ICAS Paper No. 68-35

THE PROBLEM OF TURBINE NOISE IN THE CIVILGAS TURBINE AERO ENGINE

by

Michael J. T. SmithRolls-Royce, LimitedNottingham, U.K.

TheSixthCongressofMe

InternationalCouncilofMeAeronauticalSciences

DEUTSCHES MUSEUM, MUNCHEN, GERMANY/SEPTEMBER 9-13, 1968

Preis: DM 2.00

á

THE PROBLEMOF TURBINE110I3EIN THECIVILGAS TURBINEAEROENGINE

M. J. T. SmithDevelopmentEngineer- Noise

RollsRoyceLtd.Derby, England.

ABSTRACT

The significanceof noisefromthe turbineofa turbojetengineis examinedin thecontextofthe othercomponentnoisesources. A studyismadeof its generation,propagationand radiation,and the questionof suppressionis relatedtocurrentresearchwork.

INTRODUCTION

Duringearlyworkon thenoiseproblemsassociatedwith jetenginesin use withcivilaircraftoperatorsattentionwascentredalmostentirelyon the jetas themainnoiseproducer.Overtheyearshoweverit has becomequiteclearthatthechangingdesignphilosophyin goingtotheuse of higherandhigherbypassratioshasthrownup both thecompressorand the turbineas significant,and withinthenextfewyears,thepredominantnoiseproducers. 'Menthecominggenerationof highbypassratioturbofanenginesare in servicethisdifferencein sourceimportancewillbe strikinglyapparent,althoughtodayit ispossibleto citecleardifferencescausedby moremodestchangesin bypassratio.Thisis illustratedin figure1, whichis a plotof PerceivedNoiseLevela;;ainsttimefor four flyoversof twoair-craft. Theseaircraftwouldnormallybe identical

if it werenot for theirengineshavingslightlydifferingbypassratiosand dissimilarnozzlegeometry. The actualbypassratiodifferenceisfromjustless than1 on the onehand to about1i-on the other,and theflyoverschosenare directlycomparablein thatthereis bothan approachandtakeofftracepresentedfor eachaircraft.

In thecase of thelowerbypassratioinstallation,the enginesof whichhaverelativelyhigh jetvelocitiesand fullymixednozzleconfig-urations,theover-ridingnoisecontributioniscausedby themixingof thejetswith theatmosphere-Thisis clearlyevidentin the takeofftracewherethepostoverheadpeakis seento be an extendedexposuretypicalof theradiationpatternof jetnoise,whereasthe equivalenttracefor the higherbypassratioengineAhowsa muchquickerdecayofnoiseversustime. In theapproachtracesthereis not suchan obviousdifference,sincethe jetnoiseof eventhe low bypassratioenginesisreducedsufficientlyto revealbotha pre-overheadand post-overheadpeak,whichwe now knowis causedby the bladingof both thecompressorsand theturbines. The higherbypassratioinstallationexhibitsa rathersharperpeak,sincethecompress-ors are of a high tip speedhighrelativevelocitydesign,and as suchare theover-ridingnoiseproducers. Furthermorethisnoiseis allowedtoradiatedirectlythrougha Shortunmixedfan outletnozzle.

BYPASS ENGINE(WEED STREAMS)

&PROS 10 1.0

fun - Orr] [APPROACH!

1.11171 0001, IN, RECEDING

FLYOVER TIME - - SECONOS

TURBOFAN ENGINE(SEPARATE FAN NOZZLE)

YR. PO TO 1.1

FLYOVER TIME - — ICC O NO x

Thesearebut two of manyin-serviceenginetypes,eachof whichhas a turbinenoisecontent

-o which,dependingon the designphilosophyand actual powersetting,may or may not affectthe overall

-0 level. To considermoreexactlythe si,L;nificance of thenoisecausedby theturbineon theseandotherenginesthe subjectwillobviouslyneed

-20

amplification,and it is worthdevotinga littletimeto a discussionof the wayin whichturbinenoisewas firstnotedandisolated.

SOURCE DETECTION & DEFINITIONRELATIVE

•NLAs alreadypointedout,10 or 15 yearsago jet

-o noisewas themajorproblemwithcivilaeroengines,and an intensiveresearchanddevelopmentprogrammewas curriedout by themajoraero-enginemanufact-urersto establishbothhow thenoisewas generatedand whatstepscouldbe takento reduceit. (A

-30 numberof publicationsexistdealingsolelywith jet noiseand, for furtherconsideration,referenceworksmay be consultedelsewhere,includingtheselectionquotedat the end of thispaper).Towardsthe end of thedevelopmentprogrammeon jet noisesuppressorsit becal..eclearthatat low powersettingson evenpurejet and low bypassratioenginessomeothersourcesof noisewerecontrib-utingto theoverallenginelevel. Sincetheseothersourceswerenot obeyingacceptedjet noiselaws,and furthermorewerenot respondingto normalexternaljet noisesuppressiontechniques,it was

FIGURE1. COLPARISONOF TAKE-OFF& APPROACHFLYOVERTRACES. LOW BYPASSRATIOMIXED:MINES& UNMIXEDTURBOFANSIN SAMEAIRFRAME.

1

assumed t..atthey were the result of generatingmechanismscoitninedwithin the engine casing.

iiGUR:.2. VARIATION OF POLAR PEAK REAR ARC NOISEL.N.21 -JIM JET IrElOCITY. FULL SCALR ENGINES COMPARE) './ITHPURE AIRJETS.

