NASA / TM--1998-208661 AIAA-99-0479

Benefits of Swept and Leaned Stators for

Fan Noise Reduction

Richard P. Woodward, David M. Elliott, Christopher E. Hughes, and Jeffrey J. Berton

Lewis Research Center, Cleveland, Ohio

November 1998

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71 II i i
NASA/TMm1998-208661 AIAA-99-0479

Benefits of Swept and Leaned Stators for

Fan Noise Reduction

Richard P. Woodward, David M. Elliott, Christopher E. Hughes, and Jeffrey J. Berton

Lewis Research Center, Cleveland, Ohio

Prepared for the

37th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 11-14, 1999

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Space Administration

Lewis Research Center

November 1998
This report is a preprint of a paper intended for presentation at a conference. Because

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i:! |l
BENEFITS OF SWEPT AND LEANED STATORS

FOR FAN NOISE REDUCTION

Richard P. Woodward, David M. Elliott,

Christopher E. Hughes, and Jeffrey J. Berton

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

Abstract

An advanced high bypass ratio fan model was tested

in the NASA Lewis Research Center 9×15-Foot Low

Speed Wind Tunnel. The primary focus of this test was to

quantify the acoustic benefits and aerodynamic

performance of sweep and lean in stator vane design.

Three stator sets were used for this test series. A

conventional radial stator was tested at two rotor-stator

axial spacings. Additional stator sets incorporating sweep

+ lean, and sweep only were also tested. The hub axial

location for the swept + lean, and sweep only stators

corresponded to the location of the radial stator at the

upstream rotor-stator spacing, while the tip axial location

of these modified stators corresponded to the radial stator

axial position at the downstream position. The acoustic

results show significant reductions in both rotor-stator

interaction noise and broadband noise beyond what could

be achieved through increased axial spacing of the

conventional, radial stator. Theoretical application of these

results to acoustically quantify a fictitious 2-engine aircraft

and flight path suggested that about 3 EPNdB could be

achieved through incorporation of these modified stators.

This reduction would represent a significant portion of the

6 EPNdB noise goal of the current NASA Advanced

Subsonic Technology (AST) initiative relative to that of

1992 technology levels. A secondary result of this fan test

was to demonstrate the ability of an acoustic barrier wall

to block aft-radiated fan noise in the wind tunnel, thus

revealing the acoustic structure of the residual inletradiated noise. This technology should prove valuable

toward better u nderstandi ng inlet liher design, or wherever

it is desirable to eliminate aft-radiated noise from the fan

acoustic signature.

Introduction

A major source of aircraft engine noise comes from

interaction of the rotor viscous wake with the exit guide

vanes, or stalors. The most prominent component of this

interaction noise are tones at multiples of the rotor blade

passage frequency, although there also exists a broadband

component of this rotor-stator noise. Traditional methods

of reducing this interaction noise have been to select

blade/vane ratios to satisfy the cut off criterion for

propagation of the fundamental rotor tone t and increased

axial spacing between the rotor and stator. 2 Increased

rotor-stator axial spacing may somewhat degrade the fan

aerodynamic performance and increase the overall engine

weight.

The current Advanced Subsonic Technology ('AST)

noise initiative calls for a 6 EPNdB (Effective Perceived

Noise) engine noise reduction relative to 1992 technology

levels to be _Jchieved by 1999. This work calls for a

comprehensive understanding of engine noise generation

mechanisms accompanied by analytic and experimental

validations.

Stator vane lean and/or sweep have been suggested as

a mechanism to reduce the severity of the rotor wake

interaction with the stator vane. Vane sweep is the axial

displacement of the vane with radius such that the tip

region is further downstream than the hub.

Correspondingly, lean is a circumferential displacement

of the vane stacking line relative to the radial direction.

Both of these stator modifications have been proposed as

a means to reduce the stator response to the rotor downwash,

thereby reducing the rotor/stator acoustic response. Kazen 3

demonstrated rotor/stator interaction tone reductions

associated with a stator leaned 30 ° in the direction of fan

NASA/TM--1998-208661 1
rotation.Noisereductionsinthe2BPFtonefrom1.5to

3.5dBwiththeleanedstatorwereobservedinthisstudy.

Analyticalstudies4havesuggestedthatbothstator

leanandsweep,if properlyapplied,maysignificantly

reducerotor/stator interactiontonenoise.Optimalstator

leanandsweepoffersthepossibilityofreducingthe

overallengineweightthroughdecreased axialrotor-stator

spacingorachievingadditional tonenoisereductionfora

particularrotor-stator spacing.

An advancedhighbypasssubsonicfanmodel

incorporatingstatorsweepandleanwasdesignedand

builtbytheAllisonEngineCompanyundercontractto

NASALewisResearchCenter(Contract NAS3-25950).

TheAllisonfanwastestedin theNASALewis9-by

15-Foot LowSpeedWindTunnel 5-7(9×i5LSWT),which

islocatedinthelow-speedreturnlegofthe8-by6-Foot

Supersonic WindTunnel (8×6SWT).Thesefantestswere

conductedatafreestreamMachNumberof0.10.The test

section walls are acoustically treated to provide anechoic

conditions down to a frequency of 250 Hz, which is lower

than the range of test fan acoustic tones.

The emphasis of this fan test was to evaluate the

aeroacoustic performance of the swept + leaned, and

swept only stator relative to that of a baseline radial stator.

All stators had the same vane number and were designed

for equivalent aerodynamic performance. Acoustic data

are presented in terms of sideline directivities and spectra.

These data were also used to generate flyover and sideline

EPNL estimates for a fictitious two-engine aircraft and

flight path to give an estimate of the EPNL benefit

associated with these stator modifications.

Description 9f Fan Test

ResearchFan. An advanced high bypass subsonic fan

model incorporating stator lean and sweep was designed

and built by the Allison Engine Company under contract

to NASA Lewis. 7 Figure 1 is a photograph of the fan

installed in the NASA Lewis 9×15 LSWT. The fan was

tested at a freestream Math Number of 0.10 in the test

section, which is sufficient to achieve acoustic flight

effect 8and provides acoustic data representative of takeoff/

approach operation. All data were taken at 0° fan axis

angle of attack.

The Allison fan was driven by the NASA Lewis Ultra

High Bypass (UHB) drive rig. The UHB drive rig was

powered by a high pressure air turbine drive with the drive

air and instrumentation supplied through the support strut,

shown in Fig. 1. The drive turbine exhaust air was ducted

downstream through an acoustically treaded diffuser and

exited the end of the treated test section. There was little

indication of acoustic contamination of the aft fan data

from the turbine exhaust.

Table I shows design characteristics of the Allison

Fan. The 18 blade rotor had a diameter of 55.9 cm (22 in.).

Three research stator sets were fabricated-a conventional

radial stator and modified stators with sweep + lean, and

sweep only. (A leaned only stator set would have been

desirable, but was eliminated from the contract due to cost

considerations.) All stator sets had 42 vanes and were

designed for equivalent fan stage aerodynamic

performance. The fan stage did not have a core flow

simulator. The baseline slator configuration was with the

radial stator at the closer axial rotor-stator spacing (Fig. 2).

