7

=:_ Acoustic Mode_ Measurements in the -

: Inlet of a Model Turbofan Using a

-_......... -C0nt :lnuousl ' ............ y-Rotating Rake ..................................

Lau.r__ence J._Heidelberg

National A eronautics and Space Admu__stration

Lewis Research Center ........................

Cleveland, Ohio

and

David G. Hall

Sverdrup Technology, Inc.

Lewis Research Center Group

Brook Park, Ohio .......... :......

=

Prepared for the .............

31st Aerospace Sciences Meeting

spons_ored_ by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 11-14, I993

i

N/ A

(NASA-TM-IOfi989) ACOUSTIC MODE

MEASUREMENTS IN THE INLET OF A

MODEL TURBOFAN USING A CONTINUOUSLY

ROTATING RAKE (NASA) 31 p

G3107

N93-16705

Unclas

0140774

ACOUSTIC MODE MEASUREMENTS IN THE INLET OF A MODEL TURBOFAN

USING A CONTINUOUSLY ROTATING RAKE

Laurence J. Heidelberg

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

David G, Hall

Sverdrup Technology, Inc.

Lewis Research Center Group

Brook Park, Ohio 44142

Abstract

Comprehensive measurements of the spinning

acoustic mode structure in the inlet of the Advanced

Ducted Propeller (ADP) have been completed. These

measurements were taken using a unique and previously

Untried method which was first proposed by T.G. Sofrin.

A continuously rotating microphone system was employed. The ADP model was designed and built by

Pratt & Whitney and tested in the NASA Lewis 9- by

15-Ft Anechoic Wind Tunnel. Three inlet configurations were tested with cuton and cutoff stator vane sets.

The cutoff stator was designed to suppress all modes at

the blade passing frequency. Rotating rake measurements indicate that several extraneous circumferential

modes were active. The mode orders suggest that their

source was an interaction between the rotor and small

interruptions in the casing tip treatment. The cuton

stator produced the expected circumferential modes plus

higher levels of the unexpected modes seen with the

cutoff stator.

Introduction

The next generation of fan engines will likely

employ a marriage of turbofan and propeller technologies to achieve significant noise and fuel consumption

reductions. The Advanced Ducted Propeller (ADP)

model used in this investigation was designed and built

by Pratt & Whitney, Division of United Technologies,

and tested in the NASA Lewis 9- by 15-Ft Anechoic

Wind Tunnel (Fig. 1). Typical of propeller technology,

the ADP allows for the in-flight adjustment of the blade

pitch angle. This provides reverse thrust and optimum

performance over a wide range of conditions. The duct

provides the noise suppression advantage of a conventional fan engine. Since future engines axe expected to

use still higher bypass ratios, fan noise is likely to be the

dominant engine source.

One of the most important features of fan tone

noise is its modal structure. Knowledge of these spinning modes helps to identify the generation mechanism,

control duct propagation (thus mode knowledge is

needed for acoustic treatment design) and control faxfield radiation angles. Previous attempts at direct mode

measurements 1"3have faced formidable practical difficulties such as: very large axial and circumferential arrays

of wall microphones that axe not practical for the short

ducts of ultrahigh bypass engines, and radial measurements upstream of the fan that introduce a wake that

interacts with the rotor thus causing extraneous modes.

A continuously rotating microphone technique first proposed by T.G. Sofrin 4 overcomes the problem of wake

generated modes, reduces the number of microphones

and duct length required. This technique has been

implemented for the first time in this investigation.

Two important features of this technique axe: (1) each

circumferential mode order generates a signal at a different frequency and (2) the interaction of the rotating

rake wake with the rotor produces a signal at only one

frequency, the one that corresponds to the rotor locked

mode. This mode cannot propagate in a duct when the

fan tipspeed issubsonic(suchas the ADP). A complete

discriptionof the measurement technique and data analysisispresentedin a companion paper.5

The objectof the present study is to measure the

modal structurein the inletof the ADP forthree inlet

lengthsand two statorvane blade numbers. One stator

was designed to have no cuton modes at the blade passing frequency (BPF) while the other was designed fora

single circumferentialorder. The fax-fieldacoustic

resultsas well as a preliminary look at some mode

measurements forthe ADP were reported in Ref. 6.
Apparatus and Procedures

Anechoic Wind Tunnel

The NASA Lewis 9- by 15-Ft Anechoic Wind

Tunnel is located in the low-speed return leg of the

supersonic 8- by 6-Ft Wind Tunnel. The test section

Mach number was held at 0.20, which provides a

takeoff/approach test environment. The tunnel acoustic

treatment provides anechoic conditions down to a

frequency of 250 Hz, which is well below the lowest BPF

of the ADP model. Figure 1 shows the ADP model

installed in the wind tunnel during a far-field acoustic

test.

