NASA / TM--2001-210699

Fan Noise Reduction: An Overview

AIAA-2001-0661

Edmane Envia

Glenn Research Center, Cleveland, Ohio

February 2001
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NASA/TMm2001-210699 AIAA-2001-0661

Fan Noise Reduction: An Overview

Edmane Envia

Glenn Research Center, Cleveland, Ohio

Prepared for the

39th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 8-11, 2001

National Aeronautics and

Space Administration

Glenn Research Center

February 2001
NASA Center for Aerospace Information

7121 Standard Drive

Hanover, ME) 21076

Price Code: A03

Available from

National Technical Information Service

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Price Code: A03

Available electronically at htt-p://gltrs.grc.nasa.gov/GLTRS
FAN NOISE REDUCTION: AN OVERVIEW

Edmane Envia*

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

Fan noise reduction technologies developed as

part of the engine noise reduction element of the

Advanced Subsonic Technology Program are

reviewed. Developments in low-noise fan stage design,

swept and leaned outlet guide vanes, active noise

control, fan flow management, and scarfed inlet are

discussed. In each case, a description of the method is

presented and, where available, representative results

and general conclusions are discussed. The review

concludes with a summary of the accomplishments of

the AST-sponsored fan noise reduction research and a

few thoughts on future work.

INTRODUCTION

With the advent of high bypass ratio turbofan

engines, the fan has become a major source of modem

commercial aircraft propulsion noise. In fact, engine

system noise studies [1] indicate that, at both takeoff

and approach operations the fan noise tends to

dominate the engine total flyover noise signature even

when noise suppression due to acoustic liners is

included {see Fig. 1). The anticipated growth in the

engine bypass ratio is likely to increase the importance

of the fan noise even further. Therefore, any significant

reduction in the level of noise produced by modem

aircraft power plants must include provisions for

controlling and reducing the fan noise.

The early work in the area of fan noise reduction

developed along two distinct lines: (1) noise source

control and (2) noise level reduction. Examples of

source control methods include, blade-vane count

selections to achieve "cut-off" of the rotor-stator

interaction tone noise caused by the fan wakes

impinging on the core inlet and bypass outlet guide

vanes, rotor-stator spacing optimization to weaken the

impinging wakes, clean inlet designs to minimize

*Aerospace Engineer, Senior Member, AIAA.

inflow distortions ingested by the fan, and minimizing

the potential pressure fields from engine struts and

pylons in which the fan has to operate. The noise

reduction methods on the other hand have mainly

involved the use of inlet and exhaust fan duct acoustic

liners to absorb the noise radiated by the various fan

sources. However, while, for the most part, these

methods have proven effective, they have also tended

to suffer from inherent limitations. For example, the

cut-off method is primarily used to eliminate rotorstator tone noise at the blade passing frequency (BPF),

since the blade-vane counts required for cutting off the

higher harmonics of the BPF are usually not practical.

Similarly, the rotor-stator spacing optimization method

is always constrained by the size and weight penalties

associated with increasing the engine length. As for the

,¢,

I"

._ ¢_

Figure 1.

t15 '

lo,_!

I00 _°

70 !_

t;5 L,

i,<

d

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.2

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i

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-, treatment}

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-_ , With

, acot=stit tteat n)ent

1

Takeoff

Representative high bypass ratio turbofan engine

flyover noise levels on a component basis. Figure

reproduced from Ref. 1.

NASA/TM--2001-210699 1
liners,theireffectiveness ilikelytodiminishasengine

bypassratioisincreased. Thisismainlyduetothefact

that an increasein the bypassratio is usually

accompaniedbya decreasein thenacellelengthand

thicknessand,hence,a decreasein the available

treatment area[2].Lesstreatment areameanslessnoise

reductionbenefitsfromliners.

Tocircumvent theselimitationsanddevelop new

noise reduction technologies, NASA in partnership

with the FAA and the U.S. aerospace industry began a

comprehensive program of aircraft noise reduction

studies in 1992. These efforts were undertaken as part

of the Advanced Subsonic Technology Noise

Reduction Program and included both airframe and

engine noise reduction research. Specifically, the

engine noise reduction element called for 6 EPNdB

(Effective Perceived Noise dB) reduction in the level

of the engine system source noise relative to 1992

technology by the end of the last decade [3].

The engine noise element of the Advanced

Subsonic Technology (AST) program included work

on reducing both the fan and jet associated noise. The

fan noise reduction portion itself was comprised of

research in such areas as low-noise fan stage design,

swept and leaned outlet guide vanes, active noise

control, fan flow management, scarfed inlets, and

advanced liners. In this paper we shall summarize these

efforts and provide representative results. One notable

exception is that we will not touch upon the acoustic

liners which saw significant development under the

AST program. This is an extensive area deserving of a

separate review. Furthermore, since this review will

focus on the AST work exclusively, it will also not

include the research that was conducted outside of the

purview of the AST program or that which was carried

out in Europe or Japan during the same time period.