Figure 2 illustrateshow,in plotting the overallnoise level from u series of pure jet and lowbypass ratio engines,theobserved laws of jet noisegenerationwere not accounting for the measurednoise levels at low jet velocities. The correl-

ating parameter in this illustrationis thatusually accepted, the vulocity of the jet relativeto the oltside environment,andestablished theorysuggests that the noise level variation with thisparameter should follow a power law with an indexof somewhere in the re don of 8. The theory isbacked up by small scole pure jet test evidencewhich allows the constructionof the experimentaljet line indicated. Clearly at lower jet veloc-

ities there is a significantdivergencefrom thislaw in the case of the full-sczdeengine results.For a long time it was felt that this divergencecould be accounted for by errors in the theory, butcloser examination of the results revealed otherdifferencesthat led tO d.cconclusionthat therewas one or more other noise sources contributineto he overall level at the lower velocities.

180

170

160

ISO ANGLE TO

ANLET A XIS

140

130

120

110

1002.5 2.6 27 2 8 2.9 30 3.1 3-2 33 3 4

Log ,. JET VELOCITY

FIGURE 3. VARIATIO: OFAMA.. OF RADIATIONOFREAR ARC PEAK flOISE LEVEL JET VELOCITY. FLLL SCALE E.LINE:,COMPARED WITH PURE AIRJETS.

One significantpiece of evidence arose fromconsiderationof the angle of radiation of the peaknoise. It was found that in plotting angle of peak radiation against jet velocity the pure airjet and full-scaleengine results diverged signif-icantly, and this is iflustratedin figure 3.Clearly at high jet velocitiesthe engines and theairjets agree in radiation angle, just as they doin peak intensity,but as the reduced volecitiescorrespondingto the area of divergence in peaklevel are approached,there commences a significantdivergence in peak angle. This is a gradualprocess, until the point is reached where theanticipated intensity of the jet noise is wellbelow that of the observedresults. Here thepeak angle of radiation from the full-scaleenginessettles to an alm8st constantdifference ofsomething like 40 from the pure air jet angle.

Another piece of evidence was that exposed byclose inspection of the spectra associated withactual engines at very low velocities. This revealed that there was a clearly defined discretetone content resultingfrom turbine blade inter-action, and thereforeit was concluded that theturbine was probably a major contributorto theoverall noise level. It was felt at the time of these early conclusionsthat the co:Llpressorwasnot a significantcontributorin the rear arc,since it was not possible to detect any compressordiscrete tones even with the aid of fairly narrowband analysis. Tests on later engines, which were subjected to !,orecritical analysis,sub-sequently showed this conclusion to be quiteincorrect, as will be discussed,due mainly to thefact that lone bypass duct and mixer system on alow bypass ratio engine causes an attenuationofdiscrete tones. The conclusion was presumablyright however in the case of a pure jet engine,where there is of course no bypass system to carryand radiate the downstreamcompressornoise.

From all the early work outlined and thesubsequent investigationsto date it is nowpossible to indicate what the major sources are ina typical low bypass ratio engine, and how thesevary with engine power setting. Such an indication i3 given in figure 4, where the linearPerceived Noise field shapes are plotted asfunctions of engine power setting.

130

FULL SCALE ENGINE .DIVERGENCE FROMPURE AIRJET LINE

28 2 9 3-0 31Log,. JET VELOCITY

120

PNdB-10 log ,08AASSFUJINI

LWE FROM HO1PURE AIRJETTESTS

100

3-2 33

0

UNEAR

PNdB

20

30

PERCENTAGE

POWER

40

. . .COMPRESSOR

INLET

60

50

40

30

20

TURBINE COMPRESSOR

EFFLUX

200 30 40 50 73 90 120 1500ANGLE TO INLET AXIS

FIGLRE 4. VARIkTION OF LINEAR NOISE FIELD SHAPES

WITH POWER SETTIKG: LOW BYPASS RRTIO ENGINE.

ocet" 80•

',;-; • •••

10

JET

V

100

PURE AIRJETS

'`O'n c: •••- • lit •h•XiX X

0 0 0 )0(

X.

TURBOJE TENGINES WI y X

XX >XX X

2

Forwardsof the eaginethecompressornoiseisseento be thedominantsource,whilstrearwardsof the enginethepeakis attributableto eitherthecombinedeffectof thecompressorand theturbineor to the jet,dependingupontheabsolutepowersettingof the engine. At thehigherpowersettings,in thiscaseabove60, the jet is ofcoursethedominantfeatureof practicallythewholenoisefield.

EXTRACTIONOF TURBINE NOISE AS A _UNIUU4SOURCE

If we considerthe natureof the low powernoiseat theangleof peak radiationin thereararc noisefieldof thelow bypassratioenginecnsideredin figure4, we see thetypeofspectrumillustratedin figure5.

0

6

Nig

- N

\,

SW =Se

1 (46-̀&

L114 ma

60 SOD 600 WOO

IIIIWWWv-OKLIS/WC.

FIGURp, 5. PEAKHEAR ARC SP,CTRUMAT 14,, POdERLOWBYPAS RAIO MIAED 1NLIN. 6;, BANDWIDTHkNALnIS.

Herejet noiseis very low butneverthelessis stilla contributorto portionsof the overallspectrum. Its contributionis quitereadilypredictablefrommodeljet evidenceand isrestrictedto freplenciesbelowapproximately600cycles/sec. Abovethisfrequencythe wholespectrumis a combinationof the turbinenoiseandthecompressornoisecarrieddownthebypassduct,

: bothradiatedat substantiallythesameangle,forreasonswhichwill be discuesedlater

It is thereforenot possibleto simplydeduceandextractthe separateturbineand compressorcomponentsfrom sucha combinedspectrum.