Model Fan

The ADP model featured a 16-bladed rotor

43.81 cm (17.25 in.) in diameter. Three inlet lengths

were tested as shown in Fig. 2. The rotating rake was

always installed so that the measurement plane was at

the minimum diameter of the inlet (throat). The

cowling was supported by the stator vanes and there

were no struts or pylons crossing the fan stream. Two

stator vane sets were tested. The 40 vane stator should

have had no cuton modes at the BPF for any speed in

the operating range: The 22 vane stator should have

produced only the "6 circumferential order. The maximum tip speed of this fan is 275.2 m/s (903 ft/sec) at

12 000 rpm. This corresponds to a rotor tip Mach

number of 0.81.

The mechanisms which generate acoustic modes in

an axial flow fan may be divided up into three classes;

rotor-alone modes, rotor/stator interaction and inlet

flow distortion/rotor interaction. It may be shown that

the rotor-alone mechanism generates a rotor-locked

circumferential mode and its harmonics. These modes

have m-numbers equal to integer multiples of the

number of blades, B.

The effect of rotor/stator and inlet distortion/rotor

interaction is found by modifying the rotor-alone

analysis to include a periodic inflow disruption with

spatial order V. In the case of rotor/stator interaction,

V is equal to the number of equally spaced stator

vanes. In the case of inlet flow distortion, V is equal to

the number of distortions, assuming equal spacing in the

circumferential direction or the special harmonic content

of nonuniform spacing. The m-modes generated by

interaction phenomenon are given by the following

(k = + any integer, h = BPF harmonic, 1,2,3...):

m = hB - kV (1)

Positive m-numbers indicate rotation in the same

direction as the fan. Negative m-numbers indicate

rotation in the opposite direction. The spin rate of each

m-mode depends on its order (all spinning modes

produce tones at the blade passing frequency and/or its

harmonics):

mn m = hBw (2)

Background--Ducted Fan Acoustic Modes

This sectionpresentsa briefoverview of the topic

of ducted fan acousticmodes and fan noise generation

mechanisms. A generalunderstanding of thismaterial

is required in the discussionto follow. This topic has

been the subjectof an extensivebody of researchover

the past several decades. 7"9

The pressure field in a cylindrical duct containing

an axial-flow fan consists of a spinning pattern with

distinct maxima and minima. This pattern exhibits a

characteristic, repetitive ("modal') shape. This shape

can be described mathematically as a superposition of

periodic functions, containing m periods in the circumferential direction and n periods in the radial direction,

A rotating pressure pattern whose shape can be

described using a characteristic function at one unique

spatial frequency is referred to herein as a mode. Sing|efrequency patterns of this type in the circumferential

direction are referred to as m-modes, and in the radial

direction as n-modes.

where

tim mode speed

w fan speed

The far-field tone noise generated by an axial flow

fan results from the propagation of spinning modes out

of the duct. Duct propagation is studied analytically by

solving the homogenous wave equation subject to

boundary conditions at the duct walls. If the spin rate

of a mode is sufficiently high, the sound pattern propagates in a spiral fashion down the duct and radiates to

the far-field. If the pattern spin rate is low, the wave

equation gives a solution where the modal amplitude

decays exponentially with distance from the source. The

decay rate is usually very large over a small distance,

and modes with spin rates less than the critical minimum produce negligible far field noise. This _mini_mum

spin rate is referred to as %utoff," Various simple

formulas exist to predict the occurrence of cutoff. The

following general rules may be stated:

2
(1)As fan speed increases,so does the circumferentialMach number at the wall. This allowsmore and

more modes to cut on and begin propagating. This is

seen in far-fieldnoiselevels,which increasewith higher

fan speeds.

(2) For a given fan speed, a larger duct throat

diameter will allow more modes to propagate.

(3) As fan speed increases, modes with low m- and

n-orders cut on first.

(4) The rotor locked mode (order m = B), and all

modes of greater order, can only propagate for blade tip

roach numbers above 1.0.

To summarize, the occurrence of a given mode in

the duct depends on the mode generation mechanisms

(rotor alone, rotor/disturbance interaction and rotor/

stator interaction). The propagation of a mode to the far

field depends primarily on duct geometry and fan speed.

Rotating Rake

Figure 3(a} is a photo of the rotating rake installed

on the inlet of the ADP model. The rake is attached to

a ring gear surrounding the inlet. This gear rotates at

exactly 1//250 of the fan speed and is phase locked to the

fan. A control system using a stepping motor drives the

ring gear and rake. This control system can be

described as an electronic gear box. The angular relationship between the fan and the rake is in general held

to within ±0.2 ° of the ideal. Five miniature pressure

transducers are mounted on the rake (Fig. 3(b)). The

pressure signals are transmitted across the rotating

boundary by telemetry. By running the rake at two

different radial positions, measurements at nine different

radii were obtained, along with one repeat radial

location. The design, installation, and operation of the

rake and control system are described in Ref. 10.