In what follows, the various noise reduction

techniques will be listed in no particular order. In each

case, a description of the method and its underlying

principles will be presented. Where final assessments

have been completed, a discussion of the relevant

results, issues and conclusions will also be presented.

Highlights from several efforts that were initiated

under the AST engine noise reduction program but

have not yet been fully assessed will also be included.

The paper will conclude with a summary of current

accomplishments and a few thoughts on future work.

FAN NOISE REDUCTION TECHNIQUES

Advanced Ducted Propulsor

Incorporating all of the proven fan noise

reduction technologies of the time, Pratt and Whitney

designed and built [4] a scale model fan stage known

as the Advanced Ducted Propulsor (ADP), shown in

Fig. 2, to demonstrate the feasibility of a propulsion

system capable of meeting the AST noise reduction

goal of 6 EPNdB.

Figure 2. The Advanced Ducted Propulsor fan. Pictured is

one of the 22" variants of the concept called Fan 1

shown installed in the NASA 9'xl 5' wind tunnel.

The ADP, which is built around a low tip-speed t

variable-pitch fan, features large rotor-stator spacing

and cut-off vane counts for both the bypass and core

stators. The design also takes advantage of advanced

liners in the inlet, mid-stage and exhaust sections of the

fan duet to further mitigate the noise (see Fig. 3).

While finalized system noise studies are not yet

available, results from a number of NASA wind tunnel

tests (see, for example, Refs. 5 and 6) indicate that the

ADP is likely to fulfill its original design goal of

meeting or exceeding the AST engine noise reduction

target. Of course, the ADP represents a departure from

the conventional cycle design and it remains to be seen

whether it will be embraced by the industry.

. [ Mlct-Stage Ltner] ,

l ,n,et uner / ' / ' ]Att Une-e I

Figure 3. Acoustic liner locations inside the ADP fan duct.

_ Estimates based on the (Vtip)8 rule suggest substantial noise

benefits from lowering the fan tip speed significantly.

NASA/TM--2001-210699 2
Outlet Guide Vane Sweep and Lean

One of the great success stories of the AST

engine noise reduction program has to be the proof that

guide vane sweep and lean s is an effective means of

reducing fan noise. Starting in the early '70s, several

studies had hinted at the potential acoustic benefits of

stator vane sweep and lean for reducing fan tone noise

[7-11], but it wasn't until the AST program that the

effectiveness of vane sweep and lean was convincingly

demonstrated.

In a NASA/Allison wind tunnel test [12], farfield

radiated noise levels produced by four aerodynamically

equivalent outlet guide vane (OGV) configurations

[13] were measured. The configurations included: a

radial OGV (see Fig. 4a), the radial OGV but with

increased rotor-stator axial spacing (see Fig. 4b), a 30-

degree swept OGV (see Fig. 4c), and a combination

30-degree swept and 30-degree leaned OGV (see Fig.

4d). The radial stator, representing the standard OGV

design practice, served as the baseline against which

the acoustic performance of the swept and leaned stator

could be compared. The radial stator in the "aft"

position was included to isolate noise reductions due to

increased spacing that are realized when sweep is

introduced, and the swept-only stator was included in

an attempt to separate the sweep effects from those due

to lean.

The test showed significant tone noise reductions

with a swept and leaned OGV as illustrated by the

2BPF directivity results shown in Fig. 5. In this plot the

noise benefits (i.e., tone level attenuations) are plotted

relative to the radial OGV noise levels (a positive

number is benefit) at both the approach and takeoff

conditions. The swept and leaned stator shows

significant noise benefits for all angles with reductions

on the order of 5dB in the inlet quadrant and over 10dB

in the exhaust quadrant at both conditions. On an

EPNdB basis the results are equally impressive (see

Fig. 6) showing more than 3 EPNdB noise reductions

over the entire range of fan tip speeds for the swept and

leaned OGV compared with the radial OGV in its

nominal (forward) position.

The test results also indicate that the swept and

leaned stator is quieter even when compared with the

radial stator in the aft position. This suggests that the

effectiveness of sweep and lean is not solely due to the

additional viscous wake decay that is realized through

the increased rotor-stator spacing for the swept and

leaned stator as compared with the radial stator in its

_tSweep is the axial and lean the circumferential displacement

of the vane leading edge from its radial position.

Figure4. Photographs of partially assembled fan stages

showing the four stator packs used in the sweep

and lean study. (a) Baseline radial stator, (b) radial

stator in aft position, (c) swept stator, and (d) swept

and leaned stator.

