NASA/TM--1998-208662

Design Selection and Analysis of a Swept

and Leaned Stator Concept

Edmane Envia

Lewis Research Center, Cleveland, Ohio

M. Nallasamy

NYMA, Inc., Brook Park, Ohio

National Aeronautics and

Space Administration

Lewis Research Center

December 1998

https://ntrs.nasa.gov/search.jsp?R=19990018027 2018-05-11T17:25:46+00:00Z
AcknowledgmeJIts

The authors wish to thank Mr. Richard E Woodward and Dr. James E. Bridges, both of the Acoustics Branch at

NASA Lewis Research Center, for providing the acoustic data shown in this report and the routines used for

computing the 1-foot lossless acoustic power from the measured spectra.

NASA Center for Aerospace Information

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TABLE OF CONTENTS

1. INTRODUCTION

. SWEEP AND LEAN DESIGN STUDY

2.1 Background

2.2 Design Tool

a) Fan Noise Code

b) Fan Wake Description

2.3 Design Methodology

a) Procedure

b) Results

c) Selected OGV Design

. ANALYSIS OF SWEPT AND LEANED STATOR

3.1 Analysis Tools

3.2 Sideline Directivities

4. CONCLUSIONS 12

5. ACKNOWLEDGMENTS 13

6. REFERENCES 13

7. APPENDIX 14

DESIGN SELECTION AND ANALYSIS OF

A SWEPT AND LEANED STATOR CONCEPT

Edmane Envia

NASA Lewis Research Center, Cleveland, Ohio

M. Nallasamy

NYMA, Inc., Brook Park, Ohio

1. INTRODUCTION

The commercial air traffic is expected to increase

substantially in the coming years, as air travel becomes

more affordable worldwide. At the same time, increasingly

more stringent community noise regulations will continue

to place severe limits on the acceptable levels of aircraft

noise near airports. To address these concerns and develop

low-noise propulsion technologies, promising concepts for

suppression and/or reduction of noise emissions from

subsonic aircraft are being investigated under the auspices

of NASA Advanced Subsonic Technology (AST)

initiative.

The study reported here focuses on the reduction

of fan noise which is a significant source of engine noise

[Ref. 1]. The targeted source is the so-called rotor-stator

interaction tone noise generated as a result of periodic

impingement of fan wakes on the outlet guide vanes

(OGV). The suppression method involves incorporating

swept and leaned stators in the design of the OGV. As

shown in Figure 1, sweep is defined as the axial

displacement of the vane leading edge from its baseline

position. Similarly, lean is defined as the circumferential

displacement of the vane leading edge from its baseline

position. Since the early '70s, several theoretical and

experimental studies have hinted at the potential of sweep

and lean for reducing rotor-stator tone noise [Refs. 2-9]. In

a more recent experimental study, the benefits of OGV

sweep and lean for reducing fan noise were convincingly

demonstrated for a representative modern low-speed fan

stage [Ref. 10]. The results show that, compared to a

radial one, a swept and leaned OGV provides sizable

reductions in the level of rotor-stator interaction tone noise

for a wide range of operating conditions.

The present report documents the design

procedure that was utilized to select the candidate swept

and leaned OGV used in that test. The report includes an

overview of the theoretical tools used in the study as well

as the details of the selection process. The report also

contains comparisons of the predicted and measured

sideline directivities of the swept and leaned stator against

both the baseline radial stator and a swept-only stator.

These comparisons serve to validate the design approach

and the theoretical tools used in the process. A summary

of conclusions drawn from this study is also included.

[ (a) Definition of Sweep Angle 1 c_

Radial Smtor Swept Stator

[(b) Definition of Lean Angle]

! ,

Radial Stator Leaned Stator

Figure 1. Geometry and definitions of stator vane sweep and

lean.

2. SWEEP AND LEAN DESIGN STUDY

2.1 Background

Rotor-stator interaction noise is caused by the

periodic cutting of rotor wakes by the stator vanes. The

strength of the interaction is related to the efficiency with

which unsteady pressure distribution on the vanes couples

to the acoustic modes of the bypass duct [Ref. i 1]. The

pressure distribution, in turn, depends on the upwash

induced by the rotor wake on the stator. It can be shown

that, through this dependence, the source strength is

strongly influenced by the spanwise phase of the upwash

[Ref. 12]. Significant upwash phase variation can cause

noise cancellation between contributions from different

locations along the vane span resulting in weaker

interaction tones. For the most part, variation in the

spanwise phase of the upwash is controlled by the number
of individual rotor wakes that intersect a given vane I. This

number is determined, primarily, by the kinematics of the

rotor wakes in relation to the stator vanes. Swirl variation

from hub to tip introduces a tangential shift between the

circumferential positions of the wakes along the span. The

shitt increases with downstream distance causing the tip

wakes to advance ahead of the hub wakes. Generally, this

shift becomes large enough that wake sheets from more

than one blade intersect a single vane.

For a typical fan stage, there usually occur a few

intersections per vane. However, for a fixed number of

vanes, the introduction of vane sweep and/or lean, changes

the number of intersections per vane. The change occurs

because sweep and lean alter the kinematic relationship

between the wakes and vanes for the same hub-to-tip rotor

wake tangential shift. Figure 2 shows the crucial influence

_a): a = 0", I_= 0*

{b)a = 30", I_= -30"

(c): a _ -30", I_= 30 °

Figure 2. Wake centerline traces (dashed lines) at the stator LE

(solid lines). Fan rotates CW. (View looking aft)

of sweep and lean in determining the number of

intersections. Sketch (a) in this figure depicts the baseline

kinematic picture for a radial stator. Sketches (b) and (c)

Multiple intersections cause the harmonic content of the upwash to

experience s_gnificant spanwise phase variation.

show the change in the relative kinematics for a

"beneficial" combination of sweep and lean and a

"detrimental" one, respectively. A correct choice of sweep

and lean (b) results in additional intersections compared

with the radial case, while an incorrect choice (c) reduces

the number of intersections. Basically, a beneficial

combination increases the streamwise distance between the

rotor and the stator allowing more wake tilting to occur.

Conversely, an incorrect choice reduces the rotor-stator

distance resulting in less tilting before the wake

impingem_t. As will be shown later, the most important

message of Figure 2 is that a correct combination of sweep

and lean is crucial if maximum noise benefits are to be

realized. In fact, an incorrect combination can increase the

noise levels. Therefore, the goal of the design procedure

was selection of a proper combination of sweep and lean to

ensure maximum noise reductions for all operating

conditions of interest. For this study, these conditions

correspond to the engine power settings for takeoff,

cutback and approach.