The radial stator was also tested at a larger rotor-stator

axial spacing. The swept + leaned, and swept only stators

were designed such that the hub was located at the same

axial rotor-stator spacing as the baseline stator, and the tip

was located at an axial location corresponding to the radial

stator at the larger axial spacing, (Fig. 3). These stators

were designed with 30 ° of sweep and 30 ° of lean. The

swept + leaned stator was leaned in the direction of rotor

rotation.

Figures 4 to 7 are photographs of the partially

assembled fan stage. Figure 4 is a photograph of the stage

showing the rotor and the swept and leaned stator. Figure 5

shows the rotor and the swept-only stator. Figure 6 is a

downstream view of the swept and leaned stator. Figure 7

is a downstream view of the swept and leaned stator seen

through the rotor.

Anechoic Wind Tunnel and Aco0._ti¢ Instrumentation.

The NASA Lewis 9× 15 LSWT is located in the low speed

return leg of the 8×6 SWT (Fig. 8). The tunnel test section

walls, floor and ceiling had acoustic treatment to produce

an anechoic test environment. Figure 9 is a sketch of the

test fan installed in the 9× 15 LSWT. Sideline acoustic data

were acquired with a computer-controlled translating

microphone probe (also seen in the photograph of Fig. 1)

and with three aft microphone assemblies mounted to the

tunnel floor. The translating microphone probe acquired

data at 48 sideline geometric angles from 27.2 to 134.6 °

relative to the fan rotor plane. The translating probe

traverse was at 224 cm (88 in.) from the fan rotational axis

(four fan diameters). A wall microphone assembly placed

a reference microphone adjacent to the translating probe

home position (i34.6 '_,maximum aft travel). Three fixed

microphone assemblies were mounted to the tunnel flow

at this same axial position to acquire aft acoustic data at

geometric angles of 140, 150, and 160 °. The acoustic data

were acquired through a digital computer system and

stored for post-run analysis.

Rcsuhs and Discussion

Aerodynamic Performance

The three fan stator sets were designed for equivalent

aerodynamic performance. Figures 10 and 1I present a

brief overview of the fan performance with the three stator

sets. The baseline radial stator showed the highest corrected

NASA/TM--1998-208661

'_1 !1
weightflowasafunctionofcorrectedfanspeed, withthe

swept+ leanedstatorshowingthelowestweightflow

value(Fig.10).However,theweightflowdifferences

betweenstatorsetsat thesamefanspeedisrelatively

small,ontheorderof 1to1.5percent.

Figure!I showsthepercent systemlossasafunction

ofpercentcorrectedfanspeedforthefanwithswept+

leaned,andsweptonlystators.Theresultshownare

normalized to the baseline stator configuration

performance. The system losses with the swept + leaned

stator were 2.5 to three percent greater than losses with the

swept only stator. These losses are thought to be associated

with the high pressure and velocities due to the

supercharged nature of the flow in the hub region without

a core simulator. These flow conditions appear to be

causing large separated regions on the stators that increase

system losses. In addition, large viscous wall losses are

associated with the corner flow at the stator hub and tip for

the swept + leaned stator. A somewhat different design

methodology was employed for the swept only stator,

which was partially optimized using area ruling at the tip

region to help relieve the high velocity region there caused

by the flow stacking up in the outboard region of the stator.

Consequently, the swept only stator showed less system

losses.

These limited aerodynamic results are included to

better understand the associated acoustic performance of

the fan with the non-radial stators. A point for consideration

is that the swept + leaned and swept only stator designs

were not optimized for the fan or design point performance.

These were technology demonstrator, proof-of-concept

designs, and the aerodynamic performance losses

associated with them are not representative of the

performance potential for this technology. Encouragingly,

acoustic benefits associated with the swept + leaned stator

are in spite of the somewhat higher system losses associated

with this stator. A swept + leaned stator with lower system

losses may show additional noise reduction relative to the

results presented in this report.

Acgustiq Performance

All of the fan acoustic data were acquired at a tunnel

test section velocity of 0.10 Mach. Sideline data are

presented in terms of emission angles. The emission

angles are related to the geometric, or observed angles by

the relationship:

Oem = Ogeo m - sin -1 (M o sin Ogeom)

where ®era and ®geom are, respectively, the emission and

observed sideline angles, and M o is the test section Mach

number. The observed angles for the sideline translating

microphone probe are then 25 to 130'1,and the three fixed

microphones measure aft observed angles of 136, 147, and

158°. This angular range was sufficient to define the

sideline noise profile for this aft-dominated fan for

subsequent EPNL calculations.

Digital acoustic data were processed as constant

bandwidth spectra. Spectra were acquired and averaged at

each translating probe or fixed mie position with 6 and

59 Hz bandwidths. These constant bandwidth spectra

were electronically merged and used to generate 113

octave spectra. The results presented herein are in terms of

both constant bandwidth and 1/3 octave spectra.

Swept + leaned, and swept only stators were expected

to reduce rotor-stator interaction tones by relieving the

severity of the rotor wake interaction with the stator vanes.

An additional observed benefit was a reduction in fan

broadband noise. Two techniques were employed to

separate the interaction tone and broadband components

of the noise. A software technique was used with the

digital data reduction to produce constant bandwidth, and

consequently, I/3 octave spectra with minimal interaction

tone content. This technique investigated the spectra at

interaction tone frequencies and eliminated tones which

were 6 dB or more above adjacent spectral levels.

Broadband levels at the first four interaction tone

frequencies were also deduced by manually inspecting

selected constant bandwidth spectral arrays.

Effective Perceived Noise Levels. The effective

perceived noise level (EPNL) provides a subjective

measure of the aircraft flyover and sideline noise levels.

This value is derived from the flyover or sideline sound

pressure level profiles and is a function of frequency,

duration, and tone content.

Effective perceived noise levels were calculated for a

fictitious 2-engine aircraft and flight profile based on the

Allison fan model acoustic results. A 3.5 scaling factor

was assumed, and calculations were made for a 0.25 Mach

flight speed. EPNL calculations were made for the full

1/3 octave spectra, and for representative broadband noise

using the 1/3 octave spectra with the interactions tones

electronically removed. FAR 36 Stage 3 sideline EPNL

calculations are for an observer on a 450 m (1476 ft)

sideline. EPNLs were evaluated every 30.5 m (100 It)

along this line to ensure that the sideline noise reported

was indeed the maximum level. FAR36 Stage 3 culback

EPNL calculations are for an observer 6500 m (21325 ft)

from brake release in line with the runway.

Figure 12 shows the aircraft EPNL on the 450 m

(1476 ft) sideline. Although the throttle setting used at

takeoff would be at or near the fan design speed, the

sideline noise is evaluated for the range of speeds

investigated for illustrative purposes. There is about a

1.5 EPNdB decrease associated with moving the radial

stator from the forward position to the aft position at all fan

speeds except 110 percent of design, where the change in

noise level is negligible. However, the addition of sweep

NASA/TM--1998-208661 3
+lean,orsweeponlyresultsinabouta3EPNdBreduction

fromnoiselevelsrelativetothatfortheforwardradial

statoratfanspeedsuptoabout75percent ofdesign.The

sweeponlystatormaintainsthis3EPNdbreductionrelative

tobaselineinthemidspeedrangeof 75to95percent

designspeed. Theswept+leanedstatorshowedthemost

noisereductionatdesignandabovefanspeeds. Similar

resultsareseenfortheflyoverEPNLcalculationsof

Fig.13.Theuseofarangeoffanspeedsismoreapplicable

forflyoverEPNL,sinceathrottlecutbackisoftenusedin

thatsegment. Theanalytical EPNLpredictionsforsideline

andflyoverobserversdifferduetogeometricinputstothe

extragroundattenuationandgroundreflectionmodels.