NASA/TM--2001-210699 3
a • _---_- _=_t_ ,

_0 30 40 :5o 61) 70 tic gO 100 I10 I_0 130 140 150 160

Emisli_ a_g_s _g a 224 c_ _ _.)

I ([hi T_lceoff -.-r_ t_dlalslltorin afll_lllorl] _ .m ! _ l _

20 30 40 50 60 70 _) 90 100 110 f2O 130 140 150 160

._._lOrl ingJes alo_ a _4 cm [_8 inJ _e

Figure 5. 2BPF sideline directivities showing noise

reductions relative to the baseline stator (radial

OGV in its nominal forward position). Benefits

shown for (a) approach condition and (b) for

takeoff condition. Figure reproduced from Ref. 12.

Figure 6. Sideline EPNL for fictitious twin-engine aircraft

and flight path. Maximum relative noise levels on a

2000 fl sideline are shown. Figure reproduced from

Ref. 12.

forward position. Part of the noise benefit is due to the

additional variation that occurs in the phase of the

incident wake along the vane span due to the

introduction of Sweep and/or lean: More sp_nwise

phase variation of the wake means more noise

cancellation that can occur between the contributions

from different locations along the vane span resulting

in less interaction noise. Viewed in terms of the

kinematics of wakes in relation to vanes, the noise

benefits come from having more wakes intersecting a

single vane with sweep and lean than without [14]. As

shown in Fig. 7, there are more wake-vane

intersections for the swept and leaned stator compared

with the radial one.

One unexpected result was the apparent acoustic

advantage of the swept-only stator over the swept and

leaned stator for some fan tip speeds (say, 70% to

95%). A theoretical design study [15] had indicated

that the combination of sweep and lean was more

effective than sweep alone. Analysis of the

aerodynamic performance of the OGVs showed that

the swept and leaned stator had somewhat higher

aerodynamic losses than had been anticipated. This

suggests that an improved aerodynamic design would

have probably realized the full acoustic benefits of the

swept and leaned stator. Nevertheless, the test did in

fact prove the potential for significant noise reductions

through the use of vane sweep and lean.

r

la) _)

Figure 7. Schematic of the kinematic relationship between

fan wakes and slator vanes. On the left. the picture

depicts a typical relationship for a radial stator and

on the right, for a swept and leaned stator. There

are more wake/vane intersections for the swept and

leaned stator. Figure reproduced from Re£ 14.

Active Noise Control

Motivated by the idea that a given acoustic field

can be cancelled by another acoustic field of equal

amplitude but opposite phase, a number of studies were

carried out to determine the feasibility of active control

of fan noise §. Owing to the complicated nature of the

noise field inside a fan duct, all of these "firstgeneration" techniques were aimed at canceling only

fan noise with well-defined modal qualities. For this

reason a dedicated active noise control fan (ANCF) rig

was designed and built [ 17] as the testbed for assessing

these techniques. The 4-foot diameter fan, shown in

Fig. 8, has the unique capability for generating specific

rotor-stator interaction mode or modes" at frequencies

similar to those produced by large turbofan engines. At

the same time, the rig can also accommodate a wide

variety of active noise control systems. Despite their

variety, however, each of the active noise control

§ An early theoretical system study [16] indicated that active

control could reduce fan noise by up to 2 EPNdB.

"* These are the classical duct modes distinguished by their

circumferential (or spinning) order ra and radial index n.

NASA/TM--2001-210699 4
Figure8.TheNASAGlennActiveControl Fanrigwasthe

testbedfor mostof theactivenoisecontrol

experiments conductedundertheASTfannoise

reductionprogram.

conceptsestedwascomposedof threebasicelements:

(1) an "actuator"arrayto producethe canceling

acousticfield,(2) anerrorsensor(e.g.,microphone)

arrayto monitorthelevelof cancellation, and(3)a

controlalgorithmtoanalyzetheoutputfromthesensor

arrayand synthesizethe appropriateinputfor the

actuatorarrayinacontinuousself-correctingloop.

Theactuatorarraywasgenerallycomprisedof an

arrangement of resonant-typedriversor conventional

electromagnetic drivers(i.e.,speakers). Theparticular

arrangement ofhedriversusedwaspredicatedon:(1)

thenumberof spinningmodesthathadtobecancelled

simultaneously, and(2)onwhetherlocalcontrol(i.e.,

inletor exhaust noise cancellation) or global control

(i.e., simultaneous inlet and exhaust noise cancellation)

was desired. Depending on the particular concept, there

were single or multiple actuator rings in the inlet duct

upstream of the fan [18, 19], or in the exhaust duct

downstream of the outlet guide vanes [20], or flanking

the outlet guide vanes [2 l, 22]. The drivers in this type

of arrangements would be flush-mounted within the fan

duct walls as shown by the examples in Figs. 9 and 10.

