Theeffectofcavitationonthenaturalfrequenciesofahyd

Proceedingsofthe8thInternationalSymposiumonCavitationCAV2012–SubmissionNo.95August13-16,2012,Singapore1TheeffectofcavitationonthenaturalfrequenciesofahydrofoilOscardelaTorreUPC-CDIFXavierEscalerUPC-CDIFEduardEgusquizaUPC-CDIFMatthieuDreyerLMH-EPFLMohamedFarhatLMH-EPFLSUMMARYTheobjectiveofthepresentpaperistoshowtheeffectofcavitationonthenaturalfrequenciesofaNACA0009hydrofoil.Theexistenceoflargeportionsofthehydrofoilsurfacecoveredbyvaporisexpectedtoreducetheaddedmasseffectsofthesurroundingflowingwater.Forthat,aspecificexcitationandmeasuringsystembasedonpiezoelectricpatcheshasbeendevelopedandvalidated.Withthisexperimentalsetup,thethreefirstnaturalfrequenciesofanaluminumNACA0009truncatedhydrofoilhavebeendeterminedinvariousconditions.Thehydrofoilhasbeentestedwithpartialcavitationandwithsupercavitationat14m/sandtwoincidenceanglesof1ºand2º.Analogoustestshavebeencarriedoutwiththehydrofoilinairandsurroundedbystillwater.Thecomparisonofalltheresultshasconfirmedthesignificantrolethatcavitationplaysinthemodalresponseofthehydrofoil.INTRODUCTIONThenaturalfrequenciesofastructuresubmergedinwaterexperienceasignificantdecreaseduetotheaddedmasseffect[1,2].Inthefieldofhydraulicmachinery,thefrequencyreductionaffectingrunners/impellersshouldbeestimatedaprioritoforeseeresonanceproblemsduringoperation.Forinstance,recentinvestigationsdemonstratethatthefrequencyreductionratiodependsonthemodeshape[3].However,noattentionhasbeengivenuptonowtothepossibleinfluenceoflargescalevapor/gascavitiesorpocketssurroundingthestructureundercavitatingflows.Onlythecasesinstillfluidofpartiallysubmergedbodieshavebeeninvestigated.Forinstance,Lindholmetal.carriedoutin1965themodalanalysisofsubmergedbeamsatvariousdepths[4].Therefore,inthepresentworktheeffectsofpartialcavitationandsupercavitationonthenaturalfrequenciesofahydrofoilhavebeenexperimentallyinvestigatedinahighspeedcavitationtunnel.EXPERIMENTALSETUPAseriesofmeasurementshavebeencarriedoutwithanaluminumNACA0009hydrofoilindifferentsituationsattheLMH-EPFLHighSpeedCavitationTunnel[5]showninFigure1inordertodetermineitsfirstnaturalfrequencies.Figure1:LMHHighspeedcavitationtunnelTheprofilehasachordof100mmandaspanof150mm.Itstrailingedgeistruncatedat90%ofthechordleavingatrailingedgethicknessof3.22mm.Thecurrentpaperstudiestheeffectonthefirstandsecondbendingnaturalfrequencies(f1andf3)andonthefirsttorsionnaturalfrequency(f2)previouslyidentifiedanddeterminedbyAusonietal.[6].Severalscenarioshavebeentestedtoclarifytherelativeimportanceofcertainvariablesonthenaturalfrequenciesofthehydrofoil.Inparticular,thedifferenthydrodynamicconditionsconsideredfortheexperimentsarethefollowingones:Hydrofoilsurroundedbyair.Hydrofoillowersurfacewettedwithstillwater.Hydrofoilcompletelysubmergedinstillwater.Hydrofoilcompletelysubmergedinflowingwaterandwithattachedpartialcavitation.Hydrofoilcompletelysubmergedinflowingwaterandsurroundedbysupercavitationmainlyonthesuctionside.Thetestswithcavitationconditionshavebeencarriedoutat14m/sandfortwoincidenceanglesof1°and2°.Aspecificset-upandprocedurehasbeendevelopedtocarryouttheexperimentalmodalanalysis.Inordertoexcitetheprofile2andmeasureitsresponse,anewexcitationsystemhasbeendesignedandprovedtobeabletoexcitethehydrofoilwhenmountedonthecavitationtunneltestsectionfullofwaterand,hence,withoutexternalaccess.Apreliminarywork,whichgavepromisingresults,wasdoneinlaboratoryconditionsonmetallicplatestoevaluatethesuitabilityofusingpiezoelectricpatches.Thepiezoelectrictechnologywaschosenduetoitscapabilitytobeusedonboardeitherasanexcitationsystemorasasensingdevice.Thispreviousworkpermittedtoselecttheadequatepiezoelectricpatchandvoltageinputcapableofgeneratingalargeenoughexcitationforcethatcouldprovokeasignificantresponse.Theamplitudeoftheresponsehadtobesufficientlyhightobemeasuredinsuchanoisyenvironmentasacavitationtunnelunderflowconditions.Ontheotherhand,thepatchusedasasensorshowedagoodmeasuringcharacteristicsanditwasproventobeasreliablyasanaccelerometer.Sincepiezoelectricdevicesarepresentinawidevarietyofconfigurations,flexiblepiezoelectricpatcheswereselectedduetotheiradaptabilitytocurvedsurfaces.Theywereintegratedonthehydrofoilsurfacewithoutalteringthehydrodynamicgeometryandthereforetheflowfieldwasnotaffectedwiththeiruse.Inparticular,twopiezoelectricpatches(61mmx35mmx0.5mm)wereembeddedontheuppersurfaceofthehydrofoil(Figure2).Theirmountingrequiredaprecisemechanizationandfixationwork.Specialattentionwasgiventotheiroptimallocationwhichdependsonthemodetobeexcitedandinvestigated.Inthecurrentwork,acompromisehadtobereachedtoexcitebendingandtorsionmodes.Duringtheteststhepatchclosertotheleadingedgewasusedasanexciterandthepatchclosertothetrailingedgeasasensor.Figure2:PiezoelectricpatchesmountedontheNACA0009profile.Inordertoexcitetheprofileasinusoidalchirpsignalwasfedtotheleadingedgepatch.Thischirpwaschosentosweepfrequencybandscontainingtheexpectednaturalfrequencies.Then,thedetectionoftheamplifiedresponseduetotheresonancespermittedtoidentifythenaturalfrequencies.InFigure3andexampleoftheexcitationsignal(onthet