2.2 Design Tool

a) Fan Noise Code

The theoretical tool used in this study for

selecting the candidate low-noise stator is the BBN/V072

fan noise prediction code [Refs. 13-15]. It is based on an

analytical model for predicting tone levels produced inside

the bypass duct due to the interaction of fan wakes with the

OGV. The code combines a 2D-strip description of the

unsteady aerodynamic interaction between the rotor wakes

and stator vanes with a 3D acoustic response of a cascade

to an incident gust. The bypass duct is assumed to be a

constant-area annulus containing a uniformly moving

medium. Both upstream-radiated (inlet) noise and

downstrea_n-radiated (exhaust) noise are computed by the

code.

Noise computations in the BBN/V072 code are

done via a two-step process. First, the unsteady surface

pressure distribution induced on the vanes by the rotor

upwash is determined. Then, the duct noise levels

resulting from the unsteady pressure distribution is

calculated Following the standard practice, noise results

are expre., sed in terms of the fan blade passing frequency

(BPF) to aes. The code provides a mode-by-mode

descriptiol of the inlet and exhaust noise for each tone.

The mode results include cutoff ratio, sound pressure level

(SPL), phase, and acoustic power.

qhe input needed to run the code includes

geometric (blade and vane definitions) and fan stage steady

aerodynaraic performance information. An important

element o_the input information is the definition of viscous

wakes of the rotor at the stator leading edge from which
upwash on the vanes is determined. Wake definition in

BBN/V072 is based on a set of empirical correlations that

are essentially two-dimensional 2 in nature and, as such, do

not fully account for the complex nature of the flow

downstream of the fan. This restriction can be avoided if

wake data are used to determine the upwash. However,

such a description can only be utilized if the following two

requirements are met. The first is that data must be taken

at the fan operating condition being considered or over a

range that brackets the condition of interest. Wake data

from operating conditions that are significantly different

from the condition of interest can not he used for noise

predictions. The second requirement is that the wake

measurement station be at or near the stator leading edge.

The complicated nature of the flow behind the fan prohibits

extrapolation of wake description from the axial station

where measurements are taken to another axial location. In

the present study wake correlations were used since wake

data were unavailable prior to the test.

b) Fan Wake Description

Wake specification in the BBN/V072 code begins

in the rotating (i.e., relative) reference flame. Here, for the

sake of developing a mathematically tractable

representation, any radial flow that might occur is

neglected. Furthermore, the flow is envisaged as a smalldeficit wake profile superimposed on a parallel and locally

uniform stream at each radius. Within this framework, the

tangential position of the wake centerline at each radius

accounts for any wake sheet tilting that might occur due to

swirl. In BBN/V072, the tangential position of the wake

centerline (at each radius) is determined by the relative

flow angle, which is specified as input to the code. The

change in the relative flow angle from hub to tip, therefore,

represents the tilting of the wake sheet. The introduction

of vane sweep and lean can enhance or diminish the wake

tilting as seen by the OGV.

As was mentioned earlier, the wake profiles used

in the BBN/V072 code are developed from empirical

correlations. There are currently two correlation-based

wake models in the code that can be used to supply wake

centerline velocity deficit and half-width information. The

user can choose between the correlations developed by

Philbrick and Topoi [Ref. 16] or those developed by

Majjigi and Gliebe [Ref. 17].

Aside from the wake centerline velocity deficit

and half-width, it is also necessary to choose a shape for

the rotor wakes. The available choices in the code are a

hyperbolic secant profile, a Gaussian profile, and a loaded2 When using correlations, wake descriptions have an explicit

dependence on the axial and tangential coordinates, but depend only

parametrically on the radial coordinate.

rotor wake profile. In principle, any combination of the

correlation parameters and shapes can be used to provide a

complete description of wakes. In practice, however, only

certain combinations produce reasonable predictions. For

this study, the correlations defined in Reference 16 along

with the loaded-rotor wake profile were used. The chosen

wake characteristics are plotted in non-dimensional form in

Figure 3. The decision to use this particular combination

was based on experience with similar fan stages.

>

i.

z

1.0

08 iii IZZIIZIIi : _/IIIIZIilIILIIIIIIIIIIIII!ZII

0406 i ii i I

021 .....

: : [

0"00t0. 0.5 1.0 1.5 2.0 2.5 3.0

Normalized Axial Distance

N

z

0.30

........ i i .... ]

 ̧ ̧/:iiiiiiiiiiiii i i!!i//iiiiL:iii

0.05/i i: :ii::illif:!:! :iii

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Normalized Axial Distance

0611111! i. iiiiiiiilfiiiiiiill

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Normalized Tangential Distance

Figure 3. Rotor wake characteristics used in this study.

Once rotor wake characteristics are specified, the

resulting description is transformed to the stationary (i.e.,

absolute) reference frame for computing upwash on the

vanes. The upwash is then Fourier decomposed for use in

the cascade unsteady pressure calculations. From this

information, the BBN/V072 code determines acoustic

pressure and power for each of the propagating duct

modes.

2.3 Design Methodology

a) Procedure

In this section, we outline the design methodology

that was used to identify the optimum vane sweep and lean
combination. The fan stage considered for this study has

18 blades and 42 radial vanes with a design tip speed of

1,000 fps. The fan hub-to-tip radius ratio is 0.3 and that

for the stator is 0.5. The calculations were performed for

takeoff (84% of the design tip speed), cutback (70% of the

design tip speed), and approach (50°,6 of the design tip

speed). Both upstream- and downstream- radiated noise

contributions were considered. Pressure and power levels

for all propagating modes contained within the 2BPF 3

through 5BPF tones were computed for various

combinations of sweep and lean angles. Since, on a modeby-mode basis, the amount of information produced is

enormous, it was decided that a global noise metric would

be more useful in judging the benefits of sweep and lean.

The metric chosen is the change in the computed tone

power level from that for the radial stator. With this

definition of the metric, a negative change (i.e., reduction)

means acoustic benefit. The selection process involves a

case-by-case examination of a comprehensive matrix of

practical sweep and lean combinations. The acoustic

"performance" of each potential configuration is gauged by

examining its calculated metric for the selected BPF

harmonic at takeoff, cutback and approach. The optimum

stator design is one that provides maximum overall tone

noise reductions for all operating conditions.

We begin by defining sweep and lean via the two

angles shown in Figure 1. Sweep parameter a is defined as

the angle, in the meridional plane projection, between the

leading edge lines of the straight and swept vanes. Positive

sweep is defined as that for which vane tip is downstream

of its root. Lean parameter [3 is defined as the angle, in the

axial plane projection, between the leading edge lines of

the straight and leaned vanes. Positive lean is defined in

the direction opposite to rotor rotation. The matrix of the

cases used for this study has sweep and lean angles ranging

from -30 ° to 30 °, in increments of 5°.

b) Results

In Figures 4 through 9, the change in acoustic

power (i.e., P(a, 13)- P(0,0)) as a function of sweep angle

is shown for a range of lean angles. In each figure, the

results for the 2BPF through 5BPF tones are identified.