A somewhat unique type of an arrangement was that

involving actuators embedded within the vanes t*

themselves as shown in Fig. 11. This approach is

described in detail in Ref. 25.

One so-called hybrid concept was also tested

which utilized both active and passive elements. The

active element was an arrangement of resonant-type

drivers while the passive element was a conventional

liner [26]. The working principle of this concept is

tt The initial concept study and development of candidate

vane actuators for this work may be found in Refs. 23 and 24.

Active Helmholtz

Resonators

(exterior view)

i Active Helmholtz7

_,, Resonators I

Figure 9. Active Helmholtz resonators drivers in a four-ring

arrangement around the duct outer wall upstream of

the fan in the inlet duct (see Ref. 18). View is from

inlet duct looking downstream.

Radiator Ring ]

Face Sheet [

(in-duct V!ew)[

Plate Radiator8

(exterior view)

"bC9 5 - 3 213 ...... _. __ ._,: _._:L2L;:._

Figure 10. Plate radiator drivers in a ring arrangement around

the outer wall of the fan duct. The location is

downstream of the OGV in the exhaust duct (see

Ref. 20). View is from exhaust duct looking

upstream.

schematically depicted in Fig. 12. An optimized

uniform (single-segment) liner (Fig. 12a) provides

some attenuation commensurate with the orientation of

the incident acoustic wave shown by the arrow. In a

tandem two-segment liner arrangement (Fig. 12b) the

first segment not only attenuates some of the incident

wave, it also redirects the remaining portion toward the

wall so that the second segment can more effectively

attenuate the remaining energy. So, for equal treatment

length, the two-segment liner system is more effective

than the uniform liner. The hybrid active-passive

system (Fig. 12c) improves on this scheme by allowing

the system to adapt to the changes in the orientation of

the original incident wave caused by the changes in the

engine operation ensuring that the benefits of the

passive portion are always optimized.

NASA/TM--2001-210699 5
C97-3073

Figure 11. In this technique, the actuators are embedded

within the profile of the stator vanes (see Ref. 25).

View is from the exhaust duct looking upstream.

(a) i/

Figure 12. Conceptual development of the hybrid activepassive system. Performance improvements over a

uniform liner (a) can be realized through the use of

a tandem two-segment liner (70).The hybrid activepassive system (c) not only provides comparable

performance to the two-segment liner, it also adds

the capability to adapt to the changing engine

environment. (See Ref. 26 for more details).

A summary of all of the AST active control tests

conducted using the ANCF rig is shown in Table 1_*.

The tests are organized in the order of increasing

complexity as defined by the number of spinning

modes that had to be controlled simultaneously. For

each entry, the particular spinning mode(s) at which

control was targeted and their relevant frequencies are

tabulated. The last column indicates whether local

2, There were other AST-sponsored active noise control tests

that were carried out on scale model engines or other rigs.

See, for example, Refs. 27, 28 and 29.

Table 1- Summary of Active Noise Control Tests

Oar

_i$,2i_ 1[ 4 1_ .... ]2BPF [tn!_:_:_;;l

6'25311 4 I ]2BPFII,n.+Ex.

II I 2 :

7_'_'_ 4 [(_, _0 ), (.,), _l (,.) 2 "_ I 2BPF In./Er, t

II ...... i](2,oL t2,i),(2,2"h (-.2,3) 1 [ In:,_X_:l

7b 1221 Same as 7a but with different control algorithm.

* Indicates the Ref. source for the test.

control (inlet or exhaust) or global control (inlet and

exhaust) was considered. In each case, control was

applied over a range of fan speeds to assess the

robustness of the system in adapting to the changes in

the mode characteristics as a function of the fan rpm.

To varying degrees, every one of the active noise

control tests demonstrated measurable reductions in the

level of the targeted mode(s). An example of the

results §§ from one of the earliest tests is shown in Fig.

13, which depicts the reduction in the level of exhaust

duct acoustic power level (PWL), denoted by the

shaded area, due to the application of active noise

control. The reduction is clearly significant averaging

around 18 dB over the range of fan speeds tested. In an

attempt to provide a summary of all of the results,

average total PWL reductions versus the number of

targeted modes are plotted in Fig. 14. The average is

over the range of fan speeds in each case and the total

is the sum of the power levels in all targeted modes (in

the inlet and/or exhaust ducts). While this may be

somewhat of a crude metric with which to gauge the

noise reductions via active noise control, it does

nonetheless serve as an indication of the potential of

the active control technology in its current stage of

development. In plotting the results, distinction is made

between the local control in the inlet only, local control

in the exhaust only, and global control in both inlet and

exhaust simultaneously. For each data point, a label

identifies the corresponding test listed in Table 1. For

the test number 7, over the range of tip speeds tested

there was an increase in the number of cut-on spinning

§§Detailed results from most of these tests were presented at

a recent meeting on active noise control [30].