Thesedifferences, however, donotsignificantlyaffectthe

trendswithrespectto fanspeed.Thus,althoughthe

magnitudesof thesidelineandflyoverEPNLsareof

coursedifferent,hetrendsarenearlyidentical.

Therelativelypoorperformance oftheswept+leaned

statoratfanspeedsnear90percent designmaybeexplained

bytherelativelyloweraerodynamic performanceofthat

stator(Fig.i 1).Systemlossesassociatedwiththeswept+

leanedstatorarethoughttoarisefromlessthanoptimal

flownearthehubandtipregions.It isquitepossiblethat

refinementsin theaerodynamicdesignof theswept+

leanedstatorwouldresultinsuperiorperformanceforthis

conceptthroughout thefanspeedrange.

Thetheoretical studyofRef.4concludesthatsweep

shouldbemostbeneficialttakeoffconditions, whilelean

shouldbemostbeneficialatapproachconditions. This

referencedoesconcludethatcombiningsweepandlean

shouldbecomplementary towardoverallnoisereduction.

TheresultsofFigs.12and13areonlymarginallysupportive

ofthisprediction. Itwouldappearfromthedatathatsweep

alone,ratherthansweep+lean,achievedessentiallyallof

thenoisereductionatthelowerfanspeeds. Atthehigher

fanspeedsadditional noisereductionwasachievedwith

sweep+leanbeyondwhatwasobservedbysweeponly.

However, it isclearfromthesefiguresthatincorporation

ofstatorsweep+leanresultsinsignificantoisereductions

throughout thefanoperatingrangerelativetowhatcould

beachievedthroughsimplyincreasingtheaxialspacingof

theradialstator.

Figures14and15presentcorresponding broadband

resultsforthefictitious2-engineaircraftbasedonthe

acousticdatawiththerotor-slatorinteractiontones

electronicallyremoved.Thiscomputertoneremoval

tcchniqueonlyremovedtoneswhichwere5dBormore

abovetheadjacent broadband, andassuch,mayriotfully

represent thespectral broadbandlevels.Theoverallpattern

of thedataissimilarto whatwasgeneratedfromthe

inclusivespectra(Figs.12and13),althoughthenoise

reductionsaresomewhat less.Inparticular, noisereductions

associatedwithincreasingtheaxialspacingoftheradial

statorareonlyseenatfanspeedsbelow85percentof

design.Reference2notesthatexperimental broadband

noiselevelshowedlittlechangewithrotor-stator spacing

(foraradialstator).Thismaybeanotherindicationthat

thereis sometonalcontaminationin thelowerspeed

resultsofFigs.14and15.

$oond Pressor¢ Level Directivities. Sideline sound

pressure level (SPL) dircctivities provide a useful tool for

evaluating acoustic differences associated with changes

in thc stator configuration. These directivities were

achieved by combining results from the traverse

microphone and the three aft fixed microphones, resulting

in 224 cm (88 in.) sideline directivities for 25 to 158 °

emission angles relative to the fan upstream axis and

centered on the fan rotor plane. These results are for

constant bandwidth (59 Hz) spectra.

Figure 16 shows representative SPL directivities for

the four test configurations. These results are for the fan

operating at 50 percent of corrected design speed. These

data are for the 2BPF tone, which falls within the 3150 Hz

1/3 octave band. Advanced high bypass ratio fans, such as

that reported herein, tend to have aft-dominated

directivities. The results of figure 16 clearly show that there

is a significant noise rcduction associated with increased

radial stator spacing, and additional noise benefits to be

realized with a swept + leaned, and swept only stator.

Thc noise reduction trends shown in Fig. ! 6, are more

easily understood in terms of changes in noise level

relative to that observed for the baseline radial stator in the

upstream position. The sound pressure level (SPL)

directivities for the four stator configurations will now be

explored in this manner at four representative fan speeds.

Constant bandwidth (59 Hz) spectra were used for this

analy sis to facilitate separation of the rotor-stator interaction

tone from adjacent broadband noise. The following test

conditions will be reported:

Percent corrected Corrected rotor tangential Tip Relative Mach

sf.ao...w_¢_

50 (approach) 152 m/s (500 ft/s) 0.507

84 (takeoff) 256 nJs (840 ft/s) 0.900

100 (sideline) 305 m/s (1000 ft/s) 1.080

I10 335 m/s (I 100 ft/s) 1.187

Results for each test speed will be presented in terms

ofrepresentative spectra at a 126° emission angle followed

with directivities showing the tone and broadband

reductions relative to noise levels observed for the baseline

radial stator in the forward axial position. The broadband

levels at rolor-stator interaction frequencies were manually

extracted through inspection of the individual noise spectra

and should provide a reasonably good representation of

these levels.

Figure 17 compares spectra acquired at 126° emission

angle along the 224 cm (88 in.) sideline for thc fan

NASA/TM--1998-208661

!1 il i7
operatingat50percentdesignspeed. The fundamental

rotor- stator interaction tone (BPF) is cut-offand essentially

not present in the spectra. Strong 2 and 3BPF tones are

evident for the radial stator in the upstream position. These

overtones are essentially attenuated either by moving the

radial stator to the aft position or employing sweep and/or

lean. However, there is clear indication that additional

broadband noise- on the order of 4 dB- is removed by the

modified stator sets.

Figures 18 and 19 show, respectively, the directivity

effects on the 2 and 3BPF fan overtones. SPL reductions

in the tone and broadband levels are plotted against the

sideline emission angle. Positive noise levels represent

noise reductions relative to what was observed for the

baseline configuration with the radial stator in the upstream

position. Tone reductions for the 2BPF tone (Fig. 18) are

greatest at aft angles, showing 12 dB reduction associated

with moving the radial stator to the downstream location.

Noise reductions of 15 and 19 dB were associated,

respectively, with the swept only, and swept + leaned stator.

There were also broadband noise reductions associated

with the swept + leaned, and swept only stators, showing

up to 4 dB broadband noise reduction. Moving the radial

stator to the downstream location produced little to no

reduction in broadband noise. This result is consistent

with that reported in the stator spacing study of Ref. 2,

which likewise noted little change in broadband noise

level with (radial) stator spacing.

There has been some concern regarding the periodic

nature of the tonal directivity data taken in the 9× 15 LSWT.

While it is possible that this behavior arises from tunnel

wall reflections, it is much more likely that the data

accurately shows a real interference pattern between aft

and forward radiated noise at a particular frequency.