The rotating frame of reference provided by the

rake separates each circumferential mode order, m, into

a different frequency by Doppler shift. This happens

because each m order spins at a different rate which is

inversely proportional to its order. For m - B, where

B is the rotor blade number, the mode rotates at the

same speed as the rotor. This mode is referred to as the

rotor locked mode. On the rotating frame of reference

the rake wake-rotor interaction is seen at the same

frequency as the rotor locked mode. Thus, the rake

wake-rotor interaction only contaminates one potential

mode measurement. This mode only appears when the

rotor tip speed is above the speed of sound, which is not

the case in the ADP model.

The data for each radial position is fan synchronously digitized. A very narrow band complex spectrum

is obtained by time domain ensemble averaging synchronous to the rake. This spectrum contains a cluster of

tones centered on the BPF and its harmonics. Each

tone that corresponds to an m order, the phase and

magnitude at each radius is used to calculate the radial

order. A leastsquarescurve fitusingBesselfunctionsis

employed forthiscalculation.A more complete descriptionof the data collectionand analysisispresentedin a

5

companion paper.

Results and Discussion

The long and medium length inlets were tested with

both the 40 and 22 vane stators while the short inlet

was tested with the 22 vane stator only. All the test

data were obtained with the short spinner and nozzle,

takeoff blade setting angle (11 ° less than cruise} and the

axial skewed grooved rub strip. Mode measurements

were made for four different rotor tip Mach numbers,

Mr, 0.65, 0.73, 0.77, and 0.81 corresponding to the

following corrected rotor speeds: 9600, 10 800, 11 400,

and 12 000 rpm. Both the BPF and 2-BPF modal

structures will be presented. Although the spectral data

was available for the 3-BPF modes, there were an

insufficient number of radial measurement locations

available to resolve the highest order radial modes

present at this frequency.

SpectralData

The effect of rotation on the rake signal can be seen

in Fig. 4. These spectra were obtained using an on-line

signal analyzer during testing. They have a very narrow

resolution bandwidth (0.0005 shaft orders} and are a

result of fan synchronous digitizing. When the rake is

stationary all of the modes of the BPF appear at one

frequency (16.000 shaft orders). When the rake rotates,

a Doppler shift occurs that splits the BPF tone into

many different frequencies, each representing a different

spin rate, and thus circumferential order. It should be

noted that the tone corresponding to m = 16 is spurious

and a result of the rake wake interaction with the rotor.

The separation between m orders in terms of shaft

orders is equal to the rake-fan speed ratio. In this case,

the m orders are 1/250 or 0.004 shaft orders apart.

As illustrated above the circumferential modal

decomposition was obtained from a very narrow band

spectral analysis. This analysis used fan rotor synchronous digitizing and time domain rake synchronous

ensemble averaging to produce a magnitude and phase

for each m order at each radial location. Next, a

radial mode analysis was performed using the radial
distributiondataforeachmorderastheinputtoaleast M = -6 isveryobvious.Thism orderisaresultof the

squarescurve fitting program. Bessel eigenfunctions

were used in calculating the radial coefficients. A complete description of this process can be found in Ref. 5.

Modal Data

When all significant m orders are resolved into

their radial components, a complete modal picture can

be seen in the form of a three-dimensional bar graph as

in Fig. 5(a). The mode amplitudes in Pascals are plotted against both circumferential, m and radial, n

orders. The case plotted here is for the 22 vane (cuton}

stator and the long inlet at BPF and a rotor tip Mach

number of 0.77. Before a discussion of the modal plot

begins it would be helpful to normalize the amplitudes,

The large amplitudes seen at the higher m and n

orders (cutoff ratio near unity) are misleading when it

comes to evaluating their contribution to far-field noise

or total power. The modes (10,0), (11,0), and (12,0) are

typical near-cutoff modes with high levels and little or

no acoustic power. These near cutoff modes are propagating at high angles to the duct axis and therefore contribute much less to the overall noise signature for a

given mode amplitude compared to modes propagating

at angles close to the axis. To compensate for this

effect, the amplitudes of all modes were normalized by

multiplying them by the cosine of their respective propagation angles. The normalized amplitude represents the

component of the amplitude in the axial direction. Figures 5(a) and (b) can be compared to see the effect of

this normalization. The largest differences occur at the

higher values of m and n. Note that modes (11,0) and

(12,0) do not appear in the normalized plot since they

are cutoff. The {10,0) mode is cuton but has a greatly

reduced amplitude. All the remaining modal data in

this paper will be presented using the normalized

amplitudes.

One limitation of the mode measurements in this

investigation is that, with only one axial rake location,

the axial direction of travel for any mode is not certain.

A mode could, in some cases be traveling towards instead of away from the fan. Since the net acoustic

power is traveling away from the fan it might be reasonable to assume that most of the modes are doing

likewise. It should be kept in mind that any strong

reflections near the end of the fan duct could lead to

erroneous interpretation of results for a particular mode.

BPF Modal Structure. --Figures 6 to 9 show a comparison of the BPF modal structure of the 22 and 40

vane stator sets, for both the long and medium inlets.

Since the short inlet was only run with the 22 vanes it

will be shown separately. Figure 6 shows the effect of a

cuton stator set (22 vanes} for both inlets at a low rotor

speed, M t = 0.65. The expected circumferential order of

interaction of the 16 blade wakes with the 22 vanes.

