Measurement24(1998)
43–54
Vibrationmeasurementsonbladesofanavalpropellerrotatinginwaterwithtrackinglaservibrometer
P.Castellini,C.SantoliniDepartmentofMechanics,UniversityofAncona,viaBrecceBianche,I-60131Ancona,Italy
AbstractTheapplicationoflaserDopplervibrationmeasurementsisapowerfultechniqueformonitoringandanalysingmechanicalsystems.Frequentlyexperimentsinoperativeconditionsmaybeausefultoolforanalysingtherealbehaviourofanobject.However,inseveralapplicationswemayfindthattheobjectisrotatingduringitsworkandintheseconditionsthemeasurementwithtraditionaltechniquesbecomesdifficult.Forthesereasonsasystemforthetrackingofrotatingobjectshasbeendevelopedinordertoperformvibrationmeasurements.Inthepresentworktheapplicationofthetrackinglaserscanningvibrometeronanoperativeconditionproblemisshown.Atrackinglaserscanningvibrometerhasbeenusedformeasuringthevibrationofamapofpointsontheobjectsurfaceduringacompletecircularmotion.Theanalysedobjectwasamodelofanavalpropellerworkinginwater.Acomparisonhasbeenfoundwiththestaticanalysis,inwaterandair,ofthesamepropeller.Someresultsareshown.©1998ElsevierScienceLtd.Allrightsreserved.Keywords:Laservibrometry;Navalpropeller;Rotatingmachinery;Specklenoise
1.Introductionvibrationcanplayanimportantroleinpreventingfailuresduetofatigue.Theresolutionofthiskindofproblemisofsignificanteconomicimportance,Inmachinery,thecontinuoustrendistogeneratemorepowerperpoundofequipmentwithalowerlinkedtotheavailabilityandservicelifeofsuchequipment[1].consumption.Inrecentyears,theinterestofthemechanicalengineerhasbeenorientedtowardstheInmechanicalapplications,therotatingmachineryrepresentsalargepartofinterestindevelopmentofdeviceswithreducedacousticemis-sionsforlowacousticpollution.Inordertoobtainmechanicalsystems.Therotating‘‘disk’’isafun-damentalcomponentofmodernrotatingtheseresults,themachinesmustworkwithclosertolerancesathigherspeeds.machinery:adiskisagoodmodelofmanydifferentelementsofcomplexsystemssuchasAnotherimportanttaskfortheresearcheristoimprovethereliabilityofmachines.Thisstudymagneticandopticalcomputermemoryunits,automotivecomponents,propellers,fans,etc.Indeterminestheneedforcontrolofstaticanddynamicdeformationofmovingparts,i.e.analysisthecaseofnavalpropellers,theproblemsarealsocorrelatedtothebehaviourofwaterwherehighofthevibrationbehaviourofthemachineinoperativeconditions.Inanumberofcases,compo-variationofpressureisinduced.Cavitationisaveryimportantproblemaffectingthereliabilitynentfailuresarecausedprincipallybyvibration-relatedfatiguestresses.Monitoringofcomponentandacousticemissionofpropulsionsystemsin
0263-2241/98/$–seefrontmatter©1998ElsevierScienceLtd.Allrightsreserved.PII:S0263-2241(98)00044-X44P.Castellini,C.Santolini/Measurement24(1998)43–54civilandmilitaryships,wherecomfortandAnothertypeofvibrationisthatinducedbyclearance,i.e.areducedspacebetweenthehull‘‘stealth’’capabilitiesarerespectivelyrequested.Severalimportantsourcesofvibrationtransmit-andthepropeller.Thisproblem,veryevidentinhullswithheavydisplacement,isduetothegenera-tedtothehullareduetothepropulsionsystem.Thepropellerproducesanexcitationoftheboat,tionofpulsesbythewaterflow,whichisforcedtopassinareducedspace.Thesepulsesareprincipallybythepassageofeachbladeinfronttosternpostandbytransmission,throughshafttransmittedtothepoopofthehull.Traditionalcontactsensors,suchasaccelerome-andbearing,ofvibrationofblades.Dynamicandhydrodynamiceffects,i.e.duetosolidorliquidters,arenotabletoresolvethiskindofproblem.Infact,solidsensorsalterthefluid-dynamicfieldmasses,cangeneratevibrations.Aknowledgeofthepressurefieldonbladesaroundtheobjectandaresubjectedtocentrifugalloads.Themassofthesedevicescanchangethecouldallowustocalculatetheexcitationforcesonthepropulsionsystem,andthisshouldbeverybehaviourofanalysedbladesandtheirvolumetheflowaround.Theextractionofasignalfromaimportantinordertoanalysethevibrationalbehaviourofthepropeller.rotatingdevicecanbedifficult,andtheuseoftelemetryorothertechniquescanbenecessary.Iftheeffectsofsolidmasses(shaftandblades)canbescheduledandanalysedthroughanalyticalInordertostudytheaxialvibrationpatternonarotatingdisk,twonon-contactapproachescantechniques,thevibrationsinducedbythehydrody-namiceffectareverydifficulttopredict.Infact,beused,theLagrangianortheEulerianone,i.e.eitherwiththemeasurementpointfollowingthetheeffectsofthewaterarelinkedtotherealpositionofbladeswithrespecttothehullandmovingpointorwiththemeasurementpointsteadyandthesurfacemovinginfrontofit.Insmalldifferencesinprojectparameterscaninducelargeproblems.bothcases,itispossibletoextractmodalinforma-tiononthevibratingstructures.Hydrodynamicexcitationsourcesaremainlythevariationofwakeupstream,thecavitationandtheInReinhardetal.’swork[1],themeasurementisobtainedusingastationarylaserinterferometerturbulenceintheabaftofthepropeller.Thefirstisduetothenon-stationaryconditionofflowobservingdifferentpointsatdifferenttimes.Therotationalspeedcanbeabout7000rpm,butaaroundthepropeller.Itinduceslowfrequencyvibrationsandveryoftenatransientresponseofgoodfinishingofthesurfaceoftheanalysedobjectisrequestedtoavoidspeckleeffects.Acomplexthepropellerduetothepassageofacrestinthewake.dataanalysisinthepost-processingstepisneces-sarytoextractmodalinformation[2]fromtheseCavitationoccurswhenalargepressurevaria-tiononthebladesurfaceispresent,duetothemeasurementswhentheshapeoftherotatingdeviceiscomplex.Inanycaseitisnotpossibletohigh-speedwaterflow,determiningaseparationbetweenwaterandsteamorgasesdissolvedinextractthetimehistoryofthevibrationinasinglepointofthestructure.them.Theabruptvariationofpressure,duetotheMoreover,iflaserinterferometryisused,theEulerianapproachshowssomeproblems;the‘‘in-generationandcollapseofbubblecloudsintheproximityofthebladesurface,inducesaveryplane’’componentofthesurfacevelocitygeneratesaverytediousnoiseduetothelaserspeckle.Whenintensiveexcitationatthehighestfrequencies.Theseparationandattachmentofflowcausealargecomponentwaveletsofanincidentlaserbeamaredephasedonscatteringfromanopticallyroughvariationofloadatlowerfrequencies,too.Inparticular,whenthespeedofthehullisincreasing,target,aspecklepatternisgenerated.Whenthetargetmotioncauseschangesinthespecklepatternthepropellercavitationcanbecomeabigproblem.Vibrationsaretheturbulenceintheabaftoftheonthephotodetector,theresultingnoisecannotbedistinguishedfromgenuinevibrationanditpropeller,whichdetermineageometricaldistortionofthewake.Theexcitationobtainedisinarepresentsa‘‘pseudo-vibration’’[3].Thescatteredlightpatternduetotheinteractionbetweenthemediumrangeoffrequency.