Figures 4 and 5 show the upstream and downstream results

for the takeoff condition. Similarly, Figures 6 and 7

correspond to the cutback condition, and Figures 8 and 9

for the approach condition. For the sake of clarity, the

levels corresponding to only five lean angles (i.e., -30 °,

-15 °, 0°, 15°, and 30 °) are plotted. The trends for the

intermediate lean angles follow closely those presented

here. The results summarized in these figures clearly show

that sweep and lean have a significant impact on the

predicted tone levels. It is also obvious that the operating

3 BPF is cut-off for this fan stage.

2BPF

20 .....

: _ i i O---O-30 i -

15 .._. =..: ...... i .i. _--o-l_i ......

"%o _-_o o lo 2o

la. o

•._ -I0

Sweep Angle, (z (deg.)

4BPF

30 -30 -20 -10 0 10 20

Sweep Angle, ct (deg.)

3G

3BPF

20 "_ ! : i

_5iiii!_iii iiiiii:_ iii iiii ii_

i

-30 -20 -10 0 10 20 30

Sweep Angle, _ (deg.)

Figure 4.

5BPF

-30 -20 -10 0 10 20 3(3

Sweep Angle, ct (deg.)

Influence of sweep and lean on the in-duct tone

power. Takeoff condition (upstream noise).

2BPF 4BPF

20 , ,

30 -30 -20 -10 0 I0 20

Sweep Angle, a (deg.)

3G

5BPF

Figure 5. Influence of sweep and lean on the in-duct tone

power. Takeoffcondition (downstream noise).
2BPF 4BPF

2015 ,- Leaa Angle)_((_)o_..o.30 J _

F ' ' • I i

.1o: ; i:i i : ......

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 313

Sweep Angle, ct (deg.) Sweep Angle, a (deg.)

3BPF

2O

15c_ : !

i , i . 1

5BPF

-20 L

-30 -20 -IO 0 io 20 30 -30 -20 -I0 0 I0 20 30

Sweep Angle, a (deg.) Sweep Angle, ct(deg.)

Figure 6. Influence of sweep and lean on the in-duct tone

power. Cutback condition (upstream noise).

2BPF 4BPF

2o, ...... ....... '

i L_m AnOe, _(_) i

-20 I

-30 -20 -I0 0 I0 20 30 -30 -20 -I0 0 I0 20 3(3

Sweep Angle, ct(deg.) Sweep Angle, ct(dog.)

3BPF 5BPF

20 • 1 _ .......

2ol 2

-30 -2o 40 o io 20 30 -30 -20 -to Io 20 3c

Sweep Angle, ct (deg) Sweep Angle, ct (deg.)

Figure 8. Influence of sweep and lean on the in-duct tone

power. Approach condition (upstream noise).

3BPF 5BPF

20

__. _o ili iiiiiii112 i i ..ill

5 ''' 2

iii

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 3(1

Sweep Angle, (x (deg.) Sweep Angle, ¢x (deg)

Figure 7. Influence of sweep and lean on the in-duct tone

power. Cutback condition (downstream noise).

2BPF 4BPF

_0 --" i

-15 ...... i ?,

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 3(1

Sweep Angle, ct (deg.) Sweep Angle, _ ((:leg.)

20 r

15

i0

c5

3BPF 5BPF

_Z

-15 i

-20

-30 -20 -IO 0 10 20 30 -30 -20 -10 O I0 20 3(1

Sweep Angle, ct ((leg.) Sweep Angle, ct (deg.)

Figure 9. Influence of sweep and lean on the in-duct tone

power. Approach condition (downstream noise).
condition, tone harmonic order, and direction of noise

propagation influence the effectiveness of sweep and lean.

Upon a closer examination of these results, the

following conclusions can be drawn. Positive sweep

angles (i.e., aft sweep) always reduce the tone levels while

negative sweep angles increase them. The influence of

lean, on the other hand, is more subtle. In most cases, for a

given (positive) sweep, negative lean angles (i.e., lean in

the direction of fan rotation) enhance the benefits of sweep

by causing additional noise reductions. In contrast,

positive lean angles tend to diminish, or even offset, the

benefits of sweep (see the 5BPF plot in Figure 5, for

example).

As was stated earlier, the observed behavior with

sweep and lean can be explained in terms of their influence

on the harmonic phase of the upwash along the vane span.

Consider, for example, the predicted noise level changes in

the inlet at 2BPF for the takeoff condition (Figure 4). For

this condition, consider the beneficial combination of

sweep and lean (it = 30 °, 13 = -30 °) for which noise is

reduced, and the detrimental combination (ct = -30 °, 13=

30 °) for which noise is increased. In Figure 10, plots of the

spanwise harmonic phase variation of the upwash for these

three sweep and lean configurations are shown. Note that,

compared with the radial stator, the favorable configuration

has significantly more spanwise phase variation than the

detrimental one. In fact, the latter has less variation

compared with the radial stator. It is instructive to

compare these phase plots with the kinematic description

shown in Figure 2. Compared with the radial stator, the

combination (ct = 30°, 13= -30 °) allows for more wake-vane

intersections and, therefore, more spanwise phase

variation. Conversely, for the combination (o = -30 °,

13= 30 °) fewer intersections lead to less phase variation.

Now let's examine the impact of the upwash

phase variation on the predicted vane unsteady surface

pressure distribution. In Figure 11, phase contours of the

unsteady surface pressure corresponding to the three sweep

and lean combinations of Figure 10 are shown. Compared

with the radial stator, the correct choice of sweep and lean

produces significantly more variation in the phase of

surface pressure. In contrast, the incorrect choice actually

reduces the phase variation. Now, since the tone levels

are related to the surface integrals of the vane unsteady

pressure, the more phase variation there is in the unsteady

pressure, the more cancellations will occur in these

integrals resulting in weaker tones as seen in Figure 4 for

the combination (a = 30 °, 13= -30°). On the other hand, if

there is less phase variation, there will be less cancellation

leading to higher tone levels as seen for the combination

(a = -30 °, 13= 30°). These plots corroborate the argument

that was advanced in section 2.1 regarding the role of

(a): ct= 0 °, 13= 0 °

°

0 20 40 60 80 I O0

Percent Vane Span

(b):ct= 30 °, 13= -30 °

IS°1 !/11 _._ .. ......!

-is0

100

c_ 18o

oo

o

-180

Percent Vane Span

(c): a = -30°,13= 30°

20 40 60 80 100

Percent Vane Span

Spanwise phase of the 2BPF component of the

upwash at the takeoff condition.

