2000-209916 730
NASA/TM—2000-209916 731
NASA/TM—2000-209916 732
NASA/TM—2000-209916 733
NASA/TM—2000-209916 734
NASA/TM—2000-209916 735
NASA/TM—2000-209916 736
NASA/TM—2000-209916 737
NASA/TM—2000-209916 738
NASA/TM—2000-209916 739
NASA/TM—2000-209916 740
NASA/TM—2000-209916 741
NASA/TM—2000-209916 742
NASA/TM—2000-209916 743
NASA/TM—2000-209916 744
NASA/TM—2000-209916 745
NASA/TM—2000-209916 746
NASA/TM—2000-209916 747
NASA/TM—2000-209916 748
NASA/TM—2000-209916 749
NASA/TM—2000-209916 750
NASA/TM—2000-209916 751
NASA/TM—2000-209916 752
NASA/TM—2000-209916 753
NASA/TM—2000-209916 754
NASA/TM—2000-209916 755
NASA/TM—2000-209916 756
NASA/TM—2000-209916 757
NASA/TM—2000-209916 758
NASA/TM—2000-209916 759
NASA/TM—2000-209916 760
NASA/TM—2000-209916 761
NASA/TM—2000-209916 762
NASA/TM—2000-209916 763
NASA/TM—2000-209916 764
NASA/TM—2000-209916 765
NASA/TM—2000-209916 766
NASA/TM—2000-209916 767
NASA/TM—2000-209916 768
NASA/TM—2000-209916 769
NASA/TM—2000-209916 770
NASA/TM—2000-209916 771
NASA/TM—2000-209916 772
NASA/TM—2000-209916 773
NASA/TM—2000-209916 774
NASA/TM—2000-209916 775
NASA/TM—2000-209916 776
NASA/TM—2000-209916 777
NASA/TM—2000-209916 778
NASA/TM—2000-209916 779
NASA/TM—2000-209916 780
NASA/TM—2000-209916 781
NASA/TM—2000-209916 782
NASA/TM—2000-209916 783
NASA/TM—2000-209916 784
NASA/TM—2000-209916 785
NASA/TM—2000-209916 786
NASA/TM—2000-209916 787
NASA/TM—2000-209916 788
NASA/TM—2000-209916 789
NASA/TM—2000-209916 790
NASA/TM—2000-209916 791
NASA/TM—2000-209916 792
NASA/TM—2000-209916 793
NASA/TM—2000-209916 794
NASA/TM—2000-209916 795
NASA/TM—2000-209916 796
NASA/TM—2000-209916 797
NASA/TM—2000-209916 798
NASA/TM—2000-209916 799
NASA/TM—2000-209916 800
NASA/TM—2000-209916 801
NASA/TM—2000-209916 802
NASA/TM—2000-209916 803
NASA/TM—2000-209916 804
NASA/TM—2000-209916 805
NASA/TM—2000-209916 806
NASA/TM—2000-209916 807
NASA/TM—2000-209916 808
NASA/TM—2000-209916 809
NASA/TM—2000-209916 810
NASA/TM—2000-209916 811
NASA/TM—2000-209916 812
NASA/TM—2000-209916 813
NASA/TM—2000-209916 814
NASA/TM—2000-209916 815
NASA/TM—2000-209916 816
NASA/TM—2000-209916 817
NASA/TM—2000-209916 818
NASA/TM—2000-209916 819
NASA/TM—2000-209916 820
NASA/TM—2000-209916 821
NASA/TM—2000-209916 822
NASA/TM—2000-209916 823
NASA/TM—2000-209916 824
NASA/TM—2000-209916 825
NASA/TM—2000-209916 826