For the 40 vane stator no modes should be cuton at

the BPF. As seen in Fig. 6 there are many modes

present for the 40 vane stators. These extraneous modes

also show up in the 22 vane cases. A closer examination

of the extraneous modes, especially at the higher speeds

shown in Figs. 7 to 9 reveal a pattern of four difference

in m order. This Am -- 4 pattern, and Eq. (1), suggest the interaction of the rotor with a fixed four lobed

distortion pattern in the inlet. No obvious cause for

such an inlet distortion pattern could be found. Farfield tone directivity patterns s also suggest the presence

of extraneous modes. It is not unusual to see residual

tones in fans with cutoff stators, though the tones seen

in this investigation may be unusually strong.

A careful examination of the fan casing tip treatment used for these tests revealed two possible sources

for extraneous modes. The casing treatment and rub

strip consists of two rows of axial slots skewed relative

to the radius. The treatment was fabricated in eight circumferential segments. At each location where the segments join there was a small area with no slots. This

symmetrical interruption of the treatment could lead to

a periodic loading change at the blade tip and thus

extraneous modes of Am = 8 centered on m = 8. In

addition to these eight segments, four blade tip clearance probes were installed through the treatment, 90 °

apart around the casing, in the center of every other segment. This could lead to a periodic blade loading

change of four per revolution as well as 16 (the distance

between the probe and either end of the segment is 1//16

of the circumference). These periodic loading changes

lead to modes of Am=4 centered on m= 12 and

Am = 16 centered on m=0. The m=0 mode is

often the largest in the BPF data. The m = t16

modes for the 2-BPF data will later be shown to be very

strong.

Figure 7 shows the modal structurefor M t --0.73.

For the 22 vane case,the m = -6 order ismuch larger

then the previous speed. Also, the higher speed is

responsibleforthe mode (6,1) tojuststartto be cuton.

Not only can the effectof rotorspeed on cuton be seen

but, the inletcan alsoeffectmode cuton. The medium

inlethas a slightlylargerthroatdiameter then the long

inlet.This allows a mode to be cuton firstin the medium inletas can be seen by the presence of the (10,0)

mode in Fig. 7(b). At stillhigherspeed,M t -- 0.77,still

more modes appear as shown in Fig. 8. Higher speed

allowshigher radialorders,n to be cuton as well,as

can be seen for the long inletwhere n -- 3 appears for

m = 0. Only the long inlethas the n -- 3 mode because

ithas a higher hub-tip ratiowhich allowsthismode to

propagate. The Am = 4 pattern is more obvious at
this speed. The importance of the m = 0 mode seems

to be growing, particularly for the long inlet. For this

inlet, the (0,1} mode is the largest, even for the %uton"

22 vane stators.

It should be noted here that, the importance of an

individual mode cannot always be determined where

more then one radial order is present. The far-field

radiation pattern is greatly effected by the phasing of

the individual radial orders. Also note that the amplitude scale used on the plots is linear, thus tending to

exaggerate differences between configurations.

Figure 9 shows the modal structureforthe highest

speed,M t= 0.81. The resultshere are similarto the

previousspeed,with the exceptionof a few more modes

being cuton. The m = -{-12orders are now cuton for

the medium inlet.

The medium inlet appears to be quieter based on

the trends shown in Figs. 6 to 9. The far-field measurement do not generally show this, but the far-field trends

depend on speed and whether sideline or polar directivity comparisons are made.

Another trend in the modal data is that the

Am = 4 modes axe higher for the 22 vane (cuton)

stators. It is not known what is responsible for these

increased levels.

In another investigation, as yet unpublished, the

complex mode amplitudes for some of the configuration

just presented were used as input to a finite element farfield radiation code. Both 40 and 22 vane, as well as

long and medium inlet cases were run and yielded good

agreement with the far-field measurements. These

results add confidence to both the mode measurements

and the code.

The modal structure for the short inlet, 22 vane

configuration is shown in Fig. 10 for all four speeds.

When these levels are compared to the medium inlet

with 22 vanes in Figs. 6 to 9, they are generally lower.

Far-field results 6 show little or no difference. Due to the

very short length (length to fan diameter ratio of 0.21)

the measuring plane is extremely close to the fan, less

then a wavelength. There may be a llear-field effect

that in some way is responsible for the lower levels. One

expected near-field effect is higher levels of cutoff modes.

This effect was not observed in the data; perhaps

because, at decay rates of the order of 200 dB per L/D,

the measurement was not close enough to the fan to

observe these cutoff modes.

2-BBF Modal Structure.--Figures 11 to 14 show a

comparison of the 2-BPF modal structures for the 22

and 40 vane stator configurations for both the long and

medium inlets. Not only are there twice as many m

orders cuton for this harmonic, but twice as many n

orders are cuton as well. There are seven n orders (0

to 6) cuton in some cases. This presents a problem, in

that the least squares Bessel function solution for radial

mode decomposition can become less suitable and accurate with data for only nine radial locations available.