Figure 10.

--, gx,

-}

Cl_d _ Chord

Figure 11. Phase contours of vane unsteady surface pressure

(theory). Vane planform is mapped to a rectangle to

facilitate comparisons.

sweep and lean in changing the tilt of the rotor wakes

relative t3 the stator vanes. They also clearly demonstrate

the importance of the proper choice of sweep and lean

combination. Of course, as was stated earlier, the realized
sweepand/orleanbenefitsvary with fan tip speed and tone

harmonic order. In general, the predicted noise reductions

tend to be more significant at higher tip speeds and for

higher tone harmonics. Both of these observations are

consistent with the kinematic argument. The tip speed

trend is justifiable because over the same streamwise

distance the higher fan tip speed at takeoff produces more

wake tilting than does the lower tip speed at approach.

Similarly, the benefits are larger for higher tone harmonics,

because harmonic phase variation is directly proportional

to the harmonic order whether or not there is sweep and/or

lean. So, for example, if there are three sign changes in the

spanwise phase for the 2BPF tone there will be twice as

many for the 4BPF tone and so on.

c) Selected OGV Design

The theoretical results presented above indicate

that, to reduce rotor-stator interaction tone noise, stator

vanes should have aft sweep and should be leaned in the

direction of fan rotation. Furthermore, they also suggest

that there are more acoustic benefits for larger sweep and

lean angles. Therefore, it was decided to implement the

largest sweep and lean angles that were structurally and

aerodynamically feasible. These requirements resulted in a

swept and leaned vane design having 30° of aft sweep (i.e.,

a = 30°) and 30° of lean in the direction of fan rotation

(i.e., 13-- -30°). To separate the benefits of sweep and lean,

a swept-only stator (i.e., a = 30 °, I_ = 0°) was also selected.

Photographs of the fan stage assembly 4 with the swept-only

and swept and leaned stators are shown in Figure 12.

The selected stator configurations were tested 5 in

the NASA Lewis 9' x 15' Acoustic Wind Tunnel where

detailed farfield noise measurements were obtained at

several fan operating conditions. The data unequivocally

show that, compared to the radial OGV, the swept and

leaned OGV is quieter at all tested conditions. The sweptonly stator also shows sizeable acoustic benefits. Later in

this report detailed comparisons between the measured and

predicted sideline directivities of the radial, swept-only,

and swept and leaned stators will be presented.

4The Allison Engine Company, undercontract to NASA Lewis Research

Center, built the fan stage and the tested stator configurations [Ref. 18].

5The baselineradial stator was tested in two axial positions, a "forward"

position and an "aft" position. The forward position corresponds to the

hub axial location of the swept/leaned stators. The all position

corresponds to the tip axial location of the swept/leaned stators. This was

done to separate the noise reduction due to sweep-induced phase

cancellation from the noise reduction due to the increased axial spacing

for the swept/leaned stators compared with the radial stator. In this

report, the results for the radial stator in the all position are not

considered.

Figure 12. Photographs of the partially assembled fan stages.

(a) is the side view of the swept stator and (b) and (c)

are different views of the swept and leaned stator.

3. ANALYSIS OF THE SWEPT AND LEANED

STATOR

3.1 Analysis Tools

The predicted sideline directivities are calculated

using two codes designed to predict the farfield tone noise

radiation from inlet and exhaust sections of a fan bypass

duct. As shown in Figure 13, the inlet and exhaust codes

compute the radiation fields in the forward and aft arcs,

respectively. In each case, the arc covers a region

extending from the fan axis to a location past the 90 °

position. In the overlap region the linear nature of the

wave equation permits the addition of the two solutions

(with the phase taken into account) to obtain the complete

sideline directivity.

Both of these codes are based on a frequencydomain finite element formulation of the problem of

acoustic radiation from termination of an axisymmetric

duct [Refs. 19 and 20]. They solve for the acoustic field

(both inside and outside of the duct) once the internal

geometry of the duct and the in-duct pressure are specified.

The specification of the internal acoustic field is in terms

of amplitudes of the cut-on duct modes at the internal

boundary of the computational domain. The mode input
Fartiled Boundary for

Exhaust Noise Computations

i Fro'filed Boundary for •

Inlet Noise Computations .... - ..............

[

Baffle for Exhaust '. , h,,n _ " " t '_

Noise Computattons • _ 1341me Noise Computallons IOf lnte ';

lnt_fi'or Boundary

(OGV leading edge)

Figure 13. Computational domains for inlet and exhaust

computations.

can be supplied from in-duct measurements or from

predictions (from the BBNN072 code, for example). The

duct internal and external flowfields are assumed to be

irrotational and are calculated as part of the solution. The

computational domain for each code is the region that

starts at the input plane inside the duct and extends outside

of the duct between the fan axis and a suitably chosen

"baffle" (see Figure 13). The baffle is a pressure release

boundary that permits outgoing waves only. Both lined

(i.e., absorbent) and unlined (i.e., hard) boundary

conditions can be specified on the duct walls and a

Sommerfeld radiation condition is applied on the farfield

arcs and the baffles.

For the exhaust problem, the presence of the

exhaust jet shear layer requires implementation of

additional constraints in the calculations. For the mean

flow, since the dynamics of the shear layer is highly

complicated, a simplified condition is implemented. The

layer is modeled as though the bypass duct extends some

distance downstream of the exit plane. Therefore, in the

extended region the flow is composed of two parts, an

inner flow and an outer flow, and the velocity potential is

discontinuous across the layer. But, beyond this extended

region, the velocity potential is assumed to be continuous

and the internal and external flows are permitted to mix on

a potential flow basis. The extent of the "fictitious duct" is

chosen so as to provide realistic acoustic wave diffraction

effects across the shear layer while at the same time

minimize the influence of the artificial mixing on the

acoustic field (see Reference 20). This treatment of the

shear layer is found to work reasonably well for the

moderate flow Mach numbers considered in this report.

For the acoustic calculations, the presence of the shear

layer necessitates the specification of two continuity

conditions along the interface. These are the continuity of

acoustic pressure across the shear layer, and the continuity

of displacement of the interface itself. The latter is a

kinematic condition arising from the assumption that the

interface acts as an impermeable membrane across which

acoustic perturbations are transmitted by virtue of its

motion.

gecent applications of this type of an approach to

the prediction of farfield fan noise may be found in [Refs.

21-24]. In the present study, using mode predictions from

the BBN/V072 code as input for the radiation codes,

sideline directivities for the radial, swept-only and swept

and leaned stators were computed and compared with the

measured data. The comparisons were carried out for the

2BPF tone at the approach, cutback and takeoff conditions

and for the 3BPF tone at the approach condition only. The

sideline, where noise data were taken, is 88" away from the

fan axis. The tunnel Mach Number is 0.1 chosen to

guarantee that the background tunnel noise level was well

below the noise from the fan model [Ref. 25].