To extractthe individualcomponentsit isnecessaryto measurethemindividually,and todo so on an engineof thistype,whereboth thecompressorand gas generatorflowsaremixedbeforeemissionthroughthepropulsionnozzle,it is necessaryto carryoutmodificationstothe engine. This in factwas doneearlyin 1967on a Conwayengine,whichwas re-engineeredtoprovideseparatecompressorbypassand gasgeneratorflowexitnozzles. To accomplishthisit was necessaryto extendthe gas generatorjetpipeand providea separateannularexitfor thebypassflow. NoisemeasureiLentswere thentakenalternatelywiththe gasgeneratorflowcarriedto a detunerin one case,and thenwitc.thebypassflowdeflectedand screenedfrom the noisemeasuringareain theother. An illustrationofthe rig arrangementfor the firstof thesetestsis shownin figure6. Herethelargeopenairtestbed at Hucknallis seenwithitsmovabledetunerrolledup behindthe gas generatornozzle,and a screeningsystembuiltaround'theinletofthe engineto provideisolatedmeasurementson thebypassflow. Both thisconfiguration,and theconfigurationfor gas generatorflowmeasurementaloneis illustrateddiagrammaticallyin figure7.

FIGURE 7. SCHEMATICARRANGOIENTFOR ISOLATIONOF COMPRESSa AND TURBINE NOIS,1;SOURCESIN LOWBYPASS RATIO ENGINE.

COMPROZORDMCHARGEDEFLECTOR

•::1,11RBINE DISCHARGE NOISE RADIATES

CASECD

ACOUSTIC

SCREENS

CASE (ZCOMMOMCR JET MUFFLERNSCHARGENOM RADIATES

20

61.6

I.

30

-. o

SO

20000

.4.;4i;!4enr---.-•. ..•

FIGURE6. LOWBYFASS RATIO ENGINE INSTALLATIONFOR E.::Asultaliza OF ISCLTD JC:1-SOR COKPONENTIN REAR

ARC. INLET MOWN SCREINED AND GAS GENERATORSYSToMDETUNET. MEASURNIENTSTAKENON REXOTESIDS OFINSTALLATION.

3

The resultsof thesetests,whichwereextensiveand coveredthe:Tholeoperatingrangeof theengine,are typifiedby thesamplespectrashownin fieure8.

FIGURE8. ISOLATIONOF SPXTRAL CONTiNTOFCOMPRESSOREFFLUXIN LOWBYPASSRATIOENGINE.

Heretwo spectraareillustratedat a fairlylowenginepowersetting,wherejetnoiseis not aproblem,and expressthecharacterof thenoisefromthebasicenginewitha separatenozzlesystemand from theisolatedcompressordischargesystem.

00 100 100 30,00 6030 10000

300131311C3 - CYCLES /

FIGLRE9 (TOP).SPECTRALCHhRACTEROF COMPRESSORINLETNOISEAT SAMECONDITIONSAS TESTSRETERREDTO IN FIGURE8.FIGURE9 (BOTTOM).DEDUCEDTURBINENOISESPECTRALCHARACTER.

Consideringthe spectraindividually,thefirstpointis thatthebasicenginewithoutanymixersystemexhibitsa significantdiscretetonecontent,not onlyfromthe turbinebutalsofromthe oompressor. Thisjustifiestheconclusionthatthereis a significantcontributionfromthecompressorin theoveralllevelto therearof theengine,whereasearliertestswitha mixedenginehad suggestedthe oppositeconclusion. Thespectrumfrom the testswith thegasgeneratorsystemfed to a detunerrevealsjusthowmuch ofthispeakis in factcompressornoise,andbydeductionthe contributionfromtheturbine.

Sincethe rearwardspropagatedcompressornoiseis clearlya significantpartof thetotalturbomachinerynoisein thereararc,it is ofinterestto comparethespectrumobtainedto therearof the enginewiththeactualforwardradiatednoiseat thesameenginecondition. Thereforeaspectralanalysisof theforwardscompressornoiseis presentedin figure9 (top)and it is evidentthatthereis a reasonableresemblanceto thecorrespondingrearwardscompresorspectrumin thesignificant600 to 6000cycles/secondrange:Thereare expecteddifferencesin discretetonecontentdue to thefactthatthe earlystagesareheardforwardsand thelatterstagesrearwards,but therandomcontributionsare alike.

Acceptingthereforethatit is possibletoextract,if not physicallythenby examiningthevariousspectra,theareasattributableto turbinenoise,one arrivesat thetypeof spectrumillus-tratedin thelowerhalfof figure9. It canbeseenthatthisisquitesimilarin characterifnot in frequencycontentto thecompressorspectrumin the sameillustration,andit is quitelogicalto supposethatit consistsof thesamecomponentnoisesas thecompressorspectra. Thereforeinthe samewey thatcompressornoiseis brokendown,intodiscreteandvortexor "white"componentsk°)

(7) the turbinespectrumhasbeenshownas theresultof suchcomponentsin theillustration.Discussionof thegenerationof thesecomponentswillfollowin duecourse,but it is of interestto note herethaton a Strouhalnumberbasisthebladedimensionsand streamtemperaturessucgesta 4: 1 characteristicvortexnoisefrequencyseparationbetweenthe turbineand compressor.This is in verygoodagreementwiththecharacter-isticsof the spectraillustrated.

The spectrumdiscussedis but one typicalexamplefroma multi-stageturbine,and to try anddeducein the samemannerspectrafromotherturbines,and makesensibleconclusionsabouttheway in whichturbinenoiseis generated,isvirtuallyimpossibleif the onlyavailabledataisthatfromfullscalemultistageenginetests. Itwas thereforedecidedthatthe anlyway to pursueturbinenoiseresearchsatisfactorilywas tocommissiona specialfacility,so thatworkcouldbe carriedout on lesscomplexnoiseRoUrces. Thisinvolvesgoingto a minimumof a singlestageturbinewiththeappropriateguidevanesystems. It wouldprobablybe sensibleto goevenfurtherand workwithan isolatedrotororaerofoil,but at thispointin timeworkiscentredarounda representativesinglestagewiththesubsequentincrementaladditionof otherstages. The facilitybeingusedfor thisworkwillbe discussedin thenextsection.