The quality of some of the 2-BPF solutions for high n

orders is less reliable than the rest of the modal data

presented.

Both the 22 and 40 vane stators have cuton rotorstator interaction modes for 2 BPF. From Eq. (1) the

22 vane configuration produce two m orders, m = 10,

-12, while the 40 vane configuration produce only one,

m = -8. The m = 10 for the 22 vane configuration

can be seen in Fig. 11 where the lowest speed data,

M t = 0.65 is shown. Only the medium inlet data for the

22 vane configuration is available here. This data shows

three radial modes for m _- 10, all over 100 Pa. The

other expected m order, m = -12 also happens to be

one of the extraneous modes (Am = 4), and so the level

due to the rotor-stator interaction is uncertain. In any

case, it has a low amplitude, and this counterrotating

(negative m orders rotate opposite the direction of the

rotor} is not a significant contributor to the overall

noise. The expected mode for the 40 vanes is also

counterrotating, m = -8, and again it has a relatively

low level, which may not completely be due to rotorstator interaction. Low levels of counterrotating modes

may be a result of these modes being reflected from the

rotor blades as they try to propagate upstream through

the rotor. 11 Unfortunately, it is expected that the

energy these modes carry would be transferred to the aft

duct. The addition of aft mode measurements for this

fan could prove valuable in understanding the rotorstator reflection and transmission problem.

At the higher speeds (Figs. 12 to 14) the overall

modal structure for both vane sets remained similar to

the lowest speed except for some drop in levels at the

highest speed, M t = 0.81. The Am = 4 pattern is very

evident at most speeds and for both inlets. Just as in

the BPF data, the 2-BPF data for 22 vanes have higher

levels of the extraneous modes particularly near and

above the large rotor-stator mode of m = 10. For the

40 vane stators, the dominant modes are the extraneous

modes (Am = 4) even though there is a cuton rotorstator mode. A pattern of Am = 8 can be seen in

much of the 40 vane data. The m-- 16, 0, and -16

orders axe usually some of the most prominent. This is

further evidence that the extraneous modes might be a

result of the casing tip treatment interruptions of 1/4,

1/8, and 1/16 of the circumference.

In most cases, the differences between the long and

medium inlet were small. For the short inlet, only
radialmode solutionsfor one speed,M t = 0.81 could be

obtained. Much of the spectraldata for the short inlet

had phase shiftsbetween the two rake immersions.

With onlyfiveradialpositionsavailablefora singlerake

immersion {consistentphase angle data),thereisinsufficientinput for a solutionfor the seven radialorders

requiredforlow m orders. Since only the modal structure for the short inletwith 22 vanes is availableat

M t= 0.81,Fig. 15 shows a comparison of allthe inlets

for thisspeed and vane number. Although the levels

may be somewhat lower forthe shortinlet,thismay not

be significant,especiallyforthe expected m = 10 modes

which are the highest. When allthe normalized amplitudes of the radialmodes are summed for m = 10, the

largestdifferencebetween inletsislessthen 20 percent,

which is small in terms of decibels. The extraneous

modes particularlyat m = +16, 0, and 20 can be fairly

strong.

There was some tendency for the mode levelsto drop

with length. This was most evidentin the shortinletat

BPF.

Since mode measurements were made in only one

axiallocation,the directionof travelof a mode isnot

certain. It isgenerallyassumed that most modes are

travelingaway from the source. Duct termination

reflectionscould confuse the interpretationof the data.

Additionally,the presenceof many radialordersmakes

itdifficultto projectmodal changes to far-fieldchanges.

These limitationsrestrictthe interpretationof modal

data. There are many toolsthat could prove usefulto

overcome these limitations.One is a duct to far-field

radiation code. Preliminary and as yet unpublished

resultsfrom such a code indicategood agreement with

far-field data, when mode data from this investigation

was used as input.

Concluding Remarks

This paper presented the acousticmodal structure

in the inletof the ADP turbofan model as measured by

a unique and previouslyuntriedmethod. The BPF and

2-BPF modal structureforfiveconfigurationsinvolving

the inietlength_and statorvanennmber were measured

in the NASA Lewis 9- by 15-Ft Wind Tunnel. The continUous|y rotatlng rake technique employed here was

able to measure both circumferentialand radialmode

order with no apparent interferencewith the noise

sources.

The modal structureforconfigurationwith a cuton

statorexhibitedhigh levelsof the expected m order.

A complicated and unexpected modal structurewas observed in addition to the expected one. These extraneous modes were present with both cuton and cutoff

stators. The extraneous modes had a pattern of

Am = 4, 8, and 16, which Was found to be the same as

a pattern of symmetrical interruptions of the fan casing

tip treatment. It seems that these small interruptions in

the treatment may have caused periodic loading changes

at the blade tip which result in extraneous modes. Further evidence is needed to confmrn this rotor-casing

treatment interaction source. Higher levels of the

extraneous modes were observed when the rotor-stator

interaction was cuton.