There are several observations to support the second

interpretation. The fan is aft dominated, therefore one

would expect the cancellation pattern to be more

pronounced toward the forward angles where the relative

noise levels are more nearly equal in level. This is, in fact,

what is observed in the sideline data. An analytical study

of predicted sideline-noise levels was performed which

considered a case for inlet and exhaust radiation for an aft

dominated fan. Again, a similar noise interference pattern

was observed for these analytical results. Finally, results

for another advanced fan model which was tested in a

large anechoic free jet facility showed similar interference

in the sideline results-in this case there was no nearby

tunnel wall to provide possible reflections. This

phenomenon will be further explored in a later section of

this report in which an acoustic barrier wall was placed

adjacent to the fan model and effectively blocked aftradiated noise from reaching the sideline microphone.

The 3BPF results of Fig. 19 show significant tone

reductions, which are now greatest toward the forward

angles. Different tone orders are associated with different

radiation mode structure, and therefore changes in the

directivity patterns arc expected. In particular, acoustic

interaction modes which are just above cut-off tend to be

more forward radiating than more highly cut-on orders.

Tone reductions associated with simply moving the radial

stator downstream are nearly as great as those associated

with the swept + leaned stator (up to 18 dB). The swept

only stator was slightly less effective for tone removal at

forward radiation angles, but essentially equivalent to the

swept + leaned stator at the aft angles. The swept + leaned

stator was most effective in reducing broadband noise

levels at all measured sideline angles.

Figures 20 to 22 present corresponding acoustic results

for the fan operating at 84 percent design speed. The

spectral overlay of Fig. 20 is similar to the 50 percent

speed results of Fig. 17 in that the fundamental tone Is

essentially cut-off, and most overtone energy is associated

with the radial stator in the upstream position. The

"haystacking" nature of the swept + leaned spectra near

3BPF may be associated with flow disturbances caused by

the poorer aerodynamic performance of that stator.

Figure 21 shows sideline noise reductions for the

2BPF tone and broadband. The two modified stators were

essentially equivalent in terms of tonal noise reduction.

Tone reductions associated with the radial stator in the

downstream position were almost as good as those for the

modified stators except for downstream sideline angles

beyond 100°. Broadband noise reductions for the modified

stators were about 2 dB at upstream angles, increasing to

4 to 5 dB at further aft angles.

Tone reductions at 3BPF and 84 percent design speed

(Fig. 22) showed similar reductions for the modified and

further downstream radial stators. Broadband noise

reductions at 3BPF were greatest with the swept only

stator. The swept + leaned stator generated increased

broadband noise at sideline angles from 90 to 110 °.

The fundamental rotor-stator interaction tone

remained cut-off at 100 percent design speed (Fig. 23).

However, higher-order tones are now present in the spectra

for the radial stator in the downstream position and for the

swept and leaned stator. Data were not taken at this speed

for the swept only stator due to aeromechanical avoidance

zones for this stator and fan speed. There is essentially no

interaction tone for the swept + leaned stator until 4BPF

(and higher) harmonics.

Fundamental (BPF) tone and broadband reductions

as a function of emission angle are shown in Fig. 24. The

interesting observation here is that, although cut-off, there

is a significant noise increase associated with the swept +

leaned stator. Again, the suspect cause is the lesser

aerodynamic performance of that stator.

The swept + leaned stator had much more effect than

the radial stator in the downstream position in reducing

NASA/TM-- 1998-208661 5
2BPFtone noise at 100 percent fan speed (Fig. 25).

However there was essentially no broadband reduction at

2BPF. A similar result was observed at 3BPF (Fig. 26).

All stator configurations produced significant tone

noise at the 110 percent overspeed condition. The

fundamental rotor-stator interaction tone is now weakly

cut-on and is evident for the radial stator at the two axial

locations and for the swept only stator. However, this tone

is not evident for the swept + leaned stator (Fig. 27). The

swept + leaned stator essentially eliminated the 2 and

3BPF tones from the spectra. The swept only stator was

marginally effective in reducing acoustic energy at these

tone orders.

The fundamental tone directivity results of Fig. 28

likewise shows significant noise reductions associated

with the swept + leaned stator, with somewhat increased

noise levels (relative to the radial stator in the forward

position) seen for the swept only and radial stator in the

downstream position. The broadband levels at BPF were

essentially unchanged by stator configuration, except for

small reductions at upstream angles with the swept +

leaned stator. A corresponding noise increase was

associated with the downstream radial stator at these

forward angles.

The directivity results at 2 and 3BPF (Figs. 29 and 30)

were somewhat similar, showing that the swept + leaned

stator was most effective in reducing tone and broadband

energy at this fan speed.

These results for the constant bandwidth tone and

broadband directivities are consistent with those presented

earlier in this report for the fictitious aircraft effective

perceived noise levels (Figs. 12 to 15). Stators correctly

redcsigned with sweep + lean, or possibly sweep only

have been shown to significantly reduce both rotor-stator

interaction tone and broadband noise levels. However,

there is not a consistent story as to whether sweep + lean

or sweep only is the preferred modification. The relative

stator performance varied with fan speed. This story is

further complicated by the observation that the swept +

leaned stator showed greater aerodynamic losses than did

the other stators, suggesting that its acoustic performance,

likewise, was compromised. On the other hand, one could

infer that the expected acoustic benefits of a better designed

swept + leaned stator would be at least as good as were

shown herein, and perhaps better.

Data repeatability. The modified stator sets showed

significant reductions in fan tone levels, therefore there is

a need to validate the repeatability of these results. Repeat

data runs for two stator configurations were made to

quantify repeatability of the acoustic data. In each instance,

the second set of data represents a fan rebuild and was

acquired at a totally different test time. Thus, the following

comparisons are rather rigorous toward validating the

acoustic data. Successive data were taken for the swept +

leaned stator and for the radial stator in the downstream

axial position.

Sideline 1/3 octave directivities comparing repeat

data sets are shown in Figs. 31 and 32, respectively, for the

2500 and 20000 Hz frequency band. In each instance the

data repeatability is excellent.

Acoustic barrier wall. Noise levels for modern highbypass ratio subsonic turbofans tend to be aft-dominated.

That is, the highest flyover noise levels radiate from the

fan exit. Measurement of fan inlet sound radiation without

aft radiation contamination requires selective suppression

of the aft noise. An acoustic barrier was used in the NASA

Lewis 9xl5 LSWT to effectively isolate the inlet noise

field from the fan exit noise. The acoustic barrier was

mounted on tracks on the tunnel floor and ceiling at a

sideline distance of 15 cm (6 in.) from the fan nacelle.

Tests were made with the wall leading edge at the fan inlet

highlight plane and 15 cm (6 in.) further aft. The wall

extended downstream essentially to the end of the treated

tunnel test section. The barrier was constructed in sections

which were joined upon installation. The barrier was of

wood frame construction, 8 cm (3 in.) thick, with typically

0.64 cm (0.25 in.) tempered fiberboard skins. An elliptical

leading edge was faired into the upstream barrier section.