---- 0

*WSW TOMS

'PEAK REAR ARC SPECTRUMI

COMPLETE ENGINE

axwmtm31, meau

COMPRESSOR DISCHARGEA

C033.0111603

FiAK REAR ARC SPEZ75M7

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,Jer NOISE 4\

t •

0 aOO 100 10 1000 30000

FRE013DWI, - CYCL16 / SIC

(P) Numbersin parenthesisreferto references

4 at the endof thepaper.

Alr Supply

"'111"1"1"1,1"4"..ti-Atz4-.4044141

MICROPHONETROLLEY

RESULTSFROMTURBINERIG

At thetimeof writingtheresultsavailableforopendiscussionarelimitedto thosefromasingleand a two stagecoldturbine. Thesewillbe discussedin greaterdetailin a latersectionbutat thispointit is of interestto considerthttypesof peak spectrumbeingobserved. The twospectrashownin figure12 wereobtainedfrom thesingleand two stageunitswiththecommonfirststagerunningat identicalconditions. Sincegreatcarewas takento avoidhighjetvelocitiesby theuse of a largefinalnozzlebehindtheturbine,theyare substantiallydevoidof jetmixingnoise. Thecharacterof thesespectraisverysimilarto thefullscaleenginespectruMdeducedearlier,exceptfor frequencydifferencescausedby the differentbladenumbers,the scaleof bladesusedand the differenttemperatures.Thereis as expecteda clearlydefinedvortexnoise

FIGURE10. DIAGRAMOF TURBINENOISERESEARCH levelwith therotorpassagediscretefrequenciesFACILITY. superimposed. Unfortunatelythereis verylittle

iifferencebetweenthebladenumberson the firstIndsecondstages,and thereforeseparatetones

NEW TURBINEFACILITY 'romthe two stagesare not apparent.

Figure10 indicatesthe layoutof the specialturbinenoisefacility. A microphonetraversingsystemfor automaticmeasurementsis centredonthetestunit,ehichis suppliedwithair fromanexternalsource. It has beendesignedto acceptnot onlyspecialturbinesbuiltfornoiseworkbutalsoas many of theavailableand futureaero-dynamicresearchunitsas possible. The nominaldiameterof the turbinesin questionis 15",andthefacilityis run eitherhot or cold. Theinitialpartof theprogramme,whichstartedlatelastyear,has beenconfinedto coldsystems,buttheadditionof a combustionchamberhasmade itpossibleto workon existingsmallengineturbines.

SINGLESTAGE

j000r fri'''Np

- VOW E X •LE VEL

ITWO STAGEL

GROUND RIPLINCTENILIMICTS

LEVEL

xm mm moo moo vxmoPotougNa-amisimr.

Figure11 is a photographof theactualsite.The turbineand microphonetraversingtrackisShowninsidea "rosebowl"screeningsystem,whichhasbeen erectedfor tworeasons. One is toavoidthe reflectionof turbinenoisefromadjacentbuildingsbackintothemeasuringarea,and theotherto reducewindeffeote.

FIGURE12. SAMPLESPECTRAFROMSINGLEAND 2 STAGECOLD TURBINES. 6% BANDWIDTHANALYSIS.

RADIATIONPATTERNS

Beforeconsideringthemechanismsand gener-ationof the turbinenoise,and discussingthewayin whichintensityvarieswith thechangingparam-etersduringoperation,onepointwellworthdiscussionis thequestionof how the noiseGener-ated at thebladesradiatesthroughthe jet to theoutsideatmosphere.

FIGURE11. PHOTOGRAPHOF TURBINENOISERESEARCH Figure13 showsseveralfUll scaleengineFACILITYSHOWINGTRAVERSETRACEAND SCREENING fieldehapevariationsfor the7rd octavecontain-SYSTEM. ing thefinalstageturbinefundamentaldiscrete

tone,andattentionahouldbe centreduponthecharacteristiopeak angleof radiation.

5

It is fairlyeasyto postulatetheway in whichanoisesourcegeneratedupstreamof a nozzleis propagatedthroughthe jet to theoutsideatmosphere,but it is by no meanscertainthatone canaccountabsolutelyfor themeasuredanglesof radiation.

70° so 90 100 110 120 130 wo 150° ANGLE 10 N•ILETAXIS

FIGUR613. LINEARFIELDSHAPESFORFULLSCALEENGINETURBINENOISE. /rd OCTAVZ CONTAININGFINALSTAGEFUNDAMENTALZONE.

FIGUR.;14. SIMPLIFIMILLUSTRATIONOF REFRACTIONAND RADIATIONOF NOISEORIGINATINGWITHINJETPOT,,NTIALCORE.

Figure14 ahowsdiagrammaticallyhow it is feltthenoiseis refractedand radiatedfromthepotentialcore to theoutsideenvironment,anditshouldof coursebe possibleto calculatea theor-eticalangleof refractionknowingthe governingvariables. Suchcalculationsdo nothoweverappearto agreesimplywiththeobservedanglestyrifiedby figure13. Thisis especiallytrueof thecaseof thecoldturbine,whereany refrac-tionfromthepotentialcorewillmainlybe afactorof thevelocitydistributionin thejet.Usingsimplerefractiontheory,therefractedanglep is relatedto theincidentangleQ by:

WhereC1 and02 are theincidentandfinal

velocitiesof soundandV1 andV2 are the

jet and environmentalvelocities.

Thisrelationshiphasbeenusedto calculatetheangleof refractionfroma coldjetfor a rangeof incidentangles,and thecurvesarepresentedinfigure15. Alsoplottedin thisfigureare someof theactualresultsfromthecoldturbines,thepointsrepresentingtheanglewherethe1/12thoctavecontainingthetonefundamentalis seentopeak. Thisangleis generallyverycloseto theobservedwhitenoisepeakangle.