NASA/TM--2001-210699 6
Figure13.Reductionof fan duct mode power level due to

active noise control over a typical range of fan

speeds tested in the ANCF rig. (Results plotted

from the data in Ref. 20).

modes from two to three and then to four in the inlet,

and from two to three in the exhaust. Therefore, the

reductions for each set of propagating modes are

plotted separately.

The results as plotted in Fig. 14, indicate that

there are significant noise reduction benefits from the

use of active noise control, but that the magnitude of

the noise benefits tends to diminish with increasing

number of simultaneously controlled modes. While a

5O

4O

3o

t¢

[,o

Summary of Active Noise Control Results

I o L a,contro,,,n ,, I I

• Local Control (Exhaust) ! I

........... D Global Control (Inlet and Exhau.sl).] ............. l

20

........................... i....................... 30 .....................................................................................

i 20

_Tb 713

,,o $ i

,'d + U

6+ +m

..... j............................ Z__4........ h 7b

O_ I 2 3 4 5

No. of Targeled Modes

Figure 14. Fan noise level reductions achieved by active

control. The labels refer to the test configurations

listed in Table 1.

detailed investigation of the reasons underlying this

trend is outside of the scope of this review, one

possible explanation may be as follows. Due to the

nature of the rotor-stator generated modes, multiple

duct modes always have a unique phase relationship

with each other that depends on the axial location in

the duct. Therefore, the level of control will be

dependent on the accuracy with which the sensor

array(s) can measure this phase relationship, and the

accuracy with which actuator array(s) can synthesize it.

Small errors in measurement and/or synthesis can

therefore produce a canceling field that does not

exactly match the target field resulting in less noise

control (reduction). Since the complexity of the mode

phase relationship increases with the number of modes,

the control may be less effective when many modes

exist compared with the situation when only one or two

mode(s) exist.

Nevertheless, the important point to remember is

that these tests clearly demonstrate the potential of

active noise control as a means of reducing fan tone

noise, particularly in circumstances when there are

only one or two dominant modes to be controlled. A

more general assessment regarding the utility of the

active noise control techniques is not possible at this

time since, to date, only one system analysis study"*

has been carried out that incorporates the results from

these tests,

Fan Wake Management

A novel approach for reducing fan tone noise

involves the use of mass injection (or "blowing") at the

blade trailing edge to reduce fan wake deficit. In

principle, this should render the flow impinging on the

downstream stator more uniform leading to lower

levels of unsteady loading on the vanes and, hence, less

rotor-stator interaction tone noise. Early experiments

on fiat plates [31] and 2D cascades [32] had established

the feasibility of this approach, but issues remained in

applying the method to realistic fan geometries. These

issues were first tackled in a research effort carried out

at MIT in the late 90's [33, 34]. Building on a series of

numerical and experimental investigations, a method

was developed for designing a fan to study flow (and

That system study (see Ref. 22) predicted minimal

benefits from the use of active noise control. However, this

conclusion is colored by the particular choice made for the

aircratVengine combination used in the system study, which

had de-emphasized the impact of tone noise reduction on the

system flyover noise. An aircraft/engine combination for

which tones are a more significant spectral component is

likely to show more benefits from the application of active

noise control.

NASA/TM--2001-210699 7
byimplication)noisecontrolinarealisticsetting.The

resultwasthefanshowninFig.15whosebladeseach

havea labyrinthof internalpassagesthatstartatthe

bladeroot, wheretheyreceivethe flow supplied

throughtheshaft,andterminateata seriesof trailing

edgeports,wherethesuppliedfluidisdischargedinto

thefanwakeflow.Provisionsweremadeto allowfor

spanwisetailoringoftheinjectionprofile.

Figure15.Close-upviewoftheMITblownrotor(left)anda

detailedviewof thebladeinternalpassages.

(Reproduced fromRef.34).

Combinationsof severalinjection rates and

profiles ttt were tested using this fan. In each case, the

flow downstream of the fan and the duct wall unsteady

pressure levels were measured. A typical flow result is

shown in Fig. 16. The trailing edge blowing has "filled

in" the original wake (solid line) to produce a more

uniform mean flow profile (dashed line). On a

harmonic basis (see the inset), the trailing edge

blowing has reduced the wake harmonic amplitudes by

more than a factor of two for the first four harmonics.

A summary of the unsteady pressure results is

shown in Fig. 17. Harmonic sound pressure levels

(SPL), measured on the outer duct wall in the inlet and

exhaust, are plotted for different injection rates.