3.2 Sideline Directivities

We begin the data-theory comparisons with the

radial stator whose results are shown in Figure 14. The

figure shows plots of 2BPF tone SPL as a function of the

emission angle for each speed considered in this report.

The symbols represent the measurements and the solid

curves the predictions. The thin line connecting the data

points is drawn to aid in discerning the pattern of the

measured directivities. To help analyze the trends, the

measured broadband noise level at 2BPF is also shown

(dashed line). The SPL range for each graph is chosen so

as to provide the best overall representation of the results

but the increment is kept the same for all graphs to allow

for easy comparison.

The radial OGV results show that there is

generally a good agreement between the predicted and

measured directivities for the approach and cutback

conditions, but that the agreement is only marginal for the

takeoff :_ondition. In particular, at approach the predicted

peak SPL and peak radiation angles for both inlet and

exhaust are well predicted. In fact, over the entire range of

emissio_l angles, the predicted directivity is close to the

measured one. On the other hand, the data-theory

agreement for takeoff is rather poor. For this speed, there

are significant over-predictions in both the inlet and

exhaust portions of the directivity. For the cutback

conditk_n, there is fair agreement in the inlet but significant

over-pr,Miction in the exhaust. It should be noted that

where the measured tone is very close to the broadband

level, the tone data is not entirely reliable. By the same

token, the sharp dips in the predicted directivities are

somewhat unrealistic and should be "clipped" with the

broadband level. It is these clipped predictions that should
Takeoff Condition

130 _20 _ , i i iiii ZZIII iiil/x 171 i i t

9 (ii

Cutback Condition

8o ..i

70

Approach Condition

] 00 I] -- Prediction ; _ : .J_," ",_, ; ..

70 1 :

;:;::;;:7:7:;'\

o

Figure ! 4.

20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 2BPF tone

farfield directivities for the radial stator. Measured

broadband noise level at 2BPF tone is also shown.

be compared with the measured tone levels. With that in

mind, the data-theory comparisons are improved,

particularly, for the approach condition. Overall, given the

simplifications made in modeling the noise source and, to a

lesser extent, those employed in the radiation predictions,

the overall data-theory agreement (at both speeds) is very

encouraging.

The results for the swept-only stator are

summarized in Figure 15. For this configuration, the

absolute 2BPF levels are significantly lower than those for

the radial stator and so they are closer to the broadband

noise levels. Overall, the data-theory agreement is

remarkably good. Here the trends, as well as the levels, are

well predicted for all three speeds. There are, nonetheless,

local discrepancies especially for the cutback and takeoff

conditions for which the theory over-predicts the measured

exhaust SPL at large emission angles. Note that both the

measured and predicted levels for the swept-only stator

show sizable reductions compared to the levels for the

radial stator levels as discussed below.

Predicted and measured 2BPF tone reductions due

to sweep (i.e., swept-only OGV levels minus radial OGV

levels) are shown in Figure 16. Note that with this

definition, the 0 dB represents the level for the radial

Takeoff Condition

: i i i ! " i .............. I

loo : i .ii. 111711iiii ;/ " 7i ii'

,.-1

120

Cutback Condition

.1

6O

110

100

90

80

70

Approach Condition

110

lO0

90

8o

6O

7O

60

Figure 15.

20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 2BPF tone

farfield directivities for the swept stator. Measured

broadband noise level at 2BPF tone is also shown.

stator. The symbols now indicate the measured tone

reductions and the solid line the predicted reductions.

Here, the dashed lines represent broadband-corrected

predictions calculated using the clipped predictions that

were discussed earlier. The measured benefits, while not

uniform, are generally centered on the 5 dB level for the

approach condition, around the 7.5 dB levels for the

cutback condition, and on the 10 dB level for the takeoff

condition. As for the predictions, at approach and cutback

the bulk of the noise reductions fall in the 5-10 dB range

especially when the broadband-corrected levels are

considered. For takeoff, the predicted reductions are

somewhat larger and more erratic. This is, of course, the

result of the fact that the predicted radial stator levels do

not agree well with the data even though the levels for the

swept-only stator do. Nevertheless, the general trend of

reductions with the fan tip speed is reasonably well

predicted in the sense that the reductions are higher at

takeoff compared to those for approach.

Next, the comparisons for the swept and leaned

stator (i.e., a = 30 °, [3 = -30 °) are shown in Figure 17. Note

that for this configuration, the absolute levels for the 2BPF

tone are essentially at the broadband level. This suggests

that the sharp oscillations in the predicted levels should be

ignored when comparing the theoretical and experimental
Takeoff Condition

" !

o!, z ::: i t

_: -2o

,.-1

_ -30

-40

Cutback Condition

_._ 10 r

Approach Condition

Jo[ f-'N .... 1.| -i : -:.k{ !.7

=_ o i _: i :_ . _ _ . i

•_ ] i " _- _ ,

-30/ _ P_aict,d " _ _ ..... i

0 20 40 60 80 100 120 140 160 lS0

Emission Angle (for 88-inch Sideline), deg.

Figure 16. Comparison of measured and predicted 2BPF tone

reductions in the farfield due to sweep (radial stator

is the baseline). Broadband-corrected predicted

reductions are also shown.

results. With that in mind, the general agreement between

the data and theory is very good for this configuration with

the measured SPL's being very close to the average

predicted reductions for all three speeds.

Figure 18 shows the 2BPF tone SPL reductions

when the radial stator levels are used as the baseline. If the

broadband-corrected predictions are used, the predicted

reductions compare quite well with the measured

reductions for approach and cutback, but the comparison is

poor at takeoff. The agreement for the takeoff condition is

poor because the directivity for the radial stator is not well

predicted. The measured reductions are about 5 to 7 dB

for approach and cutback and around 10 dB for takeoff.

The predicted reductions are somewhat higher for all three

speeds. The tone reductions summarized in figures 16 and

18 clearly demonstrate the success of sweep and lean in

reducing the tone level.

To assess the benefits of swept and leaned stator

versus the swept-only stator, the 2BPF tone SPL difference

between the two configurations is plotted in Figure 19.

Note that, the 0 dB now represent the level due to the

swept-only' stator. The calculated differences in the

measured tone levels are generally centered on the 0 dB

Cutback Condition

120r

iio .................

,...1 90 ! ' '! '

Approach Condition

Figure 17.

20 40 60 80 1oo 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 2BPF tone

farfield directivities for the swept and leaned stator.