Some indication of modal reflection from the rotor

for counterrotating modes coming from the stator was

observed. This should be further investigated by measuring the modes in the aft fan duct as well as far-field

acoustics.

Much of the expected behavior of the circumferentialand radialmode orders with speed was observed.

The Effectof inlet length was difficultto estimate.

References

1. Pickett, G.F., Sofrin, T.G., and Wells, R.A.,

_Method of Fan Sound Mode Structure Determination. Final Report," NASA CR-135293, Aug.

1977.

2. Mo0re, C.J., UMeasurement _)fRadial and Circumferential Modes in Annular and Circular Fan Ducts,"

Journal of Sound and Vibration, Vol. 62, No. 2,

1979, pp. 235-256.

3. Joppa, P.D.,

Technique," AIAA Paper 84-2337, Oct. i984.

4: Cicon, D:E., Sofrin, T.G., and Mathews, D.C.,

_Investigation of Continuously Traversing Microphone System for Mode Measurement," NASA

CR-168040, Nov. 1982.

_An Acoustic Mode Measurement

5.Hall,D.G., Heidelberg,L.J.,and Konno, K., "Acoustic Mode Measurements in the Inletof a Model

Turbofan Using a Continuously Rotating Rake:

Data Collection/Anlaysis Techniques," NASA

TM-I05936, Jan. 1993 (ALso, AIAA Paper 93-0599,

Jan. 1993).

6. Woodward, R.P., Bock, L.A., Heidelberg, L.J., and

Hall, D.G., "Fax-Field Noise and Internal Modes

From a Ducted Propeller at Simulated Aircraft

Takeoff Conditions." AIAA Paper 92-0371, Jan.

1992 (ALso, NASA TM-105369, 1992).

7. Tyler, J.M., and Sofrin, T.G., _Axial Flow Compressor Studies," SAE Transactions, Vol. 70, 1962,

pp. 309-332.
8.Sofrin, T.G., "Some Modal-Frequency Spectra of Fan

Noise, _ AIAA Paper 81-1990, Oct. 1981.

9. Rice, E.J., Heidman, M.F., and Sofrin, T.G., "Modal

Propagation Angles in a Cylindrical Duct with

Flow and Their Relation to Sound Radiation,"

AIAA Paper 79-0183, Jan. 1979 (Also, NASA

TM-79030, Jan. 1979).

10. Konno, K.E., and Hausmann, C.R., "Rotating Rake

Design for Unique Measurement of Fan-Generated

Spinning Acoustic Modes," To be published as

NASA TM-!05946, 1993.

11. Topol, D.A., Holhubner, S.C., and Mathews, D.C.,

"A Reflection Mechanism for Aft Fan Tone Noise

from Turbofan Engines," AIAA Paper 87-2699,

Oct. 1987.

• ".:

Figure1.--Photograph of the advancedducted propellermodel installedinthe 9x5 ft anecho4c windtunnel.

7
L/D F D1-/DF

0.53 0.897

0.41 0.932

0.21 0.988

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Figure 2.--ADP inlet configurations.

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(a) Photograph of Installation.

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f

Rotating rake

Modal pattern (48 rpm max)

-- Pressure transducers

Fan rotor (12 000 rpm max)

fi / s.o ,n,et

_L_atlonary

rake

(b) Details of rotating rake Installation.

Figure 3.--Rotating rake Installed on the Inlet of the ADP model.
144 -

124 -

/i, LLuIJ ,_L aJ.J

lo4 _V=. q'"-""t_l-lf

,,_,-

2

I

Rake stationary

..,.L-_. u,-=..,,L_ _J

II

!

i

i I

Rake rotating, Nr/N f = 1/250

LI..- ,=.I,

_=mll'r

O.

"0

rRake wake --_ _.

104

13

]Jl

8

-.t,-

4 )

u I -4

.-12= m

15.900 16.000

Shaft orders

Figure 4.--Effect of rake rotation on spectrum. Long inlet, 22 vanes, M t = 0.81, 2.3 cm (0.91 in.} from wall.

16.100

10
(0

Q.

(9

:E

80

60

40

20

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

(a) Asmeasured.

8O

(9

Z

0

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(b) Normalized.

Figure5.mEffect of normalizationon BPFmodeamplitudes, longInlet, 22 vanes,M t = 0.77.

11
80

6O

4O

o

Z

40

20

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(a) Long Inlet.

Figure 6.--BPF modal structure at M t = 0.65.

12
8O

60

4O

20

2O

0

-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(b) Medium inlet.

Figure 6.--Concluded.