NASA/TM—2000-209916 827
NASA/TM—2000-209916 828
NASA/TM—2000-209916 829
NASA/TM—2000-209916 830
NASA/TM—2000-209916 831
NASA/TM—2000-209916 832
NASA/TM—2000-209916 833
NASA/TM—2000-209916 834
NASA/TM—2000-209916 835
NASA/TM—2000-209916 836
NASA/TM—2000-209916 837
NASA/TM—2000-209916 838
NASA/TM—2000-209916 839
NASA/TM—2000-209916 840
NASA/TM—2000-209916 841
NASA/TM—2000-209916 842
NASA/TM—2000-209916 843
NASA/TM—2000-209916 844
NASA/TM—2000-209916 845
NASA/TM—2000-209916 846
NASA/TM—2000-209916 847
NASA/TM—2000-209916 848
NASA/TM—2000-209916 849
NASA/TM—2000-209916 850
NASA/TM—2000-209916 851
NASA/TM—2000-209916 852
NASA/TM—2000-209916 853
NASA/TM—2000-209916 854
NASA/TM—2000-209916 855
NASA/TM—2000-209916 856
NASA/TM—2000-209916 857
NASA/TM—2000-209916 858
NASA/TM—2000-209916 859
NASA/TM—2000-209916 860
NASA/TM—2000-209916 861
NASA/TM—2000-209916 862
NASA/TM—2000-209916 863
NASA/TM—2000-209916 864
NASA/TM—2000-209916 865
NASA/TM—2000-209916 866
NASA/TM—2000-209916 867
NASA/TM—2000-209916 868
NASA/TM—2000-209916 869
NASA/TM—2000-209916 870
NASA/TM—2000-209916 871
NASA/TM—2000-209916 872
NASA/TM—2000-209916 873
NASA/TM—2000-209916 874
NASA/TM—2000-209916 875
NASA/TM—2000-209916 876
NASA/TM—2000-209916 877
NASA/TM—2000-209916 878
NASA/TM—2000-209916 879
NASA/TM—2000-209916 880
NASA/TM—2000-209916 881
NASA/TM—2000-209916 882
NASA/TM—2000-209916 883
NASA/TM—2000-209916 884
NASA/TM—2000-209916 885
NASA/TM—2000-209916 886
NASA/TM—2000-209916 887
NASA/TM—2000-209916 888
NASA/TM—2000-209916 889
NASA/TM—2000-209916 890
NASA/TM—2000-209916 891
NASA/TM—2000-209916 892
NASA/TM—2000-209916 893
NASA/TM—2000-209916 894
NASA/TM—2000-209916 895
NASA/TM—2000-209916 896
NASA/TM—2000-209916 897
NASA/TM—2000-209916 898
NASA/TM—2000-209916 899
NASA/TM—2000-209916 900
NASA/TM—2000-209916 901
NASA/TM—2000-209916 902
NASA/TM—2000-209916 903
NASA/TM—2000-209916 904
NASA/TM—2000-209916 905
NASA/TM—2000-209916 906
NASA/TM—2000-209916 907
NASA/TM—2000-209916 908
NASA/TM—2000-209916 909
NASA/TM—2000-209916 910
NASA/TM—2000-209916 911
NASA/TM—2000-209916 912
NASA/TM—2000-209916 913
NASA/TM—2000-209916 914
NASA/TM—2000-209916 915
NASA/TM—2000-209916 916
NASA/TM—2000-209916 917
NASA/TM—2000-209916 918
NASA/TM—2000-209916 919
NASA/TM—2000-209916 920
NASA/TM—2000-209916 921
(This page intentionally left blank)
SECTION 4