Measured broadband noise level around 2BPF tone is

also shown.

level with noticeable additional benefits only in the exhaust

region. This suggests that the swept and leaned stator is

not significantly quieter than the swept-only stator except

in the exhaust. The corresponding predicted differences

generally show quite a good agreement for most of the

emissiora angles at all speed conditions, especially when

the broadband-corrected predictions are used for

calculating the reductions (dashed lines). The exceptions

are the predicted benefits for the exhaust emission angles

greater than 140° for the cutback and takeoff conditions

where the theory predicts significant additional benefits for

the swept and leaned OGV compared with the swept-only

OGV.

A tacit assumption used in computing the farfield

directi_ities presented in this report is that the rotor

transmission losses are negligible. Despite this

approxmation, however, the general agreement between

the pre:iicted and measured noise reductions is remarkably

good. A possible explanation for this agreement is that the

2BPF :one does not suffer significant transmission losses

throug)h the rotor for this fan. It is also possible that the

rotor affects the absolute levels of the 2BPF tone equally

for all three stators, so that it drops out of the difference

calcul_ tions. However, given that with the exception of the

10
Takeoff Condition

-10 : .......... iii."" 'i" .... .....:ii .i

Cutback Condition

m

10 : : /

"_=_-to°i::i.: 7_i:_:i_ii:iii_ ,i _....: _i _ii:iiii, • _ii ii_.i ; ii/i_i

_ 10

0 0

_ -10

_ -20

_ -30

-40

Figure 18.

Approach Condition

" : ! i i ...... i ' " " i : _ " _ ' ! ....

__ Predtcted(BB-hdjusted) ; ! _ [ ! [

20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 2BPF tone

reductions in the farfield due to sweep and lean

(radial stator is the baseline). Broadband-corrected

predicted reductions are also shown.

radial stator at the takeoff condition the directivities for all

other speeds and configurations are in very good

agreement with the data, the first explanation seems more

likely.

Returning to the data-theory comparisons, we next

present the results for the 3BPF tone for which the

predictions are restricted to the approach condition only.

The sideline directivities for cutback and takeoff could not

be obtained because the mesh resolution requirements for

the frequencies corresponding to these conditions are

beyond the current capabilities of the two radiation codes.

The format of the subsequent presentation is therefore

slightly different from the results presented so far.

In Figure 20 measured and predicted 3BPF tone

sideline directivities are shown. From the top, the graphs

are for the radial, swept-only and swept and leaned stators.

The symbols indicate data and the solid lines represent the

predictions. The measured broadband noise level at 3BPF

is also shown for each stator (the dashed line). The results

indicate that while there is a reasonably good agreement

between the data and theory in the inlet region, there are

noticeable ,under-predictions in the aft region. The

magnitude of the under-prediction depends on the stator

.=

4

Takeoff Condition

10 : i : l

-3o-2°ii LIII !I:_!IIIZI/IIZZIIIZ/ZIT__'__/III _! i ii

Cutback Condition

Approach Condition

•_ 10

o 0

-10

-20 _ i .... i ,,

I__ -30 [--Pred:cted ! ! _ i _

r._ --40 [_ - - Predteled (Bn-Adjusted) 7 = " " : ! : ]

0 20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Figure 19. Comparison of measured and predicted 2BPF tone

reductions in the farfield due to sweep and lean

(swept stator is the baseline). Broadband-corrected

predicted reductions are also shown.

configuration. For the radial and swept-only stators the

discrepancies are modest (around 5 dB), but for the swept

and leaned case they are as much as 10 dB. However, it

should be noted that, except for the radial case, for which

there is noticeable tone protrusion, the levels of measured

tone and broadband are virtually equal for the 3BPF tone.

Therefore, a better data-theory comparison is obtained

when the broadband levels are taken into account in

assessing the predicted noise benefits. In that case, the

predictions agree rather well with the measured benefits.

In Figure 21, measured and predicted SPL

reductions due to sweep-only and sweep and lean are

presented. From the top, the graphs show the noise

benefits of a swept-only stator compared with the radial

stator (i.e., p(30, 0) -p(0, 0)), the benefits of the swept and

leaned stator compared with the radial stator (i.e., p(30, -

30) -p(0, 0)), and the benefits of the swept and leaned

stator compared with the swept-only stator (i.e., p(30, -30)

- p(30, 0)). Overall, the comparison between the measured

reductions (symbols) and the predicted reductions (solid

lines) is quite reasonable, especially when the broadbandcorrected predictions are used for computing the predicted

benefits (dashed lines). Similar to the 2BPF tone, there are

measurable noise benefits when the OGV is swept or is

11
Approach Condition: Radial Stator

110 t

,oo , !

z. so ' :- "<.X. :

70 !

Approach Condition:Swept-OnlyStator

110 ! _ ! : ........ i i : i . i

70, 1

6ol / TV " U'l

Approach Condition: Swept and Leaned Stator

IlOFr : : : :

|c_Data ] - , : ! : : :- .... !..

100 "-- Pred ctlon / ....... :. ;- ; ; :

70 _ :i __ __f_iiiill ii( 60

0

Figure 20.

20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 3BPF tone

farfield directivities for the radial, swept-only, and

swept and leaned stators at approach condition.

Measured broadband noise level around 3BPF tone is

also shown.

swept and leaned in accordance with the noise reductions

shown in Figures 8 and 9.

Since a direct data-theory SPL comparison for

higher tip speeds (or, for that matter, higher harmonics)

could not be made, an indirect method was employed. The

procedure involves comparing the in-duct acoustic power

levels computed in Section 2 of this report with acoustic

power level estimates based on integrating the measured

sideline SPL's for the higher speeds and/or tones. The

estimates are based on 1-foot lossless tone data computed

from the measured spectra at the 88-inch sideline. For the

sake of consistency and completeness, these comparisons

have been carded out at all three speeds and for all the

harmonic tones between 2BPF and 5BPF. Using the same

procedure, the corresponding broadband noise level in the

neighborhood of each tone was also calculated to help with

the analysis of the tone data.

The salient conclusion from these comparisons is

that the broadband-corrected duct power level predictions

agree remarkably well with the power levels calculated

from the measured sideline directivities. This is true both

of the absolute levels of the acoustic power in the

individual tones as well as the variations of tone power

p(30, 0)- p(0, 0)

ill

m p (30, -30)- p (0, o) io ............

o i i i iii:i::i:i r:i

-10

_40L : : : :

-=

pO0, -30) - p(30, 0)

I0 F : : : : i : : : I

-2o4°/i/i.ii_": of I I!III "_........ 1111111111111 II ! : _IIIIII',I : I! III '. _ I

-30/t-- _ict_

,,.40_--- Prvdictcd(BB-Adjumcd) j[ [ + : i ........ ]

o

Figure 2 i.

20 40 60 80 100 120 140 160 180

Emission Angle (for 88-inch Sideline), deg.