13
BO

6O

4O

0

Z

60

40

20

i i i i i i i i i i i i i i i i i i i i i i i i

::::;:::::::::::;:::::::

::::::;::::::;:::;::;;::

:::;::;::::::::;::::::::

:;::::::::;:::;::::::::;

_ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 22vanes; _ _ _ _

! "_ i _ g g i i i i i i i i g i g g i i i i i i i i

}Zi, _/ iiii!i!iiiiiiiiiii!ilili

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

:::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

iiiiii'-!iiiiiii!i!iiii!

i///i/ /

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

::::::::::::::::::::::::

::::::::::::::::::::::::

i i i i ! i i i i i _ i i i i i i i40vanes

.."ii!iiiiiii!!iiiiiiii!!ii

::::::::::::::::::::::::

ii!iiiiil;;i;i;;!!!;i;il

..."iiiiii!iiiiiiiiiiiii!iii

!iiii!iiiiii!iiiiiiii!i;

-12-10 -8 -6 -4 -2 0 2 ,I 6 8 10 12

Circumferential mode order, m

(a) LongInlet.

Figure 7.--BPF modal structure at M t = 0.73.

14
8O

6O

4O

2O

ill

0,,

do

I

0

Z

6O

4O

2O

0

iiiii i iiiiiiii ii ii i i i iii i! i i i iil i i i i22va._si iii

/ __.. '" ii i! i i i i i i! iii !i iii i i i i i i i i iil ii

- /i _:`_..i:_:::;_i:`.:.i:_::i_.<_:i_:.:_:_::_:_:::_+:i:`_.i:_::i_`:i_::i:_::i:.`,,::.

b

_f.: :i .- :_: :[ .. ..I .. ..[ .. ..I .. ..I ,.',..I ." .I . .l . .l ." .I I _v

-12 -10 -8 -8 .4 -2 0 2 ,q, 6 8 10 12

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

::::::::::::::::::::::::

i _ i _ i ! ! i i i i i i i i i i i i i i i i !

iiiiiiiiiiii!i_v_°°_siiiiii

42 40 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(b) Medium Inlet.

Figure 7.--Concluded.

15
16
8O

6O

4O

20

O.

d o

t.,.

o

Z

6O

4O

20

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(b) MediumInlet.

Figure8.--Concluded.

17
40 : .'

.•i : ,

..+ .+ ,+." <_..::...:-_

',,.::

0 -I

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(a) LongInlet.

Figure9.--BPF modal structureat M t = 0.81.

18
8O

6O

40

2O

i °

(9

"0

0

E

,N_

0

Z

6O

40

2O

0

-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

(b) Medium Inlet.

Figure9..--Concluded.

]9
°Iiiiiii..............

...-.--"i"...-.--! i..ili!iT i!ijiiiiiiiiiiiii

_o of...""

' .::_2::'_. .::'_:::"i:_.......7_::':: ::"=:,,"_'i ::':: _ ,/

0-12 "-10 -8 -6 -4 -2 0 2 4 6 8 10 12

60

40

2O

0

• !.." i ...""i i .! .i .i .._.._..ii:i .! ..::.J.._.i.,:.._.-_..::-_-_._._._._.::

-12 -10 -8 -8 -4 -2 0 2 4 6 8 10 12

Circumferential mode order, m

Figure10.--BPF modal structurefor the short Inletwiththe 22vane stator.

2O

100

80

60

40

8

20

a.

o

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

(9

Im

O

Z

8O

6O

4O

2O

0

-24-20-16-12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

(a) Longinlet.

Figure11.--2 BPFmodal structureat M t = 0.65.

22 PRECEDING PAGE BLANK i_iOT FILMED
IO0

8O

6O

(0

Q.

d

4O

2O

E

o

Z

80

60

40

2O

i iHiiiiiiiiiii!i_ii_ ..............................

i_iiiiii!iiii

i/i !!iii!iiiiiiiii

..." ,, •,- ,, :::::::::::;:::

iiiiiiiiiiiiiii

iiiiiiiiiiiiii2! iiiii!iii

:::::::::::::::::::::::::::::

:::::::::::::::::::::::::::::

:::::::::::::::::::::::::::::

:::::::::::::::::::::::::::::

:::::::::::::::::::::::::::::

4 8 12 16 20 24 28

8

0

-24 -20 -lS -12 -8 -4 0 4 e 12 16 20 24 2e

Circumferential mode order, m

(b) Mediuminlet.

Figure11.--Concluded.

23
0

Z

8O

6O

:. :

0

-24-20-16-12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

{a)Long inlet,

Figure12.--2 BPFmodal structureat M t = 0.73.

24
iiiiiiiiiliii iiiiii iii;ii ii ii iii iii!

iiiiiiiiiiiiiiiii{iiiiiiiiiiiiii;ili22vanes iiiiiiii

100

:i:ii._!:i:_..!!Q_.!!_..!!i_.._Q_..!!..:_;:`:...!!!_..!!_:_..!!_;`..

40 [- '_:!:._i:i:._:..i_:!;.:_.i"..[)atanotava[iabie :_.:i_i:._.i.:_:_:_.:i_.ii!'.-:_C":_:i'.-:ii._:_i:_:i_;:_."."_3 o_,v

D.