The barrier sections extended floor to ceiling and had an

axial length of 61 cm (24 in.). The upstream section had

nominal full height by 46 cm (18 in.) axial length acoustic

treatment on the fan side of the barrier just downstream of

the leading edge. This treatment consisted of a bulk

absorber with a perforated metal skin. Inlet airflow

computations indicated that the presence of the barrier

wall should have a minimal effect on fan aerodynamic

performance. The barrier wall was shown to structurally

sound up to 0.20 Mach tunnel velocity.

Figure 33 is a photograph of the Allison fan in the

NASA Lewis 9× 15 tunnel showing the acoustic barrier in

the upstream position. The sideline translating microphone

probe may be seen in the background. Figure 34 is a sketch

of the acoustic barrier wall installed in the 9xl5 LSWT.

Figures 35 to 38 show 1/3 octave directivities for the

baseline fan with the radial stator in the upstream

position, and with the acoustic barrier wall installed at the

two axial locations (leading edge at fan inlet highlight

and 15 cm (6 in.) further aft). Figures 35 and 36 show

1/3 octave directivities for the fan operating at 50 percent

design speed. Figure 35 shows representative broadband

results at 2500 Hz. The presence of the barrier wall

significantly reduced fan aft noise levels, with somewhat

better aft suppression with the wall leading edge located at

the fan inlet highlight. Similar results are seen in Fig. 36

for the 3BPF tone. Maximum wall shielding at this fan

speed is about 20 dB. It is interesting to note that acoustic

modal structure from (presumably) inlet radiation is now

exposed due to the barrier wall shielding.

NASA/TM--1998-208661 6
Figures37and38showcorresponding directivity

resultsforthefanoperatingat100percentdesignspeed.

Thebarrierwallatbothaxiallocationssignificantlyreduced

aft-radiatedbroadbandnoise(Fig.37).Likewise, the wall

was quite effective in shielding aft-radiated 4BPF tone

noise. Maximum shielding of this strong tone (Fig. 23)

was about 25 dB.

Figures 39 to 43 show the effect of the barrier wall on

the fan acoustic power. Figure 39 shows the overall sound

power level (OAPWL) calculated with 1/3 octave data

from 2000 to 20000 Hz. The data are for the translating

microphone probe for emission angles from 25 to 130°.

The presence of the barrier wall reduced the OAPWL from

about 4 dB at the lower fan speeds to over 10 dB at thc

higher speeds. The fan noise directivity becomes

increasingly aft-dominated at higher speeds where the

barrier wall is shown to significantly reduce the measured

sound power. As expected, the barrier wall is slightly

more effective at its upstream location with the wall

leading edge at the fan inlet highlight.

Figures 40 and 41 show the corresponding fan OAPWL

for forward emission angles (25 to 61 °) and downstream

angles (61 to 130°), respectively. The upstream OAPWL

results of Fig. 40 show essentially no barrier wall effect at

fan speeds below 85 percent design, where the fan noise

directivity begins to become more aft dominated. There is

some wall-induced noise reduction at higher fan speeds,

showing that dominant aft-radiated fan noise is present at

these upstream angles. There was essentially no difference

in barrier wall effectiveness for the two wall axial positions.

The barrier wall was quite effective in reducing the

downstream OAPWL (Fig. 41), with reductions typically

about 10 dB at lower fan speeds, increasing to 15 dB at the

higher speeds. The aft noise levels were sensitive to wall

location, being about 1.5 dB lower with the wall at the

forward axial location at the inlet highlight. However, the

wall axial location had no effect on noise reduction at I00

and 105 percent fan design speed where the noise is highly

aft-dominant.

The fan first overtone (2BPF) was cuton at all fan

speeds. Figure 42 shows the sound power levels (PWL)

for the I/3 octave band containing this 2BPF tone as a

function off an speed. Again, the data are for the translating

microphone probe over an emission angle range from 25

to 130°. These results are very similar to those seen for the

OAPWL in Fig. 39. Figure 43 shows the tone PWL

derived from 59 Hz narrowband data. The use of this finer

bandwidth facilitates better separation of the 2BPF tone

from other noise, such as broadband. These results are

similar to the I/3 octave tone results of Fig. 42, although

the actual noise reductions due to the wall are somewhat

higher, being 15 or more dB at the highest fan speeds.

These acoustic results show that the barrier wall can

be a useful tool for isolating inlet radiation from an aftdominated fan in the LeRC 9xl 5 LSWT. This technique

should have application for investigating inlet acoustic

treatment effects or wherever it is desirable to eliminate aft

noise from the acoustic signature in the 9x15 anechoic

wind tunnel.

Concluding Remarks

An advanced high bypass ratio fan model was tested

in the NASA Lewis 9- by 15-Foot Low Speed Wind

Tunnel. The primary focus of this test was to quantify the

acoustic benefits and aerodynamic performance of sweep

and lean in stator vane design. Three stator sets were used

for this test series. A conventional radial stator was tested at

two rotor-stator axial spacings-a relatively close spacing

and a more open spacing, axially downstream. Additional

stator sets incorporating sweep + lean, and sweep only were

also tested. The hub and tip axial locations for the swept+

leaned, and swept only stators corresponded to the hub and

tip locations of the radial stators at the two axial spacings.

In theory, the use of about 30° of sweep and lean should

significantly reduce the impact of rotor wake-stator

interaction, thus resulting in lower rotor-stator noise levels.

The results clearly showed that incorporation ofstator

sweep + lean, or sweep only can significantly reduce

rotor-stator tone levels. Tone levels for the modified

stators were significantly reduced beyond what was

achieved by simply relocating the conventional radial

stator to the downstream location. It is not clear if stator

sweep alone is typically adequate to achieve substantial

reductions in rotor-stator interaction noise, or if there are

significant additional benefits to be realized through

incorporation of both sweep + lean. In particular, the

aerodynamic performance of the swept + leaned stator

showed somewhat higher losses than that of the other

stators, suggesting that noise reductions associated with

this stator may be further improved through enhanced

aerodynamic design of a swept + leaned stator.

There is increasing interest in reducing broadband

noise levels of advanced subsonic turbofans. Noise

signatures of modern turbofan engines are increasingly

dominated by broadband noise rather than rotor-stator

tone noise. Increased axial spacing of a conventional

radial stator does not impact the fan broadband noise level,

except, perhaps, to increase the potential for broadband

noise generation through increased scrubbing surface, etc.

However, the results for the swept + leaned, and swept

only stators reported herein did show a significant reduction

(often on the order of 4 dB) of the broadband noise relative

to that generated with the radial stator.

Acoustic results scaled to a fictitious 2-engine aircraft

and flight path suggested that about 3 EPNdB could be

realized through incorporation of these modified statorsa result which could represent a significant part of the

NASA/TM--1998-208661 7
currentASTinitiativegoalofa6EPNdBreductionrelative

to 1992 technology levels.

These results suggest that incorporation of some

combination of stator sweep and lean may significantly

reduce both tone and broadband noise levels for future

advanced turbofans. Additional research in this area should

further quantify the aeroacoustic performance of these

modified stators and give insights into methodology for

additional engine noise reduction.

REFERENCES

1. Tyler, J.M. and Sofrin, T.G., "Axial Flow Compressor

Noise Studies," SAE Trans., Vol. 70, 1962,

pp. 309-332.