Clearlythe experimentalpointsdo not fallon any one singlelineof constantincidence,andthereforeit must be concludedthateithertherefractionsuppositionis wrong,or thattheeffectiveincidentanglewithinthejetis not aconstantvaluewith thevelocity. The secondsuppositionis probablythemoreviable,sincethediscretetonesare,likecompressortones,generat-ed as helicalwavesand theeffectiveincidentanglemustvarywithvaryingturbineconditions.In practicevery lowvelocitiesare not of anyreal interest,and thereforea considerationofthepointsabove200 or 300 feetper secondjetvelocityis probablysufficientat thisstage.0Here the incidentangleis apparentlyaround50 ,someof whichmay be accountedforby theconeangleof thejet core,but therestmustbe assoc-iatedat thistimewith thehelicalnatureof thewave fronts. No completeanalysisis attempted herebut thededucedincidentangleis appliedtothe "reallife"case,whereturbinesoperateattemperaturessubstantiallyaboveambient.

This is donein figure16,fora rangeof jettemperaturesup to 1000K. Plottedon herealsoare a numberof actualengineresultsforwhichtheaveragetemperatureis in theregionof 500 to600°K. The observedresultsare seento be infairlygoodagreementwiththe"theoretical"lineswhichhavebeencalculatedon thebasisof a 500incidentanglewithinthe jet.

SYMBOLS DENOTE3 SHAFT SPEEDSOF60,70 & 800/0 OFMAXIMUM.

SOUND WAVESREFRATEDFROM HOT HIGHVELOCITYCORE 10 STILL COOLAMBIENT ENVIRONMENT.

ANGLE OF RADIATIONDEPENDENTUPON VELOCrTYII TEMPERATUREOF JET

REGION OFSCATTER DUETOJET BOUNDARYTURBULENCE.

THEORETICAL FIELD SHAPE DISTR1BUTION

90° 100 110 120 BO 140 BO 160° ANGLE TO INLET AXIS

SOUNDPRESSURE

LEVEL

11•C RUNACTON

NW 0 • SW • •C.

GO

110. 40• 30. 0

COLD 140D4LTURS94 RIPATS

100 200 SOO 400 103 600 ,03 SOO 900

vtLoCrr• rr/sIC

secpCl

secQ +V1 -V2

C2 C2 FIGURE15. COMPARISONOF COLDTURBINEPEAKRADIATIONANGLEWITSTHEORETICALCURVES.

6

0 MO NM MM MM IMO MM WM MX WM E000

JET VELOCITY IT I SEC

FIGURE 16. COMPARISON OF FULL SCALE ENGINEPEAK RADIATION ANGLES WITH THEORETICALCURVES.

MECHANISMOF GENERATION

The two component forms of noise, discretetones and broad band vortex noise, are of acompletely different nature, and the mechanisms ofgenerationare different. They are howeverdirectly comparable with the correspondingmechan-ismsof generation of fan or compressornoise, themore significantof which are illustratedinfigure 17. The discrete tones result from thecyclic interception of the nozzle guide vane wakesby the rotor blades, whilst the vortex noise mayresult from random lift pressure fluctuationsonboth rotors and stators. The pressure fluctuat-ions are caused by wake and freestream turbulence,the latter resulting from both combustion systemmixing and also the burning process.

Factors contributing to the intensity ofgeneration are considerablein number, and havebeen fairly full?;outlined in an earlier paper oncompressornoise(7). The most important isundoubtedly the blade relative velocity, whilstturbulenceintensity, stream density, blade incid-ence, cascade solidity and number of stages allcontribute,with nozzle guide vane to rotor

FIGURE 17. SIMPLIFIED EXPLANATIONOF GETERATIVEMECHANISNS OF TURBINh VORTEX AND DISCRETE TONENOISE.

separationhaving an independentlyimportantbearing on discrete tone intensity. The relevantexpression for dipole noise developed from Light-hills Theory of Aerodynamic Sound is as follows:-

PA, = 50 loglioVral + 30 logio

— 22 )+ 10 loglo M p (

(v /v2)

- lo loglo(c/ ) (Va/ ) + K

( s)m( Vrel)

where PWL = vortex or "white" noise total acoustic power

Vrel = relative blade velocity

= mass flow

Va = mean axial velocity

0 = lift curve Slope

—2v / 2 = turbulenceintensity

V

a = local speed of sound

(C/s),= mean blade cascade solidity

and K = accumulated vortex noise constantembracing flow convection, turbulencegrowth and number of stages.

For a given family of turbines the factorsinvolving solidity and turbulenceintensityareprobably similar, and thereforeare assumedconstant within the context of this paper.Subsequent research is directed more at obtaininga general correlation of turbine noise, which mustof course embrace all the factors influentialingeneration, but an attempt is made in the followingsection to correlate currently available resultson simplifiedbases.

CORRELATION

At the outset it must be admitted thatcorrelation of turbine noise, and in particularthe vortex component, is probably a differentprocedure than correlating fan and compressornoise. The way in which turbines work, forinstance the high pressure system and low pressuresystem having significantlydifferent working linesand frequently being on separate shafts, and thefact that almost all the noise must be lq-cpagatedin the downstream direction,makes the deductionof the exact contributionsto the overall levelextrebely difficult. Moreover the opinion hasbeen expressed that the early stages of turbinemay only be acting as turbulence producers, andperhaps not direct contributorsto the overallradiated noise level, and if this is the case itis not merely a question of how much noise isproduced by each stage, but primarily of whichstages produce noise.

Faced with these difficultiesit is perhapssensible to look firstly at some of the experim-ental evidence in the context of "correlationsattempted to date" and then consider if it ispossible at this stage to derive some overall formof analysis.