Comparison of measured and predicted 3BPF tone

reductions in the farfield; top figure shows the sweep

benefits directivities for the radial, swept-only, and

swept and leaned stators. Broadband-corrected

predicted reductions are also shown.

levels with sweep and lean. The complete set of datatheory comparison plots for tone power levels is included

in the Appendix.

4. CONCLUSIONS

I'he principal conclusion of this study is that,

when ch3sen properly, sweep and/or lean reduce rotorstator interaction tone noise. Using a set of inlet and

exhaust radiation codes to establish a link between the

predicted in-duct pressure levels and the farfield noise

levels, the theoretical sideline directivities were shown to

be consistent with the experimental data. A kinematic

argumen:: was proposed to explain the mechanism of noise

reductio_t due to sweep and lean. This argument suggests

that to r_duce noise, sweep and lean must be chosen in

such a _ ay so as to increase wake intersections per vane.

A set of simple design rules is proposed for implementing

sweep mtd lean in practical fan stage geometries. Sweep

for which the vane tip is downstream of its root, and lean in

the direction of the fan rotation reduce the strength of the

interaction tones with size of reduction dependent on the

amount of sweep and lean chosen.

12
A secondaryconclusion is that the codes used in 7.

this work provide reasonably accurate tools for studying

the aeroacoustics performance of modern fan stages. It

should be emphasized that all of the theoretical results

shown here were plotted without any adjustments or shifts 8.

in their levels and as such they represent true predictions.

While the task of accurately calculating the absolute noise

levels for all emission angles at all relevant fan operating

conditions and stator configurations remains a challenge, 9.

the essential features of the sideline directivity, as well as

the trends associated with sweep and lean, are accurately

captured with the codes used in this study. Improvements

in noise source modeling cannot but help improve these 10.

predictions further.

5. ACKNOWLEDGEMENTS

The authors wish to thank Mr. Richard P.

Woodward and Dr. James E. Bridges, both of the

Acoustics Branch at NASA Lewis Research Center, for

providing the acoustic data shown in this report and the

routines used for computing the l-foot lossless acoustic

power from the measured spectra.

1.

2.

.

.

.

.

6. REFERENCES

Groeneweg, J.F. and Rice, E.J., "Aircraft Turbofan

Noise," Transactions of the ASME, 109, pp. 130-141,

1987.

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

Reduction," AIAA Paper 72-126, January 1972.

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

Fan Source Noise Reduction," NASA-CR 134486,

November 1973.

Adamczyk, J.J., "Passage of a Swept Airfoil Through

an Oblique Gust," Journal of Aircraft, Vol. 11, 1974,

pp. 281-287.

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, December

1977.

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

Wakes Interacting with a Leaned Vane Stator," AIAA

Journal, Vol. 20, No. 10, 1982, pp. 1352-1358.

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

Interaction of a Rotor Wake with a Swept Stator

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

Envia, E. and Kerschen, E.J., "Noise Generated by

Convected Gusts Interacting with Swept Airfoil

Cascades," AIAA Paper 86-1872, July 1986.

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

Sweep on Rotor-Stator Interaction Noise," NASA-CR

187052, December 1990.

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

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

Fan Noise Reduction," NASA-TM (TBD), October

1997.

11. Groeneweg, J.F. and Soft'in, T.G., Gliebe, P.R., and

Rice, E.J., "Turbomachinery Noise," in Aeroacoustics

of Flight Vehicles: Theory and Practice, Volume 1:

Noise Sources, Edited by H.H. Hubbard, NASA RP1258, Vol. 1 WRDC Technical Report 90-3052,

August 1991.

12. Envia, E., Huff, D.L., and Morfison, C.R., "Analytical

Assessment of Stator Sweep and Lean in Reducing

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

1996.

13. Ventres, C.S., Theobald, M.A., and Mark, W.D.,

"Turbofan Noise Generation, Volume 1: Analysis,"

NASA-CR 167951, July 1982.

14. Ventres, C.S., Theobald, M.A., and Mark, W.D.,

"Turbofan Noise Generation, Volume 2: Computer

Programs," NASA-CR 167952, July 1982.

15. Meyer, H.D. and Envia, E., "Aeroacoustic Analysis of

Turbofan Noise Generation," NASA-CR 4715, March

1996.

16. Philbrick, D.A. and Topoi, D.A., "Development of a

Fan Noise Design System, Part l: System Design and

Source Modeling," AIAA Paper 93-4415, October

1993.

17. Majjigi, R.K. and Gliebe, P.R., "Development of a

Rotor/Wake Vortex Model," NASA-CR 174849,

Volume 1, June 1984.

18. Dalton, W.N., Elliott, D.B., and Nickois, K.L.,

"Design of a Low Speed Fan Stage for Noise

Suppression," NASA-CR (No.TBD), June 1997.

19. Eversman, W. and Danda Roy, I., "Ducted Fan

Acoustic Radiation including the Effects of Non13
UniformMeanFlowandAcousticTreatment," AIAA

Paper 93-4424, 1993.

20. Eversman, W., "Aft Fan Duct Acoustic Radiation,"

CEAS/AIAA 95-155, 1995.

21. Topoi, D.A., "Development of a Fan Noise Design

System, Part 2: Farfieid Radiation and System

Evaluation," AIAA 93-4416, 1993.

22. Nallasamy, M., "Computation of Noise Radiation from

Fan Inlet and Aft Ducts," Journal of Aircraft, Vol. 43,

No. 3, 1997, pp. 387-393.

23. 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,"

A1AA Paper 96-1740, May 1996 (also NASA-TM

107213).

24. Sutliff, D.L., Nallasamy, M., Heidelberg, L.J., and

Elliot% D.M., "Baseline Acoustic Levels of the NASA

Active Noise Control Fan Rig," AIAA96-1745, May

1996 (also NASA-TM 10721).

25. Woodward, R.P., Dittmar, J.H., Hall, D.G., and KeeBowling, B., "Background Noise Levels Measured in

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

Tunnel, AIAA Paper 95-0720 (also NASA-TM

106817).

low order tones especially for the radial stator. The

opposite trend is true for the higher order tones particularly

for the swept-only and swept and leaned stators. It should

be noted that, in the under-predicted cases, the theoretical

power le_,el is generally below the measured broadband

power level.