2. Woodward, R.P., and Glascr, F.W., "Wind Tunnel

Measurements of Blade/Vane Ratio and Spacing

Effects on Fan Noise," AIAA Journal of Aircraft,

Vol. 20, No. I, January, 1983, pp. 58-65.

3. Kazin, S. B., "Radially Leaned Outlet Guide Vanes for

Fan Source Noise Reduction," NASA CR-134486,

November, 1973.

4. Envia, E., Huff, D.L. and Morrison, C.R., "Analytical

Assessment of Stator Sweep and Lean in Reducing

Rotor-Stator Tone Noise," AIAA 96-1791, May,

1996.

5. Dahl, Milo D., and Woodward, Richard P.,"Comparison

Between Design and Installed Acoustic

Characteristics of the NASA Lewis 9- by 15-Foot

Low Speed Wind Tunnel Acoustic Treatment,"

NASA TP-2996, April, 1990.

6. Dahl, Milo D., and Woodward, Richard P., "Acoustical

Evaluation of the NASA Lewis 9- by ! 5-Foot Low

Speed Wind Tunnel," NASA TP-3274, November,

1992.

7. Woodward, Richard P., and Dittmar, James H.,

"Background Noise Levels Measured in The NASA

Lewis 9- by 15-Foot Low-Speod Wind Tunnel,

"NASA TM-106817, AIAA-95_)720, January,

1995.

8. Dalton, W.N., Elliott, D. B., and Nickols, K.L., "Design

of a Low Speed Fan Stage for Noise Suppression,"

Allison Engine Company, Allison EDR 17923,

NASA CR-"TBD", June, 1997.

9. Chestnutt, D., "Flight Effects of Fan Noise," NASA

CP-2242, January, 1982.

TABLE I.--ALLISON FAN DESIGN CHARACTERISTICS

Rotor diameter ......................................................................... 55.9 cm (22 in.)

Rotor blade number ........................................................................................ 18

Rotor hub/tip ratio ....................................................................................... 0.30

Rotor aspect ratio ....................................................................................... 1.754

Stator vane number ...................................................................... Call modes) 42

Stator aspect ratio ..................................................................................... 3.073

Swept and leaned stator ....................................................... 30 ° lean/30 ° sweep

Swept only stator ............................................................................... 30 ° sweep

Design stage pressure ratio ........................................ 1.378 (I .45 tip-1.20 hub)

Design specific weight flow .............................. 210.4 kg/slm2 (43.1 Ibm/s/ft 2)

Design corrected tangential tip speed .................................. 305 m/s (1000 ft/s)

Design tip relative Math number .............................................................. 1.080

NASAfI'M-- 1998-208661

_1 I i
Figure 1 .--Photograph of the Allison fan installed in the NASA Lewis 9xl 5 LSWT.

r

I

(a)

''I" .....

i ! _.w

t ,_'-" T_I_

rT"

(b) __._-_-_

Figure 2.--Sketch of the Allison fan with the baseline radial stator in the

forward and aft positions. (a) Upstream position. (b) Downstream position.

NASA/TM--1998-208661 9
(b) _.

Figure 3.--Sketch of the Allison fan with the swept + leaned and swept only

stator. (a) Swept only stator. (b) Swept + leaned stator.

Figure 4.PPhotograph of the partially-assembled fan stage

showing the swept + leaned stator.

NASA/TM--1998-208661 10
Figure 5.RPhotograph of the partially-assembled fan

stage showing the swept-only stator.

Figure 6.--Downstream view of the swept + leaned stator.

NASA/TM--1998-208661 II
Figure 7.--Downstream view of the swept + leaned stator viewed through the rotor. The

fan direction of rotation is counter-clockwise.

2.74 by 4.58 m

(9xl 5 ft) low speed

test section

\

\

Cooler

!.44 by 1.83 m

(8x6 ft) supersonic

test section

_- Flow

control

doors

Muffler

\

_- Flexible nozzle

Air dryer --

\

_- Compressor

Drive

motors

Figure 8.mNASA Lewis 9xl 5 Low Speed Anechoic Wind Tunnel.

NASA/TM--1998-208661 12

i'till
I J-- Reference wall

_- 872 (343) ="_/ microphone at

_T_" _'_! _ home position

834 (349) _1

;.

224 (88) microphones

V

76 (30)

Flow _ 363(151) I

Figure 9.--Sketch of the Allison fan installed in the 9x15 wind tunnel anechoic test section. (All dimensions are in

cm (in.)).

4.54_kg/s

(1o.O_m/s)

0

_=

0

L_

40 50 60 70 80 90 100 110 120

Fan corrected speed, percent

Figure 10.--Fan stage weight flow comparison for the radial

(baseline), swept + leaned, and swept only stator.

NASA/TM--1998-208661 13
U)

o

E

(/)

_o

g --

8 ..... Vane l

configuration

O Swept

7 _ =-- [] Swept + leaned

..............

5 ..... _1 _1 __

[]

1 - ()

0

40 50 60 70 80 90 100 110 120

Fan corrected speed, percent

Figure 11.ESystem thrust loss relative to baseline (radial) stator.

(Note: Vanes not optimized for performance - no core flow and

sharp comer flows. Swept only vanes partially optimized using

area ruling at the tip.)

I

10

EPNdB

ition I 1

50 55 60 65 70 75 80 85 90 95 100 105 110

Corrected design speed, percent

Figure 12.ESideline EPNL for fictitious 2-engine aircraft and flight path. Maximum noise level for an

observer on a 610 m (2000 ft) sideline.

NASA/TM--1998-208661 14

i1 !li
t

10

EPNdB

ii

i i

_r

i

---e-- Swept + leaned stator

Swept only stator

Radial stator in forward position

Radial stator in aft position

50 55 60 65 70 75 80 85 90 95 100 105 110

Corrected design speed, percent

Figure 13.--Flyover EPNL for fictitious 2-engine aircraft and flight path. Maximum noise level for an

observer 3.5 nautical miles from brake release in line with runway.

t

10

EPNdB

JJ

!

l

Broadband only

--t-- Swept + leaned stator

---!!--- Swept only stator

---O--- Radial stator in forward position

Radial stator in aft position

70 75 80 85 90 95 100

Corrected design speed, percent

50 55 60 65 105 110

Rgure 14.ESideline broadband EPNL for fictitious 2-engine aircraft and flight path. Maximum noise

level for an observer on a 610 m (2000 ft) sideline.

NASA/TM--1998-208661 15
10

EPNdB

(

I

70 75

Broadband only

Swept + leaned stator

Swept only stator

--O--- Radial stator in forward position

Radial stator in aft position

50 55 60 65 80 85 90 95 100 105 110

Corrected design speed, percent

Figure 15.--Flyover broadband EPNL for fictitious 2-engine aircraft and flight path. Maximum noise

level for an observer 3.5 nautical miles from brake release in line with runway.

110 --

,,,,,, ?

,' --0--- Swept + leaned stator "

! ! _ Swept only stator II I

I

I

Radial stator in forward position _

1_ Radial stator in aft position \ I1

,00-- i '\

r

80

I

\i

4

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 16.--One-third octave directivities along a 224 cm (88 in.) sideline (50 percent fan design speed,

2BPF tone).