70 VANE HARD CONFIGURATION

NASA/TM—2000-209916
NASA/TM—2000-209916 924
NASA/TM—2000-209916 925
NASA/TM—2000-209916 926
NASA/TM—2000-209916 927
NASA/TM—2000-209916 928
NASA/TM—2000-209916 929
NASA/TM—2000-209916 930
NASA/TM—2000-209916 931
NASA/TM—2000-209916 932
NASA/TM—2000-209916 933
NASA/TM—2000-209916 934
NASA/TM—2000-209916 935
NASA/TM—2000-209916 936
NASA/TM—2000-209916 937
NASA/TM—2000-209916 938
NASA/TM—2000-209916 939
NASA/TM—2000-209916 940
NASA/TM—2000-209916 941
NASA/TM—2000-209916 942
NASA/TM—2000-209916 943
NASA/TM—2000-209916 944
NASA/TM—2000-209916 945
NASA/TM—2000-209916 946
NASA/TM—2000-209916 947
NASA/TM—2000-209916 948
NASA/TM—2000-209916 949
NASA/TM—2000-209916 950
NASA/TM—2000-209916 951
NASA/TM—2000-209916 952
NASA/TM—2000-209916 953
NASA/TM—2000-209916 954
NASA/TM—2000-209916 955
NASA/TM—2000-209916 956
NASA/TM—2000-209916 957
NASA/TM—2000-209916 958
NASA/TM—2000-209916 959
NASA/TM—2000-209916 960
NASA/TM—2000-209916 961
NASA/TM—2000-209916 962
NASA/TM—2000-209916 963
NASA/TM—2000-209916 964
NASA/TM—2000-209916 965
NASA/TM—2000-209916 966
NASA/TM—2000-209916 967
NASA/TM—2000-209916 968
NASA/TM—2000-209916 969
NASA/TM—2000-209916 970
NASA/TM—2000-209916 971
NASA/TM—2000-209916 972
NASA/TM—2000-209916 973
NASA/TM—2000-209916 974
NASA/TM—2000-209916 975
NASA/TM—2000-209916 976
NASA/TM—2000-209916 977
NASA/TM—2000-209916 978
NASA/TM—2000-209916 979
NASA/TM—2000-209916 980
NASA/TM—2000-209916 981
NASA/TM—2000-209916 982
NASA/TM—2000-209916 983
NASA/TM—2000-209916 984
NASA/TM—2000-209916 985
NASA/TM—2000-209916 986
NASA/TM—2000-209916 987
NASA/TM—2000-209916 988
NASA/TM—2000-209916 989
NASA/TM—2000-209916 990
NASA/TM—2000-209916 991
NASA/TM—2000-209916 992
NASA/TM—2000-209916 993
NASA/TM—2000-209916 994
NASA/TM—2000-209916 995
NASA/TM—2000-209916 996
NASA/TM—2000-209916 997
NASA/TM—2000-209916 998
NASA/TM—2000-209916 999
NASA/TM—2000-209916 1000
NASA/TM—2000-209916 1001
NASA/TM—2000-209916 1002
NASA/TM—2000-209916 1003
NASA/TM—2000-209916 1004
NASA/TM—2000-209916 1005
NASA/TM—2000-209916 1006
NASA/TM—2000-209916 1007
NASA/TM—2000-209916 1008
NASA/TM—2000-209916 1009
NASA/TM—2000-209916 1010
NASA/TM—2000-209916 1011
NASA/TM—2000-209916 1012
NASA/TM—2000-209916 1013
NASA/TM—2000-209916 1014
NASA/TM—2000-209916 1015
NASA/TM—2000-209916 1016
NASA/TM—2000-209916 1017
NASA/TM—2000-209916 1018
NASA/TM—2000-209916 1019
NASA/TM—2000-209916 1020
NASA/TM—2000-209916 1021
NASA/TM—2000-209916 1022
NASA/TM—2000-209916 1023
NASA/TM—2000-209916 1024
NASA/TM—2000-209916 1025
NASA/TM—2000-209916 1026
NASA/TM—2000-209916 1027
NASA/TM—2000-209916 1028
NASA/TM—2000-209916 1029