In the last three figures the predicted (open bars)

and measured (darkly shaded bars) tone power level

reductions are shown. In these figures, the broadbandcorrected theoretical reductions (lightly shaded bars) are

also shown for comparison. In computing the broadbandcorrected theoretical reductions, the predicted levels

themselves were used if they were above the broadband,

and the broadband levels if the predicted theoretical levels

were below the broadband level. Figure A4 shows the

reductions due to swept-only stator (i.e. P(30, 0) - P(0, 0)),

Figure A5 the reductions due to swept and leaned stator

(i.e., P(30, -30) - P(0, 0)), and Figure A6 the reductions

due to swept and leaned stator relative to the swept-only

stator(i.e., P(30, -30) - P(30, 0)). For nearly all

combinations of tone, speed and configuration, the datatheory comparison is remarkably good when the

broadbar_d-corrected theoretical reductions are matched

against the measured reductions. These comparisons are in

accord with those shown for the SPL directivity

comparisons in Section 3.2 of the report.

7. APPENDIX

Tone Power Level Comparisons

In this appendix, tone power level comparisons

for the 2BPF through 5BPF tones at approach, cutback and

takeoff conditions are shown. The predicted in-duct power

levels were calculated using the BBNN072 code while the

experimental levels were computed by integrating the 1-

foot lossless directivities estimated from the measured 88-

inch sideline sound pressure levels. Using the same

procedure, the corresponding broadband noise level in the

neighborhood of each tone was also calculated to help with

the analysis of the tone data.

The Figures A I through A3 show the absolute

tone power levels for the radial, swept-only and swept and

leaned stators, respectively. In these figures, open bars

represent predicted levels, darkly shaded bars the data, and

lightly shaded bars the broadband levels. The trends with

tone order, fan speed and stator configuration are

somewhat mixed, but the following observations can be

made. The theory over-predicts the measured levels for the

14
2BPF: Upstream

_ Tsk=df

140

_13oI ...... I

;LAI±JIJl i

4BPF: Up_remn

c u 0_,k T_tooff

140

.__.nO't IVI

; JlAI ..i

5BPF: Ups'roman

Cmbtek TLV_off

_13° [_ D't' '-F*" L'=L_ I 1

o,_01! ,B_!**°*_'-_? '1

I10_ t

i i i .... I

100 ....... !

Figure AI. Comparison of measured and predicted tone power

levels for the radial stator. Power level in the

broadband at each tone is also shown.

2BPF: Ulmream

Al01mme_ Cxab_.k T_keoff

3BPF:

Approteh Cutl_e.k T_kooff

4BPF: U!mmream

^l_rc_h Cutl_ T_cec_

_140,

2BPF:

C_ah_-.k T.tkeoff

_'130 '

,2° ii

Figure A3. Comparison of measured and predicted tone power

levels for the swept and leaned stator. Power level in

the broadband at each tone is also shown.

_ ,o0

3BPF: Upstream

Appro_h C=_k Tnkcoff

_:-oi

.o 110 ....

4BPF: Up_a'carn

Cutt_k Takeoff

_._ 14o F

Figure A2. Comparison of measured and predicted tone power

levels for the swept-only stator. Power level in the

broadband at each tone is also shown.

3BPF: Up,attain 3BPF: Dowmtream

p_ Al_B_aeh CutM¢,k T_k_ff _ _ _ T_k_MT

4BPF: Up_mm 4BPF: Dom_mrt=_

_1 Ap_ Ctllt_l_k Ttl_off Aplmmeh Cutback Tlff,¢off

5BPF: Upt;taxam 5BPF: Downstream

Cm_k T=k_(t

Figure A4. Measured and predicted tone power level reductions

for the swept-only stator relative to the radial stator.

Theoretical reductions based on broadband-corrected

predicted levels are also shown.

15
Figure A5. Measured and predicted tone power level reductions

for the swept and leaned stator relative to the radial

stator. Theoretical reductions based on broadbandcorrected predicted levels are also shown.

2BPF:

_ c_b.akT,,V_e

2.5 .................

oo ::,"

ff -2.s|

-SO [ ..................

-I00

2BPF: 13o_msoem_

E,ool

3BPF: _ 3BPF: Dowmlre.eun

_ c_t_,aT,,k,_r _ apr,.a, c,at_. "r.a,,oe

_.iO Oi .......................... _ -I00 _

4BPF: I._ 4BPF: Downstre, e_

-2_ ii

_,-s°i :7: :_

_-,oo .......... E,oo[

5BPF: _ 5BPF:

AIB_unda C_ Takeoff _ _ Cutl_r.,k T_off

_., o.o0 o _ _

Z,oo,l [:_ g;::_.J I

Figure A6. Measured and predicted tone power level reductions

for the swept and leaned stator relative to the sweptonly stator. Theoretical reducaions based on

broadband-corrected predicted levels are also shown.

16

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1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. RE_ORT TYPE AND DATES COVERED

December 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Design Selection and Analysis of a Swept and Leaned Stator Concept

6. AUTHOR(S)

Edmane Envia and M. Nallasamy

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

WU-538--03-11--00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11383

10. SPON SORING011_ONITORING

AGENCY REPORT NUMBER

NASA TM--1998-208662

11. SUPPLEMENTARY NOTES

Edmane Envia, NASA Lewis Research Center and M. Nallasamy, NYMA, Inc., 2001 Aerospace Parkway, Brook Park,

Ohio 44142, presently at Dynacs Engineering Corporation, 2001 Aerospace Parkway, Brook Park, Ohio 44142.

Responsible person,.Edmane Envia, organization code 5940, (216) 4 33-8956.

1211. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified- Unlimited

Subject Category: 71 Distribution: Nonstandard

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

12b. DIS_FliuUTION CODE

13. ABSTRACT (Maximum 200 words)

This report describes a theoretical design and analysis study of the benefits of vane sweep and lean for reducing rotorstator interaction tone noise. It is shown that the kinematic relationship between the rotor wakes and stator vanes is the

principal factor in determining the achievable noise reductions. Vane sweep and lean control rotor wake skewing as seen

by the stator vanes and hence influence the number of wake intersections per vane. An increase in the number of

intersections reduces noise levels. Hence, to reduce rotor-stator nois4:, van sweep and lean must be chosen in such a way

as to increase wake intersections per vane. A simple design rule is th us proposed which requires a sweep configuration

that puts the vane tip downstream of its root and a vane lean that is i i the direction of the rotor rotation. A detailed

comparison of the predicted and measured noise reductions for a sw ;pt and leaned stator is then carried out. Overall,

these comparisons show that the predicted benefits of a swept and leaned stator are in good agreement, qualitative as

well as quantitative, with the measured reductions for the fan speeds relevant to noise certification procedure. Furthermore, the results also demonstrate the validity of design criterion an t the theoretical tools used in this study.

14. SUBJECT TERMS

Acoustics

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

i 18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECJRITY CLASSIRCATION

OF (_BSTRACT

Unclassified

15. NUMBER OF PAGES

23

16. PRICE CODE

A03

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev, 2-89)

Prescribed by ANSI Std. Z39-18

298-102