Depending on the rate of injection and the particular

harmonic considered, wall SPL reductions as much as

9 dB were realized. However, sizeable increases (by as

much as 6 dB) were also observed in some cases.

While these results clearly indicate the influence of

wake management on the unsteady pressure field inside

the duct, general conclusions regarding the noise

benefits cannot be drawn. The reason is two fold. First,

since the MIT facility is non-anechoic, the wall

ttt Injection rate is defined as the percent of the fan through

flow. The injection profile refers to the spanwise distribution

of the discharge flow. Tip-biased and midspan-biased profiles

were considered in the MIT experiment.

D.65 L......... ........... ::....... j

I

L I " wlo TE Blowing'_; ; 1

060 L ..... !....... ] - w! TE Blowing _ _r ........ _'' _--''[

¢: _ ....

o

o.45L i _ _ ...... i..........i ........

)

U'° n : ......... ........ i

"

°.30i °]ittt,,!iii iiiiii] iiiiiiiiiiii

' i

L ......._ .................... _.'_...._.E.'_._.........._..........i. , .........i .......

o.:zs01. o o,1i o,2i 0.3i, 0.4_ o.si 0.6_ 0.7i o.ei o._i lo

Fractim of BladePitch

Figure 16. Typical mean relative flow profiles with and

without fan trailing edge blowing. The

measurements location is at 50% span and 1.5

chords downstream of the fan. Inset: Change in

harmonic content of the wake due to trailing edge

blowing. (Profiles reconstructed from Ref. 34 data).

ln--Phaa_ Conln_dion

IGO i.................................................. ............. .......

e) i ii

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'm 'm'| m'l'

BF'F Tone Harmonic

ln-P_ Com(_ullon

(2kll-ol-P hsr_ Col_|t_)t)1_r)

i(_]-

_ _ _ C)_% _ •

B_ Tone H_rmo_k

{:]_rl-o14_ I_ Cord)_ulioo

leo - ............................................................ i

_ F

:})mBPFmTone HarraoNc BPF Tore Harmon_

Figure 17. Measured wall tone sound pressure levels in the

inlet and exhaust as a function of injection rates.

The no-injection case is the baseline. In-phase and

out-of-phase pressure results are plotted separately.

(Based on data from Ref. 34).

unsteady pressure measurements can only be

considered as rough estimates of the associated noise

levels. Second, even in an anechoic environment,

localized wall pressure measurements are not reliable

indicators of the noise power levels in the duct.

Nonetheless, the observed reductions in the amplitudes

of the wake harmonics do indicate the potential for

NASAfrM--2001-210699 8
genuinenoisepowerlevelreductions. Naturally,more

workneedstobedonetoestablishtefullpotential of

thewakemanagement techniquefor reducingrotorstatorinteractiontonenoise.

Scarfed Inlet

An old concept that was revisited during the AST

noise reduction program is the use of a "scarfed" inlet.

In theory, the asymmetric shape of a scarfed inlet lip

with the lower portion protruding further forward than

the upper portion (see Fig. I8), should shield the

observer on the ground from the inlet noise by

redirecting the noise upward. A number of studies in

the early 80's had established the potential benefits of

scarfing, but had also indicated a possible problem.

With a scarfed inlet, the asymmetry can introduce

distortions in the flow ingested by the fan that can lead

to extraneous noise that could potentially offset the

shielding benefits of the scarfed inlet. However, recent

advances in inlet and treatment design rekindled the

interest in the concept. As a result a full-scale engine

test on a Pratt and Whitney PW4098 engine was

planned in the late 90's which incorporated an advance

low-noise scarfed inlet designed and built by Boeing

[35]. The test was completed in 1999, but inlet

aerodynamic and acoustic performance data has not yet

been fully analyzed. Therefore, an assessment of the

benefits of a scarfed inlet cannot be made at this time,

although the preliminary results appear promising.

developments in low-noise fan stage design, outlet

guide vane sweep and lean, active noise control, fan

wake management, and scarfed inlet were presented

along with representative results and relevant

conclusions (where available). For the most part,

enabling technologies for achieving all or part of the 6

EPNdB engine source noise reduction goal have been

demonstrated. Further work remains to be done in

quantifying the benefits of some of the tested concepts

such as the ADP fan, active noise control and scarfed

inlet, but the outlook appears promising.

As for continuing and future work, there is a

follow on NASA test planned for this year that is

aimed at a careful quantification of the noise benefits

from the trailing edge blowing. There has also been

some additional testing of the outlet guide vane sweep

and lean concept for fan stages with higher tip speeds

than the original NASA/Allison fan. These more recent

results should help provide a more general assessment

of the acoustic benefits of sweep and lean. There has

also been some theoretical work (not yet validated)

involving optimized multi-segment fan aft duct liners

that offer significant additional noise benefits over

comparable single-segment liners.

