Fluent噪声中文教程000_noise

1•FLUENT6.0SPL(dB):02030405060708090100110120p(Pa):0.00010.0010.010.11103=∫IdAAcousticPressure,p(t)4•SoundPressure•AcousticIntensity•SoundPowerPapppdBSPLrefrefrmsµ20,log10)(22==WWAdI)(−=•=⎟⎟⎠⎞⎜⎜⎝⎛=∫rr21200210,,log10mWIapIIIILrefrmsref−==⎟⎟⎠⎞⎜⎜⎝⎛=ρr5•Frequencyrange(20Hz~20,000Hz)ƒTemporalresolutionforacousticsisoftenordersofmagnitudegreaterthanwhatisneededforatypicalfluidmechanicssolution.•LengthScalesƒAcousticwavelengthsareoftenordersofmagnitudelongerthanturbulencescale•AcousticPressureMagnitudeƒMagnitudeoftheacousticpressureismuchlessthanthehydrodynamicpressure‹e.g.,SPL=80dB~0.2Pa,patm~0.1MPa6•RadiationtoFarFieldƒToradiatetheacousticpressuretothefarfieldrequiresgridresolutionouttotheboundaryoftheflowdomain.•Non-reflectingBoundaryConditionsƒMustbewrittentominimizenumericalreflections.•NonlineareffectsƒAthighSPL’s,nonlineareffectsmaydominaterequiringnumericsthatdonotgeneratespuriouswaves.7•Twostepprocedure(Lighthillanalogy)ƒSolvetheflowusingNSequationtocapturesoundsource(useLES)ƒSolvewaveequationwiththissourcetermtocapturepressurewaves(requireshighorderscheme,notrivial)8•AdvantagesofthetwostepprocedureƒSeparatelengthscales.NSequationdealsONLYwithshortturbulentlengthscale.WaveequationdealsONLYwithlongpressurewavelengthscale.ƒSmallacousticpressureamplitudeisseparatedwithlargehydrodynamicpressure.ƒFarfieldinONLYconsideredinsolvingwaveequation•Non-reflectingboundaryconditionisstillpresentwhensolvingthewaveequation•Forfarfieldacousticsfromcompactsource,Lighthill-Curlesolutionisusedinsteadofsolvingwaveequation9’sAcousticAnalogy•Lighthill’sAcousticAnalogyƒSoundisinducedbyfluidflowwithitsfluctuatingstressesactingonanacousticmediumƒSoundispropagatedinanacousticmediumatrest,accompaniedwithexternallyexertedfluctuatingsourcesfromtheflowstresses•Lighthill’sEquation-derivedfromN-Sequation()ijijjiijijjiapuuTTxxptpaτδρρ−−+=∂∂∂=′∇−∂′∂20222201whereTijistheLighthillStressTensor,whosevalueisprovidedbyFluent10•Includesolidsurfacesanddensityfluctuation()()()=−⎟⎟⎠⎞⎜⎜⎝⎛∇−∂∂fHata020222201ρρ()()()⎟⎟⎠⎞⎜⎜⎝⎛∂∂−−+−∂∂−jijijjjjjxHppVvvxσδρ0()()⎟⎟⎠⎞⎜⎜⎝⎛∂∂+−∂∂jjjjxHVVvt0ρρ()()fHTxxijji∂∂∂+2(Monopole)(Dipole)(Quadrupole)11’sFormulation•IncludeastationarySurface(Vi=0)()()()=−⎟⎟⎠⎞⎜⎜⎝⎛∇−∂∂fHata020222201ρρ()()⎟⎟⎠⎞⎜⎜⎝⎛∂∂−−+∂∂−jijijjijxHppvvxσδρ0()()fHTxxijji∂∂∂+2(Dipole)(Quadrupole)12•Lighthill-Curle’ssolutionforacousticpressureLighthill-Curle’ssolution()444444344444421444444344444421termdipole)(')',()(41)(termquadruple,'))((41),('2022320yyyyxdSttpRlyxadVtTtRyxyxatpsSiiiijVjjii∂∂−+′∂∂−−=∫∫ππThedipolecontributiondominatesoverthequadrupletermforsubsonicflows•Lighthill-Curle’ssolutionappliesforcompactsourceatfarfield13’sMethod•Lowsonusedthisformulationforarotatingsource.•Eveniftherearenounsteadyforcesontheblades,theforceisunsteadyrelativetotheobserver.(GutinNoise)()()()⎥⎦⎤⎢⎣⎡⎟⎟⎠⎞⎜⎜⎝⎛−∂∂−−=MrrFtMrrayxtxpiii114,0πr14•TheSearsfunctionprovidesadescriptionoftheunsteadyaerodynamicresponseofabodyduetoanunsteadyinflow.•ThiscanbecombinedwithLowson’sMethodtoexaminetheeffectsofunsteadyinflow(interactionnoise).()kCUWSLwπρ=()()()[]()()kiJkCkiJkJkS110+−=UnsteadyLiftSearsFunction15•Correlatetheflowparameterstonoiselevels.ƒe.g.,Lighthillshowedrelationsforacousticpower:‹QuadrupoleNoise~U8‹DipoleNoise~U6‹MonopoleNoise~U4ƒe.g,fansgenerallyfollow:Lw~N5D7ƒrelateturbulencelevelstoacousticpowerƒIncorporateproprietaryrelationshipsasUDF’s16’sMethodforRotatingSourcesGenerateRANSSolutionApplyFlowParameterstoCorrelations18’sMethod,UnsteadyAero,CorrelationsGenerateLESSolutionApplyLighthill-CurleSaveSolutionFromEachTimeStepSaveSolutionFromEachTimeStep19()t,xObserver(1000mmawayfromtheplate)()0,atttyxy−−=′′nyx−≡RPlatePlate)/(450smU=n30mm3mmY.Dai,S.E.Kim,FluentInc.20•Per