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CYCLIC PASSAGE OF ROTORBLADES THROUGH YAMSCREATED BY UPSTREAMNOZZLEGLADEVANES PRODUCES DISCRETE TONE ANDHARMONCS AT BLADE PASSING FREGLIOCIES

TURBULENCE IN STREAM FROM COMBUSTIONSYSTEMANDUPSTREAM BLADING WAKESCAUSES RANDOM LIFT PRESSUREFLUCTUATIONS ON BLADES WITH RESULTANT DIPOLE NOISERADIATICN

w

70

00

7

(a) Fullscaleeagineturbines

Usingtheresultsof theseparatenozzletestson a low bypassratioenginethatwereout-linedearlieras a basisfor theextractionofturbinenoisefromthepeakreararc spectraofotherengines,two simplecorrelationshavebeenmade. Therereferfirstlyto thepeak1/12thoctavevortexnoiseleveland secondlyto thefinalturbinestagefundamentaldiscretetonelevel. Thesearepresentedas figures18 and 19.The basisfor boththecorrelationsis turbineoutlettip speed,thatis to say theactualbladeabsolutetip speed,althoughsincethenoiseproducedshouldbe a functionof relativevelocitythisis not the correctfundamentalparameter.Howeverit is a sufficientlygoodbasosas to showthat,afternormalisationof all theresultsbyremovalof mass flow,thereis somesortofrelationshipwithtipspeedin bothcases.

252.6 2-7 2-8 2•9 3-0Log ,0 TURBINE OUTLET TIP SPE ED

FIGURE18. CORRELATIONOF LINEARPEAKVORTEXNOISELEVEL. FULLSCALEENGINES.

FIGURE19. CORRELATIONOF FINALSTAGEFUNDAMENTALDISCRETETONSLSVELS. FULLSCALEENGINES.

For discretetonesit is a straightlinerelation-ship,but in thecaseof thevortexnoisethereisa resultingcurve. A possibleexplanationforthisis thattheearlystages,andin thecaseofthebypassenginestheseare of courseon adifferentshaftfromthelaterstages,obeydifferentbladevelocityversusenginespeedrel-ationships. Thehighpressureturbinestendtoworkmorenearlyat a constantMachNo. thanthelowpressureturbinesand thereforemightwellbe3xpectedto havea lowerslopeof noiseversustipspeed. Thislowslopecouldwellbe makingitselffeltat thelowerturbinespeeds,wherethelow pressureturbinenoisewillhavefallento agreaterdegreethanthehighpressurenoise.

(b) Modelturbines(cold)

The availableworkto dateon modelturbinesis limitedto coldtestson a singleand twostageunit. The twostageunitconsistsof a secondstageaddedto thesinglestageunit,so it ispossibleto comparetheresultsfromthe twoturbinesdirectlyby choosingcomparableoperatingconditionsof thecommonstage.

The methodof testingon both turbinesfolloweda procedurewherebya seriesof constantnon-dimensionalspeedswereheldfora varietyofpressureratios. In thiswayoperatingconditionswellremovedfromthenormaloperatingconditionsof a fullscaleturbinewereobtained,and therebylargevariationsof bladerelativevelocityandincidencerecorded.

Neglectingthesewidevariationsof incidencefor themoment,thesameprocedureadoptedforcompressorcorrelationscan be used for thesinglestageunit. Thatis to saya correlationof thepeakvortexnoiselevelcanbe carriedoutusingsomemeasureof bladerelativevelocityas abasicparameter.Thisis attemptedin figure20,wherethemaximumrotorinletrelativevelocityisusedand the peakvortexlevelsarenormalisedbyremovalof mass flow. Whilstthescatterof theresultsmightwellemergeasan effectof bladeincidence,as it didin compressorcorrelations,itis interestingto noteat thisstagethatthemeanlinedrawnhas a slopeof aboutV3. Thisis almostidenticalwith theoriginalvalueobservedforcompressorvortexnoisek7)-

9

2

PEAKVORTEXNOISE0

511.

22 20 27 22 29 20

I-OG.(2070R MIAT Vii,

FIG.20. CORRcIATIONOF zINGLESTAGECOLDTURBINEPEAKVORTEXNOISE.

0

• 030

BWASS10dB

ENGINE0

PUREJET

2124272429LOG„, TURBINE OUTLET TIP SPEED

8

Whipid the theoreticaldipole index would beV6, or V"'where mass flow is removed as variable,some clear link is nevertheless probable betweenthe generation of compressorand turbinevortexnoise on the basis of the test results.

Moving to the results from the two stage unit,it is possible to plot these on exactly the samebasis as the single stage data. As a firstpresentationtherefore the same maximum rotor inletrelative velocity (first stage) has been used infigure 21.

LOGE esTROTOR INLET V

FIGURE 21. CORRELATION OF SINGLE AND 2 STAGEVORTEX NOISE USING FIRST ROTOR STAGE VELOCITIESAS BASIC PARAMETER.

The two sets of results fall on almostparallel lines and, as might be expected, the twostage levels are higher than the single stagelevels, by in fact about 3 or 4 dB. Thisobservationimmediately suggests a 10 logio (N)

relationship,where N is the number of stages.Unfortunatelyno 3, 4 or more stage results areavailable yet to verify this approach, but sincethe single stage turbine is working in fairlyideal conditionswith no combustion or upstreamstage turbulence,the problem of subsequentstagesis undoubtedlymore complex. If, as has beenpostulated already, the last stage is the governingfactor, and the earlier stages only act as turb-ulence producers, then the levels should be con-sidered on the basis of the final stage operatingconditions. If we choose therefore the finalstage maximum relative velocity as an alternativecorrelatingparameter a different picture emerges.This is shown in figure 22.