Given the continuing emphasis on aircraft noise

reduction, as indicated by NASA goals to provide

technology to reduce noise by 10 dB by the year 2007

and 20 dB by the year 2022, fan noise reduction is

likely to remain in the forefront of future engine noise

research.

Figure 18. Boeing scarfed inIet installed on a Pratt & Whitney

4098 engine. Photo reproduced from Ref. 36.

SUMMARY AND DISCUSSION

Fan noise reduction techniques developed as part

of the Advanced Subsonic Technology Noise

Reduction Program were reviewed. Highlights of

1.

2.

.

4.

5,

REFERENCES

Owens, R.E., "Energy Efficient Engine

Performance System - Aircraft Integration

Evaluation," NASA/CR 159488, 1979.

Smith, M.J.T., Aircraft Noise, Cambridge

University Press, 1989.

"Making Future Commercial Aircraft Quieter,"

NASA Facts, FS-1997-07-003-LeRC, or see

w-_'.lerc.nasa.gov/WWW/AST/noise, htm

Hobbs, D.H., Neubert, R.J., Malmborg, E.W.,

Philbrick, D.H., and Spear, D.A., "Low Noise

Research Fan Stage Design," NASA/CR 195382,

1995.

Dittmar, J.H., Elliott, D.M., and Bock, L.A.,

"Some Acoustic Results from the Pratt and

Whitney Advanced Ducted Propulsor - Fan 1,"

NASA/TM 1999-209049, 1999.

NASA/TM--2001-210699 9
. Elliot, D.M. and Dittmar, J.H., "Some Acoustic

Results from the Pratt and Whitney Advanced

Ducted Propulsor Model," AIAA Paper 2000-

O351,2000.

7. Rao G.V.R., "Use of Leaning Vanes for Fan Noise

Reduction," AIAA Paper 72-126, 1972.

. Hayden, R.E., Bliss, D.B., Murray, B.S.,

Chandiramani, K.L., Smullin, J.I., and Schwaar,

P.G., "Analysis and Design of a High Speed, Low

Noise Aircraft Fan Incorporating Swept Leading

Edge Rotor and Stator Blades," NASA/CR 135092,

1977.

. Schulten, J.B.H.M., "Sound Generated by Rotor

Wakes Interacting with a Leaned Vane Stator,"

AIAA Journal, no. 10, 1352-1358, 1982.

10. Envia, E. and Kerschen, E.J., "Noise Produced by

the Interaction of a Rotor Wake with a Swept

Stator Blade," AIAA Paper 84-2326, 1984.

11. Envia, E. and Kerschen, E.J., "Influence of Vane

Sweep on Rotor-Stator Interaction Noise,"

NASA/CR 187052, 1990.

12. Woodward, R.P., Elliott, D.M., Hughes, C.E., and

Berton, J.J., "Benefits of Swept and Leaned

Stators for Fan Noise Reduction," AIAA Paper 99-

0479, 1999.

13. Dalton, W.N., EUiott, D.B., and Nickols, K.L.,

"Design of a Low Speed Fan Stage," NASA/CR

1999-208682, 1999.

14. Envia, E. and Nallasamy, M., "Design Selection

and Analysis of a Swept and Leaned Stator

Concept," Journal of Sound and Vibration, Vol.

228, No. 4, December 1999.

15. Envia, E, Huff, D., and Morrison, C.R.,

"Analytical Assessment of Stator Sweep and Lean

in Reducing Rotor-Stator Tone Noise," AL4A

Paper 96-179 t, May 1996.

16. Kraft, R.E., Janardan, B.A., Kontos, G.C., and

Gliebe, P.R., "Active Control of Fan Noise -

Feasibility Study, Volume 1: Flyover System

Noise Studies," NASA/CR 195392, 1994.

17. Heidelberg, L.J., Hall, D.G., Bridges, J.E., and

Nallasamy, M., "A Unique Ducted Fan Test Bed

for Active Noise Control and Aeroacoustics

Research," NASA/TM 107213 and AIAA Paper 96-

I740, 1996.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

Walker, B.E., Hersh, A.S., Heidelberg, L.J.,

Sutliff, D.L., and Spencer, M., "Active Resonators

for Control of Multiple Spinning Modes in an

Axial Flow Fan Inlet," AIAA Paper 99-1853, 1999.

Smith, J.P., Burdisso, R.A., and Sutliff, D.L.,

"Active Control of Inlet Noise at the NASA Lewis

Ducted Fan Facility," Virginia Polytechnic

Institute and State University Report, 1997.