NASA/TM--1998-208661 16

T1 _F
100

9O

so

(/)

70

3BPF

-- 2BPF I

Swept + leaned stator

Swept only stator

Radial stator in forward position

Radial stator in aft position

U

6o I I I I I I I t I I

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency, Hz

Figure 17.--Constant bandwidth (59 Hz) spectra on a 224 cm (88 in.) sideline at 126 ° from inlet axis.

Fan is operating at 50 percent design speed.

rn

10

c

.o

o

Z

20 ---e-- Swept + leaned stator _ -=1 [ I

Swept only stator

15 , _ Radial stator in aft/position

10 _" _ - .= !_! __

0 ]Tone attenuat_onl

-5 [ 1 I

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

"_ 0

Z

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 18.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (50 percent fan design

speed, SPL at 2BPF).

NASA/TM-- 1998-20866 i I7
20

rn

o 0

z

-5

20 30

Swept + leaned stator I

--m--- Swept only stator I

Radial stator in aft position I I

- "

40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

m -.!

=" -5

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 19.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (50 percent fan design

speed, SPL at 3BPF).

120 --

110 --

"0

_r 100

Q.

O3

90

ft

2BPF

t

_m

3BPF

BPF ! ]

k

Swept + leaned stator

Swept only stator

.......... Radial stator in forward position

Radial stator in aft position

4BPF

I

m r I|,_" .,

80 I I I 1 1 1 I I I I

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency, Hz

Figure 20.wConstant bandwidth (59 Hz) spectra on a 224 cm (88 in.) sideline at 126 ° from inlet axis.

Fan is operating at 84 percent design speed.

NASA/TM--1998-208661 18

i_! I]
I

__ Swept + leaned stator

20 -----1 _ --i-- Swept only stator _

10

o

-5

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

,00 uation

t.. -5

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 21.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (84 percent fan design

speed, SPL at 2BPF).

2010

":_ 0 ITone attenuation !i

-5 I t I

20 30 40 50

/

---4l--- Swept + leaned stator t

---l--- Swept only stator _1

__Radial stator in aft ,p°siti°n, ______l ___

-I - I .

60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Z Broadband attenuation '

-520 30 40 50 60 70 80:90 100 110 120 130 140 150_-_60

Emission angles along a 224 cm (88 in.) sideline

Figure 22.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (84 percent fan design

speed, SPL at 3BPF).

NASA/TM--1998-208661 19
12O

110

rn

"10

._.f 100

13.

90

wwww====.,

Swept + leaned stator

Swept only stator (data not available)

Radial stator in forward position

Radial stator in aft position

BPF

2BPF

I

4BPF

1

3BPF

I

I!

II

LJ;J I

5BPF

I

6BPF

1

80 I I 1 I [ I I 1 I I

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Fmquency, Hz

Figure23.--Constantbandwidth _9 H_ spectra on a 224 cm(88in.)sidelineat126°from inlet axis.

Fanis operating atl00 pementdesign speed.

m

"o

o

¢.)

"O

==

.u

O

z

Swept + leaned stator ]

20 I ---!!--- Swept only stator (data not available) 1 I

/ _ Radial stator in aft posi!ion ....... I I

5 ..............

0 T°neattenuationI I I- 1 1

-5 _ _ _ _ , _ ._L , .._

20

i'

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

nn

"1o

o=o Z

5 I--Broadbandattenuationl/ I 1 / [ } I

-5

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 24.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (100 percent fan design

speed, SPL at 1BPF).

NASA/TM--1998-208661 20

i_1l i
20

15

rn

E 10

._o

= 5

.__ 0

-5 _

---e--- Swept + leaned stator

Swept only stator (data not available) |

---CF--- in aft position Radial stator l

Tone attenuation

-10 •--f----q--

20 30 40 50 60 70 80 90 IO0 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

----E3

i 5 iBroa_lband,attenu_tio n - t _

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 25.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (100 percent fan design

speed, SPL at 2BPF).

i _ Swept + leaned stator

Swept only stator (data not available) 20 ---4

I _ Radial stator in aft position

15

-10

20 30 40 50

ITone attenuation

I i

[_

60 70 80 90 100 110 120

Emission angles along a 224 cm (88 in.) sideline

_1_ I

_l_-lu -44-

-

b

130 140 150 160

"o_ ' _ L=__j_ ___lBr°adbandattenuation,_ 1----_== 1 =='_=_==

d) ==

z _' "-.--..___.__

-5'

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 26.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (100 percent fan design

speed, SPL at 3BPF).

NASA/TM--1998-208661 21
120 --

110

m

-o

j 100

0.

9O

I

-Rn|

It

n

=.........

Swept + leaned stator

Swept only stator

Radial stator in forward position

Radial stator in aft position

2BPF 3BPF 4BPF

I I 1

BPF

I

*

--N

5BPF

I

'.', l

80 [ L. I [ 1 I I I ....... I I

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Frequency, Hz

Figure 27.---Constant bandwidth (59 Hz) spectra on a 224 cm (88 in.) sideline at 126 ° from inlet axis.

Fan is operating at 110 percent design speed.

15--

m 10

0

L.

-5--

-10

20

Swept + leaned stator

---B--- Swept only stator

Radial stator in aft position _

Tone attenuation I • =

I 1 J ........

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

"_ _ _ _Broadband attenuationl

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 28.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (110 percent fan design

speed, SPL at 1BPF).

NASA/TM-- 1998- 208661 22

i_11ii
rn

"0

ff

0

"10

,=

o

z

!

i

o ' \l! -

--(b--- Swept + leaned stator

-10 ---!1-- Swept only stator ---- --

Radial stator in aft position

-15 I I I l I

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 29.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (110 percent fan design

speed, SPL at 2BPF).

20

15

m lo

-10 " -- 1

-15

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

-5 ' _.G [Broadband attenuation.

20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Emission angles along a 224 cm (88 in.) sideline

Figure 30.--Sideline constant bandwidth (59 Hz) directivities showing noise reduction relative to baseline configuration with the radial stator in the forward (upstream) position (110 percent fan design

speed, SPL at 3BPF).

NASA/TM-- 1998-208661 23
Swept + leaned stator

---C}-- Swept + leaned stator, repeat data set

--_ [ T I-- ;

20 30 40 50 60 70 80 90 100 110 120 130 140

Emission angle along a 224 cm (88 in.) sideline

10dB- __T-I' i i il --0--Rad,a, stator,na, pos,tion H

i _1 I t I tl _ Rad,a, stator,n a._osit_on. repeat datali

1 . I [ l 1 I t I 1 I I

20 30 40 50 60 70 80 90 1O0 110 120 130 140

Emission angle along a 224 cm (88 in.) sideline

Figure 31.--Data comparison for two builds of the same stator configuration showing data repeatability. (1/3rd octave directivities at 2500 Hz, 84 percent design speed.)

I V_--- =].= I

_<>_ Swe )t + leaned stator i

10dB .....