The two resulting lines are still virtuallyparallel, but have a significantdifferenceinabsolute level of about 12 or 13 dB. Thisdifference itself is extremely interesting. Asalready mentioned the single stage unit is workingunder almost ideal conditions. The nozzle guidevanes are fed with clean, smooth unburnt air froma large settling chamber, whereas the second stageof the two stage unit must be receiving fairly

2 2 2.3 2 4 2.5 26 2.7 2-8 2.9 3-0

LOG,. (FINA L ROTOR INLET VREL (u.0)

FIGURE 22. CORRELATION OF SINGLE AND 2 STAGEVORTEX NOISE USING FINAL STAGE VELOCITIES AS BASICPARAMETER.

turbulent air as a result of traversing the firststage. Compressor work and small scale aerofoilwork has led to the conclusion that the differencein the basic generative mechanisms involved inturbulent and turbulent free conditions is of theorder of 10 to 15 dB, the turbulent conditionsproducing the more intense noise. Clearly, ifit is true that a significantdifference inturbulence exists between the first and secondstages, the order of difference observed in thetest results is in very good agreement with thatanticipated.

Taking these results a stage further, thenext logical step is to compare on a common basisthe cold model results with fullscale hot engineturbine results.

(c) Modeland fullscaleturbines

The essential differencesbetween the modeland fullscale turbine tests lie in the region ofoperating temperaturesand blade velocities. Theuse of blade relative velocity as the primecorrelating parameter caters for changes in thisfunction, but a correctionmust be made fortemperature. Another difference between the testresults lies in the mode of measurement. In thecase of the model results polar traversing wascarried out at about 40 ft but in the case of theengine results traversing was parallel to theengines at 100 ft. The correlation presentedin figure 23 therefore expresses all the resultson a common basis, temperature having beencorrected to ambient conditions in the case ofthe full scale engines and the distance havingbeen extrapolated to 100 ft in the case of themodel results.

2-5 2.6 2 7 2-8 2-9 30

PEAK VORTEX NCMSE

9

LOG. (FINAL ROTOR INLET V irm. 0.))

FIGURE 23. CORREIITIONOF MODEL AND FULLSCALEVORTEX NOISE USING FINAL STAGE VEICCITIESASBASIC PARAMETER.

The degree of agreement between the 2 stagemodel and the 3 and 4 stage fullscale turbineresults is surprisinglygood for a first approx-imation. The increase over and above the twostage results for adding subsequentstages andgoing to hot conditions appears to be et a sensibleorder of magnitude. The three stage turbineslie at the bottom of the scatter band of fullscaleresults, and the overall scatter is no more than5 dB, the mean lying about 5 dB above the twostage results. This is very nearly a 10 log10(N) relationshipwith number of stages.

Moving briefly to the topic of discretetones, a similar correlationprocedure has beenadopted for the finalstage turbine fundamentaltones. This is shown in figure 24, where thenormalisingparameter other than mass flow andtemperatureis nozzle guide vane/rotor separation.This has been taken as (L/C)2 since this is theonly available parameter for use at this time,and results from work on compressorsand fans.S is the separation between the nozzle guide vaneand rotor, C being the guide vane axial chord.

Here again, as is the case in the vortexcorrelation,the agreement between model andfullscale is seen to be fairly good, There israther more scatter than in the case of vortexnoise levels, but this has always been the casein the context of discrete tone measurement,anda number of possible reasons have been discussedelsewhere.

RELATIVEPOLMITME

- S LEVEL (dS)

- 10 LOGMASS FLOW

- • 20 LOG SIC

- JO LOG el%)

2•.4 2.5 211 2.7 2.1 2.1 3-0

LOG,. FINAL STAGE

FIGURE 24. CORRELATIONOF FINAL ROTOR FUNDAMENTALTONE. MODEL AND FULLSCALE RESULTS.

To sum up this section,it is clear that alot more work has to be done to resolve thedifferencesbetween model and fullscaleresults,but this must be preceded by a fuller understandingof the basic mechanisms of generation and radiation.The correlationspresentedshow that there is goodevidence to suggest that a fairly logical overallpicture Should emerge.

SIGNIFICANCEOF TURBINENOISEIN OVERALL ENGINENOISEAND THE HEEDFOR SUPPRESSION

Using the correlationsderived in the previoussection together with existing or developed correl-ations of jet and compressorand fan noise it ispossible to generalise on the significanceof eachnoise source in overall engine noise levels.

For engines designed to produce the sameoverall thrust, an increase in bypass ratio causesthe jet component to fall in intensity due to thelowering of jet velocity, and the compressororfan noise to increase due to increasingwork.

1 1 1 1

0 I 2 3 4 5 6 7BYPASS RATIO

FIGURE 25. VARIATION OF COMPONENT NOISE SOURCESAS BYPASS RATIO IS VARIED AT CONSTANT THRUST.

1104 1/4\ ft';9 4:, • NI,

7 -sfika FANStf10 TURBINE

PERCMVEDNOMELEVEL

f10

PNL

1

/41

OVERALLTOTAL LEVEL

10

the jetnoiseis of littlesignificancethe turbinenoisemustbe affordedthesameattentionas thefannoise. Thisis particularlytrueiL tlecontextof furthersuppression,sinceevena smallreductionin fan noisewillrenderfurthersuppressionlessand lesseffective. It is thereforeclearthatresearchintofan noisesuppressionmust be accom-paniedby equivalentactivityin the turbinearea,otherwisein a very shortspaceof timemanufactur-ers willbe facedwitha suddenand possiblyunexpectedproblem. Thismay wellmean thatdelayswillbe encounteredwhenapplicationismadefor the typeof noisecertificatenow beingconsideredin theUnitedKingdom,UnitedStatesand France. Thiswouldhaveseriousrepercussionsin the saleand operationof thenextgenerationof aircraft.