_ o

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

_ _`_.._.._?;..;.._?_.._?_7_i?_.?;.;..;.._.._..;.._H;.._._._

_ _® iiiii!iiiiiiiiii!!iiiiiii!i!ii!iiiiiiiiiiiiiiiiiiiii

-_.o ......... ._..i.._?i?_.i..i.i..i?_..i?i..!..i?i..!..i..!?i..i.i'

40 •:_..i"i .i-"..{"i _.,,;:_:_,,i:;:;_,,i:;::-:._t,,i:: .:.':_-i':i':::_i':'i".::i':"::::':":i:i':!i._._ :;: ";,: ' " i :_;"!i'.::. '; :::!i:;:::::;i':!c'.!i'.:!:i'!i'.:'!::::i'.::::'.!i'.!_::!:': ,!.;:_,_-,!.;_:_::_:;::.:;::_::.i:;::_,;_:,i:i::._::. 3 4 ,,,__,_'

2O

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

(b) Medium inlet.

Figure 12.--Concluded.

25
100

80

60

4O

iiiiiiii;iii.22vanesl;iiii

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

::::::::::::::::::::::::::

0.24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

0

20

0

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

" (a) Long inlet.

Figure 13,--2 BPF modal structure at M t = 0.77.

26
iiii!i!!!i!iiiiiiiiiiiii !iiiiiiiiiiiiiiiiiiiiii!i

100

8O

6O

40

(_ 20

Q.

d o

-24 -20-16 -12 -8 -4 0 4

E

"o

m

IO0

0

Z

80

60

4O

8 12 16 20 24 28

2O

0

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

(b)Medium inlet.

Figure13.--Concluded.

2T
10000

8O

6O

40

20

0

Z

80

6O

4O

2O

0

-24-20-16-12 -8 -4 0 4 8 12 16 20 24 28

Circumferential mode order, m

(a) Long Inlet.

Figure 14.--2 BPF modal structure at M t = 0.81.

28
(b) Medium inlet.

Figure 14.--Concluded.

2g
IO0

,,o_iii iiiiiiiiiii!i!T__iiiliiiiiii!i!iii

4o _X_>°

-24 -20 -16 -12

-8 -4 0 4 8 12 16 20 24 28

too i i i_ i i iiiiii!iiiiiiiiiiiiii iiiiiiiiii!i!i .......... iiii!

- .i" : :" i -." _ . -;..]-._.-;--:'.-i..;.-._--_-._..._-..;-.i-.]..,:..i-.i.._..i..].._- _.i_.!._._i_.!.._..._.i_._.i_.i_i_._._._i_i.._i._i._

_o:il i i ii _!-i..:.'_i_iiiii_i: ii iiiiiiiiiiiiiiii{iiiiiiii!

_ ,,o_ii i_i_i_:_i_::_: ___ll_!_"_ _

2o !::S_I!! !:!:S_:_ :_!;_i_!!_;'i " !_: _.,,._

o 0

Z -24-20-16-12 -8 -4 0 4 8 12 16 20 24 28

! ! : , i:!!i!:!i;!!i!:ii,,:i/_:!:::!i:!!!!::: : : :i:!!;::i::!;:::i:!;i,:i:;!!:::- 2

Circumferential mode order, m

Figure 1 5.--Comparison of 2 BPF modal structure for all three inlets at M t = 0.81.

3O

Form Approved

REPORT DOCUMENTATION PAGE oge No. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 1992 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Acoustic Mode Measurements in the Inlet of a Model Turbofan Using a

Continuously Rotating Rake

6. AUTHOR(S)

Laurence J. Heidelberg and David G. Hall

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

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

WU-535-03-10

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-7476

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM- 105989

AIAA-93--0598

11. SUPPLEMENTARY NOTES

Prepared for the 3lst Aerospace Sciences Meeting sponsored by the American Institute of Aeronautics and Astronautics, Reno, Nevada,

January I 1-14, 1993. Laurence J. Iteidelberg, Lewis Research Center and David G. Hall, Sverdrup Technology, Inc., Lewis Research

Center Group, 200] Aerospace Parkway, Brook Park, Ohio 44142. Responsible person, Laurence J. Heidelberg, (216) 433-3859.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified -Unlimited

Subject Categories 07 and 71

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 2OO words)

Comprehensive measurements of the spinning acoustic mode structure in the inlet of the Advanced Ductcd Propeller

(ADP) have been completed. These measurements were taken using a unique and previously untried method which was

first proposed by T.G. Sofrin. A continuously rotating microphone system was employed. The ADP model was designed

and built by Pratt & Whitney and tested in the NASA Lewis 9- by 15-foot Anechoic Wind Tunnel. Three inlet

configurations were tested with cuton and cutoff stator vane sets. The cutoff stator was designed to suppress all modes at

the blade passing frequency. Rotating rake measurements indicate that several extraneous circumferential modes were

active. The mode orders suggest that their source was an interaction between the rotor and small interruptions in the

casing tip treatment. The cuton stator produced the expected circumferential modes plus higher levels of the unexpected

modes seen with the cutoff stator.

14. SUBJECT TERMS

Fan noise; Acoustic modes; Aircraft noise

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURrrY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

32

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/