NASA/TM—2000-209916 1030
NASA/TM—2000-209916 1031
NASA/TM—2000-209916 1032
NASA/TM—2000-209916 1033
NASA/TM—2000-209916 1034
NASA/TM—2000-209916 1035
NASA/TM—2000-209916 1036
NASA/TM—2000-209916 1037
NASA/TM—2000-209916 1038
NASA/TM—2000-209916 1039
NASA/TM—2000-209916 1040
NASA/TM—2000-209916 1041
NASA/TM—2000-209916 1042
NASA/TM—2000-209916 1043
NASA/TM—2000-209916 1044
NASA/TM—2000-209916 1045
NASA/TM—2000-209916 1046
NASA/TM—2000-209916 1047
NASA/TM—2000-209916 1048
NASA/TM—2000-209916 1049
NASA/TM—2000-209916 1050
NASA/TM—2000-209916 1051
NASA/TM—2000-209916 1052
NASA/TM—2000-209916 1053
NASA/TM—2000-209916 1054
NASA/TM—2000-209916 1055
NASA/TM—2000-209916 1056
NASA/TM—2000-209916 1057
NASA/TM—2000-209916 1058
NASA/TM—2000-209916 1059
NASA/TM—2000-209916 1060
NASA/TM—2000-209916 1061
NASA/TM—2000-209916 1062
NASA/TM—2000-209916 1063
NASA/TM—2000-209916 1064
NASA/TM—2000-209916 1065
NASA/TM—2000-209916 1066
NASA/TM—2000-209916 1067
NASA/TM—2000-209916 1068
NASA/TM—2000-209916 1069
NASA/TM—2000-209916 1070
NASA/TM—2000-209916 1071
NASA/TM—2000-209916 1072
NASA/TM—2000-209916 1073
NASA/TM—2000-209916 1074
NASA/TM—2000-209916 1075
NASA/TM—2000-209916 1076
NASA/TM—2000-209916 1077
NASA/TM—2000-209916 1078
NASA/TM—2000-209916 1079
NASA/TM—2000-209916 1080
NASA/TM—2000-209916 1081
NASA/TM—2000-209916 1082
NASA/TM—2000-209916 1083
NASA/TM—2000-209916 1084
NASA/TM—2000-209916 1085
NASA/TM—2000-209916 1086
NASA/TM—2000-209916 1087
NASA/TM—2000-209916 1088
NASA/TM—2000-209916 1089
NASA/TM—2000-209916 1090
NASA/TM—2000-209916 1091
NASA/TM—2000-209916 1092
NASA/TM—2000-209916 1093
NASA/TM—2000-209916 1094
NASA/TM—2000-209916 1095
NASA/TM—2000-209916 1096
NASA/TM—2000-209916 1097
NASA/TM—2000-209916 1098
NASA/TM—2000-209916 1099
NASA/TM—2000-209916 1100
NASA/TM—2000-209916 1101
NASA/TM—2000-209916 1102
NASA/TM—2000-209916 1103
NASA/TM—2000-209916 1104
NASA/TM—2000-209916 1105
NASA/TM—2000-209916 1106
NASA/TM—2000-209916 1107
NASA/TM—2000-209916 1108
NASA/TM—2000-209916 1109
NASA/TM—2000-209916 1110
NASA/TM—2000-209916 1111
NASA/TM—2000-209916 1112
NASA/TM—2000-209916 1113
NASA/TM—2000-209916 1114
NASA/TM—2000-209916 1115
NASA/TM—2000-209916 1116
NASA/TM—2000-209916 1117
NASA/TM—2000-209916 1118
NASA/TM—2000-209916 1119
NASA/TM—2000-209916 1120
NASA/TM—2000-209916 1121
NASA/TM—2000-209916 1122
NASA/TM—2000-209916 1123
NASA/TM—2000-209916 1124
NASA/TM—2000-209916 1125
NASA/TM—2000-209916 1126
NASA/TM—2000-209916 1127
NASA/TM—2000-209916 1128
NASA/TM—2000-209916 1129
NASA/TM—2000-209916 1130
NASA/TM—2000-209916 1131
NASA/TM—2000-209916 1132
NASA/TM—2000-209916 1133
NASA/TM—2000-209916 1134
SECTION 5