PrECOVIDCOO

• • 1.

20 TO TO 80 T.BO TO OD 0 CO20 AO TO•20TO 00KOODTOOK OT IOWA. ATINLAITATIORAT - TOOOCI. ALT GOAD T0 OWE TONI,

FIGURE26. VARIATIONOF RE&RiARDSPROPAGATEDNOISESOURCESWITHENGINEPOWERSETTING. FOURENGINETYPESOF DIFFERENTDESIGHPHILOSOPHYANDBYPASSRATIOSCALEDTO SAMETbRUST.

The turbinenoisewouldappearto reuainsub-stantiallyconstantsincetheunit,althoughincreasingin workoutput,reducesin massflow.A generalcurve,figure25, illustratesthesechangesand showstherelativeimportanceof eachsourcein theoveralllevelat enginedesignpoint.Frombypassratiozeroto around14-the jetnoiseis dominant,whilstthe compressoror fan noiseassumesprecedenceat higherbypassratios.Thereis a discontinuityin thecompressornoisecurvewheredesigncapabilityalloystheuse of asinglestagefan (11). The turbinenoise,in remainingsubstantiallyconstant,beginsto assumerealimportancebeyondthe pointwherethecom-pressornoisediscontinuityoccurs.

This,however,is onlyhalfthestory. Forpowersettingswellbelowdesignpoint,i.e.thoseusedduringnoiseabatementthrottlingaftertake-offand thoseusedduringapproachto land,theturbinenoiseassumesmuchmoreimportance.Figure16 illustratesthisfact,plottingin foursectionsthevariationsof intensityof therearwardspropagatingsourcesfor threeexistingin-serviceenginetypesand a highbypassratioenginetype. For thepurejetenginethe jetnoiseis all important,exceptat verylowpowerswherethe turbinenoiseis a contributoryfactorin theoveralllevel. For themixedbypassengineof up to bypassratio1, thejet is againmostsignificant,but onlyat halfpoweror more.Belowhalfpowerthecombinedeffectof copressorbypassand turbinenoiseis verysignificant.For the turbofan,of shortcowlor coplanarexitdesign,and of bypassratiobetween1 and 2, thefannoiseis dominantat almostall powersettings.Becauseof the extremeintensityof fan noisetheturbinenoise,likethejet noise,is relativelyinsignificant.

However,turningto theadvancedtechnologyengineof bypassratioof 3 or more,wherethereis a significantreductionin fannoisedue to theuse of a singlestage,the turbinenoisebecomesof importance. Over thewholepowerrangyit isonlymarginallylessthanthe fannoise,and since

COI:CLUSICNS

Turbinenoisehasbeen exposedas a signif-icantnoisesourcein the gas turbineengine.Its realimportancein thecontextof thehighbypassratioenginehasbeen indicated.

The compositionof turbinenoisehas beenshownto be verysimilarto fan and compressornoise. The two significantforms,discreteand vortex,havebeendefinedand are expectedto vary basicallywithbladerelativevelocity.

Correlationsof bothcomponentshavebeenattemptedon the basisof bladerelativevelocityand a reasonablysensiblerelation-shipbetweencoldmodeland fullscalehotengineturbinelevelshas beenestablished.

The needfor suppressionof turbinenoisehas beenexpressedfroma considerationoftherelativelevelsof fan and turbinenoiseon thehighbypassratioengine. It isimperativethatturbinenoisesuppressionresearchproceedsalongsidethecurrentprogrammeson fannoisesuppression.

ACKNOWLEDGEMENTS

The authorwouldliketo expressthankstohisChiefResearchEngineerfor encouragingthepublicationof thispaper,and to his teamofResearchEngineersat theRolls-RoyceTestEstablishmentat Hudknall.

BIBLIOGRAPHY

F. B. Greatrex,"JetNoise". 5th Internation-al AeronauticalConference,June 1955.

F. B. GreatrexandD. M. Brown,"Progressin Jet EngineNoiseReuction". 1st I.C.A.S.Congress,Madrid,September1958.

i. B. Creatrex,"NoiseSuppressorsfor Avonand ConwayEngines". A.S.M.E.Conference,Los Angeles1959.

F. B. Greatrex,"BypassEngineNoise"Publication162C,1960.

G. M. Coles,"EstimatingJet Noise".R.Ae.Soc.-.!uarterly,Vol.XIV,February1963.

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S. L. Bragg andR. Bridge,"NoisefromTurbojetCompressors".R.Ae.Soc.JournalVol.68 No.637.January1964.

N. J. T. SmithandH.E.House,"InternallyGeneratedNoisefromGasTurbineEngines,MeasurementandPrediction". A.S.M.E.Conference,Zurich,March1966,paperNo.66-GT/N-43(subsequentlyA.S.M.E.Journalof EngineeringforPower,April1967).

F. B. Greatrex,"TheEconomicsofAircraftNoiseSuppression"I.C.A.S.paperNo.66-5R.Ae.Soc.and 5thI.C.A.S.CongressLondonSept.1966.

F. B. GreatrexandR. Bridge,"TheEvolutionof theEngineNoiseProblem".InternationalNoiseConference,LondonNov.1966.

M. J. T. SmithandM. E. House,"OntheAnticipatedNoiseLevelsfromHighTipspeedCompressors. R.R. HKN.31andA.R.C. paper28.731PA 206.

G. L. WildeandD. J. Pickerell,"TheRollsRoyceThree-ShaftTurbofanEngine".CommercialAircraftDesignandOperationMeeting,A.I.A.A.June1967.

M. J. T. Smith,"TurbineNoise:Its signif-icancein theCivilAircraftNoiseProblem".A.S.M.E.Conference,Washington1968.

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