PIa, F.G., Hu, Z. and Sutliff, D.L., "Active Fan Noise

Cancellation in the NASA Lewis Active Noise

Control Fan Facility," NASA/CR 1985111, 1996.

Hersh, A.S., Walker, B.E., Leahy, R., Zhou, Z.,

and Heidelberg, L.J., "Active Control Dipole

Sound Source Cancellation of Axial Fan RotorStator Interaction Noise," U.S. Patent applied for,

April 1994.

Kraft, R.E., Hu, Z., Sommerfeldt, S., Walker, B.E.,

Hersh, A.S., Luo, H., Spencer, M., Hallman, D.,

Mitchell, C., and Sutliff, D.L., "Development and

Demonstration of Active Noise Control Concepts,"

NASA/CR 2000-210037, 2000.

Kousen, K.A. and Verdon, J.M., "Active Control

of Wake/Blade-Row Interaction Noise Through

the Use of Blade Surface Actuators," NASA/CR

4556, 1993.

Simonich, J.C., "Actuator Feasibility Study for

Active Control of Ducted Axial Fan Noise,"

NASA/CR 195412, 1995.

Curtis, A.R.D., "Active Control of Fan Noise by

Vane Actuators, "NASA/CR 1999-209156, 1999.

Parente, C.A., Arcas, N., Walker, B.E., Hersh,

A.S., and Rice, E.J., "Hybrid Active/Passive Jet

Engine Noise Suppression System," NASA/CR

1999-208875, 1999.

Thomas, R.H., Burdisso, R.A., Fuller, C.R., and

O'Brian, W.F., "Active Control of Fan Noise from

a Turbofan Engine," AIAA Paper 93-0597, 1993.

Smith, J.P. and Burdisso, R.A., "Active Control of

Inlet Noise from a Turbofan Engine Using Inlet

Wavenumber Sensors," AIAA Paper 99-1808,

1999.

Burdisso, R.A. and Smith, P.J., "Control of Inlet

Noise from a Turbofan Engine Using HerschelQuincke Waveguides," AIAA Paper 2000-1994,

2000.

NASA/'rM--2001-210699 10
30.Heidelberg, L.J.,"AnOverviewof theFanActive

NoiseControlEffortatNASA Glenn," presented

at the Joint Meeting of ASA, EAA & DEGA,

Berlin, Germany, March 19, 1999.

31. Naumann, R., "Control of the Wake from a

Simulated Blade by Trailing Edge Blowing,"

Master's Thesis, Lehigh University, Bethlehem,

PA, 1992.

32. Sell, J., "Cascading Testing to Assess the

Effectiveness of Mass Addition/Removal Wake

Management Strategies for Reduction of RotorStator Interaction Noise," Master's Thesis, M-IT,

Cambridge, MA, 1997.

33. Waitz, I.A., Brookfield, J.M., Sell, J., and Hayden,

B., "Preliminary Assessment of Wake

management Strategies for Reduction of

Turbomachinery Fan Noise," AIAA J. of

Propulsion and Power, Vol. 12, No. 5, 1996, pp.

958-966.

34.

35.

36.

Brookfield, J.M., "Turbofan Rotor/Stator

Interaction Noise Reduction Through Trailing

Edge Blowing," Ph.D. Thesis, MIT, Cambridge,

MA, 1998.

Raman, G., and McLaughlin, D.K. (Editors),

"Highlights of Aeroacoustics Research in the U.S.

- 1998," AIAA Paper 99-1915, 1999.

Huff, D.L., "Technology Development for Aircraft

Noise Alleviation - Engine Noise Reduction

Research," presentation made at the Hiller

Aviation Museum, December 9, 2000.

NASA/TM--2001-210699 11
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I February 2001 Technical Memorandum

4. TITLE AND SUBTITLE

Fan Noise Reduction: An Overview

6. AUTHOR(S)

Edmane Envia

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

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

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AIAA-2001-0661

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Prepared for the 39th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics

and Astronautics, Reno, Nevada, January 8-11, 2001. Responsible person, Edmane Envia, organization code 5940,

216-433-8956.

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Available electronically at htto://gltrs._'c.n_a._,ov/GLTRS

This publication is available from the NASA Center for AergSpace Information, 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Fan noise reduction technologies developed as part of the engine noise reduction element of the Advanced Subsonic

Technology Program are reviewed. Developments in low-noise fan stage design, swept and leaned outlet guide vanes,

active noise control, fan flow management, and scarfed inlet are discussed. In each case, a description of the method is

presented and, where available, representative results and general conclusions are discussed. The review concludes

with a summary of the accomplishments of the AST-sponsored fan noise reduction research and a few thoughts on

future work.

14. SUBJECT TERMS

Fan noise; Noise reduction

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