20 30 40 50 60 70 80 90 100 110 120 130 140

Emission angle along a 224 cm (88 in.) sideline

_ i

I_ _--O_ "adial s t_r in aft _p°siti°nj t I i

___ Radial stator in aft position, repeat_;__ !

I i

20 30 40 50 60 70 80 90 - 100 110 120 130 140

Emission angle along a 224 cm (88 in.) sideline

Figure 32.--Data comparison for two builds of the same stator configuration showing data repeatability. (1/3rd octave directivities at 20 000 Hz, 84 percent design speed.)

NASA/TM--1998-208661 24

;;1 I !i
Figure 33.--Photograph of the Allison fan installed in the 9x15 LSWT with the acoustic

barrier wall in place. The wall is shown in its upstream position with the wall leading

edge at the fan inlet highlight.

i

• L it/

Track microphone probe ....

15 cm

(6 in.) I _-- Treated section

_ , _ Barrier wall

Figure 34.--Sketch of the acoustic barder wall installed in the 9xl 5 LSWT.

NASA/TM--1998-208661 25
100

en

'lo

J

z't

03

i

Baseline, no wall 70

--0-- Wall at inlet leading edge highlight

Wall 15 cm (6 in.) downstream of inlet

leading edge highlight

I 1....

60

20 30 40 50 60 70 80 90 1O0 i I 0 120 130 140

Emission angles along a 224 cm (88 in.) sideline

Figure 35.--Effect of acoustic barrier wall (50 percent design speed, broadband at 2500 Hz).

110 L = _ L i J

Baseline, no wall

--0--- Wall at inlet leading edge highlight

Wall 15 cm (6 in.) downstream of inlet

leading edge highlight

100 -

70

20 30 40 50 60 70 80 90 1O0 110 120 130

Emission angles along a 224 cm (88 in.) sideline

Figure 36.--Effect of acoustic barrier wall (50 percent design speed, 3BPF at 5000 Hz).

140

NASA/TM--1998-208661 26

!_1!11
110

11

100 . ._=_1(

80 _ Baseline, no wall

Wall at inlet leading edge highlight

Wall 15 cm (6 in.) downstream of inlet

leading edge highlight

I L 1 t L

70

20 30 40 50 60 70 80 90 1O0 110 120 130 140

Emission angles along a 224 cm (88 in.) sideline

Figure 37.--Effect of acoustic barrier wall (100 percent design speed, broadband at 4000 Hz).

120 I I I I I I

Baseline, no wall

_ Wall at inlet leading edge highlight _VQ_D --

---C}--- Wall 15 cm (6 in.) downstream of inlet .

leading edge highlight -)_1_

100

8O

20 30 40 50 60 70 80 90 100 110 120 130

Emission angles along a 224 cm (88 in.) sideline

Figure 38.--Effect of acoustic barrier wall (100 percent design speed, 4BPF at 12 500 Hz).

140

NASA/TM--1998-208661 27
130

d

120

110

50 55 60 65 70 75 80 85 90 95 100 105 110

Design fan speed, percent

Figure 39.mEffect of barrier wall on 1/3rd octave overall sound power level measured form 2000 to

20 000 Hz. (Data from translating microphone probe.)

"o

o.

<:

O

140

130

120

110

100

50 55 60 65 70 75 80 85 90 95 100 105 110

Design fan speed, percent

Figure 40.mEffect of barrier wall on 1/3rd octave overall sound power level measured form 2000 to

20 000 Hz for emission angles from 25 to 61 °. (Data from translating microphone probe.)

NASA/TM--1998-208661 28

?] li_
m

-o

J

£

<_

o

140

130 --

J

120

J

f

j,S

Baseline radial stator, no wall

---O-- Wall at inlet leading edge highlight

--U]--- Wall 15 cm (6 in.) downstream of inlet

leading edge highlight

lOO I I 1 I I I

50 55 60 65 70 75 80 85 90 95 100 105 110

Design fan speed, percent

Figure 41.DEflect of barrier wall on l/3rd octave overall sound power level measured form 2000 to

20 000 Hz for emission angles from 61 to 130 °. (Data from translating microphone probe.)

"o

o

130 

120_

110_

50 55 60 65 70 75 80 85 90

Design fan speed, percent

95 100 105 110

Figure 42.DEflect of barrier wall on 1/3rd octave 2BPF tone sound power level. (Data from translating

microphone probe.)

NASA/TM--1998-208661 29
130 T

-ore120F-_

100_

90 L----_-

50 55

i n

60 65 70 75 80 85 90 95 100 105 110

Design fan speed, percent

Figure 43.--Effect of barrier wall on 59 Hz narrowband 2BPF tone sound power level. (Data from

translating microphone probe.)

NASA/TM--1998-208661 30

II!
REPORT DOCUMENTATION PAGE I Form Approved OM8 No. 0704-0188

Public reporting burden for this collection of information Is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of inlormation, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503,

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

November i 998

4. TITLE AND SUBTITLE

Benefits of Swept and Leaned Stators for Fan Noise Reduction

6. AUTHOR(S)

Richard E Woodward, David M. Elliott, Christopher E. Hughes,

and Jeffrey J. Berton

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

3. REPORT TYPE AND DATES COVERED

Technical Memorandum

5. FUNDING NUMBERS

WU-538-03-11-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-II382

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1998-208661

AIAA-99_0479

11. SUPPLEMENTARY NOTES

Prepared forthe 37th Aerospace Sciences Meeting and Exhibitsponsored bythe American Institute of Aeronautics and

Astronautics, Reno, Nevada, January I 1-14, 1999. Responsible person, Richard E Woodward, organization code 5940,

(216)433-3923.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified- Unlimited

Subject Categories: 02, 07, and 71 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 621-0390

13. ABSTRACT (Maximum 200 words)

An advanced high bypass ratio fan model was tested in the NASA Lewis Research Center 9xl5-Foot Low Speed Wind Tunnel.

The primary focus of this test was to quantify the acoustic benefits and aerodynamic performance of sweep and lean in stator

vane design. Three stator sets were used for this test series. A conventional radial stator was tested at two rotor-stator axial

spacings. Additional stator sets incorporating sweep + lean, and sweep only were also tested. The hub axial location for the

swept + lean, and sweep only stators corresponded to the location of the radial stator at the upstream rotor-stator spacing,

while the tip axial location of these modified stators corresponded to the radial stalor axial position at the downstream position.

The acoustic results show significant reductions in both rotor-stator interaction noise and broadband noise beyond what could

be achieved through increased axial spacing of the conventional, radial stator. Theoretical application of these results to

acoustically quantify a fictitious 2-engine aircraft and flight path suggested that about 3 EPNdB could be achieved through

incorporation of these modified stators. This reduction would represent a significant portion of the 6 EPNdB noise goal of the

current NASA Advanced Subsonic Technology (AST) initiative relative to that of 1992 technology levels. A secondary result

of this fan test was to demonstrate the ability of an acoustic barrier wall to block aft-radiated fan noise in the wind tunnel, thus

revealing the acoustic structure of the residual inlet-radiated noise. This technology should prove valuable toward better

understanding inlet liner design, or wherever it is desirable to eliminate aft-radiated noise from the fan acoustic signature.

14. SUBJECT TERMS

Noise; Swept and leaned stator; AST

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

36

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298 - 102
!1 ! i