70 VANE FULLY

TREATED CONFIGURATION

NASA/TM—2000-209916
NASA/TM—2000-209916 1136
NASA/TM—2000-209916 1137
NASA/TM—2000-209916 1138
NASA/TM—2000-209916 1139
NASA/TM—2000-209916 1140
NASA/TM—2000-209916 1141
NASA/TM—2000-209916 1142
NASA/TM—2000-209916 1143
NASA/TM—2000-209916 1144
NASA/TM—2000-209916 1145
NASA/TM—2000-209916 1146
NASA/TM—2000-209916 1147
NASA/TM—2000-209916 1148
NASA/TM—2000-209916 1149
NASA/TM—2000-209916 1150
NASA/TM—2000-209916 1151
NASA/TM—2000-209916 1152
NASA/TM—2000-209916 1153
NASA/TM—2000-209916 1154
NASA/TM—2000-209916 1155
NASA/TM—2000-209916 1156
NASA/TM—2000-209916 1157
NASA/TM—2000-209916 1158
NASA/TM—2000-209916 1159
NASA/TM—2000-209916 1160
NASA/TM—2000-209916 1161
NASA/TM—2000-209916 1162
NASA/TM—2000-209916 1163
NASA/TM—2000-209916 1164
NASA/TM—2000-209916 1165
NASA/TM—2000-209916 1166
NASA/TM—2000-209916 1167
NASA/TM—2000-209916 1168
NASA/TM—2000-209916 1169
NASA/TM—2000-209916 1170
NASA/TM—2000-209916 1171
NASA/TM—2000-209916 1172
NASA/TM—2000-209916 1173
NASA/TM—2000-209916 1174
NASA/TM—2000-209916 1175
NASA/TM—2000-209916 1176
NASA/TM—2000-209916 1177
NASA/TM—2000-209916 1178
NASA/TM—2000-209916 1179
NASA/TM—2000-209916 1180
NASA/TM—2000-209916 1181
NASA/TM—2000-209916 1182
NASA/TM—2000-209916 1183
NASA/TM—2000-209916 1184
NASA/TM—2000-209916 1185
NASA/TM—2000-209916 1186
NASA/TM—2000-209916 1187
NASA/TM—2000-209916 1188
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October 2000

NASA TM—2000-209916

E–12153

WU–522–81–11–00

1339

A99

The Alternative Low Noise Fan

James H. Dittmar, David M. Elliott, Robert J. Jeracki,

Royce D. Moore, and Tony L. Parrott

Noise; Fan noise; Acoustic treatment

Unclassified -Unlimited

Subject Category: 71 Distribution: Nonstandard

James H. Dittmar, David M. Elliott, Robert J. Jeracki, and Royce D. Moore, NASA Glenn Research Center,

Tony L. Parrott, Langley Research Center, Hampton, Virginia 23681. Responsible person, James H. Dittmar,

organization code 5940, 216–433–3921. The noise data from the Alternative Low Noise Fan is presented in Appendix C,

which is only contained on the enclosed CD.

A 106 bladed fan with a design takeoff tip speed of 1100 ft/sec was hypothesized as reducing perceived noise because of the shift of the blade passing

harmonics to frequencies beyond the perceived noise rating range. A 22 in. model of this Alternative Low Noise Fan, ALNF, was tested in the NASA

Glenn 9×15 Wind Tunnel. The fan was tested with a 7 vane long chord stator assembly and a 70 vane conventional stator assembly in both hard and

acoustically treated configurations. In addition a partially treated 7 vane configuration was tested wherein the acoustic material between the 7 long

chord stators was made inactive. The noise data from the 106 bladed fan with 7 long chord stators in a hard configuration was shown to be around

4 EPNdB quieter than a low tip speed Allison fan at takeoff and around 5 EPNdB quieter at approach. Although the tone noise behaved as hypothesized, the majority of this noise reduction was from reduced broadband noise related to the large number of rotor blades. This 106 bladed ALNF is a

research fan designed to push the technology limits and as such is probably not a practical device with present materials technology. However, a low

tip speed fan with around 50 blades would be a practical device and calculations indicate that it could be 2 to 3 EPNdB quieter at takeoff and 3 to

4 EPNdB quieter at approach than the Allison fan. 7 vane data compared with 70 vane data indicated that the tone noise was controlled by rotor

wake—stator interaction but that the broadband noise is probably controlled by the interaction of the rotor with incoming flows. A possible multiple

pure tone noise reduction technique for a fan/acoustic treatment system was identified. The data from the fully treated configuration showed significant

noise reductions over a large frequency range thereby providing a real tribute to this bulk absorber treatment design. The tone noise data with the

partially treated 7 vane configuration indicated that acoustic material in the source noise generation region may be more effective than similar material

outside of the generation region.