2015山西教师招聘语言知识目标解读

©FluentInc.1/13/20201/45FluentUserServicesCenter©FluentInc.1/13/20202/45FluentUserServicesCenterAcousticsBackgroundBasicsSimulationApproachesAcousticsToolsinFLUENT6.2DirectCalculation(CAA)Fluent-SYSNOISECouplingFfowcsWilliams-HawkingsModelBroadbandNoiseModelsSummary©FluentInc.1/13/20203/45FluentUserServicesCenterSoundisadisturbanceoftheatmospherethathumanbeingscanhear.Theoscillatingvariationsinsoundpressure(calledtheWAVEFORMofthesound)PROPAGATEintheformofaSOUNDWAVE.AcousticsFundamentals:WhatissoundWavefrontWavelength©FluentInc.1/13/20204/45FluentUserServicesCenter=20kHzAudiblerange3000HzbesthearingInfrasoundUltrasoundGrowingWavelengthEarthquakes,heavytrafficMedicalimaging,dogwhistles.©FluentInc.1/13/20205/45FluentUserServicesCenter•ThetermmostoftenusedinmeasuringthemagnitudeofsoundisSOUNDPRESSURELEVEL(SPL).Itisarelativequantityinthatitistheratiobetweentheactualsoundpressurevariationandafixedreferencepressure.SPL=10*log(p/pref)2=20log(p/pref)•ThisreferencepressureisusuallythatoftheTHREASHOLDOFHEARING.©FluentInc.1/13/20206/45FluentUserServicesCenterThresholdofhearingchangeswiththefrequencyofthesound.Ingeneralpractice,thevalue2e-5Pascalingasesand1e-5PascalinliquidsareusedasThresholdofhearing.Thereferencelevelcorrespondtothethresholdofhearingforharmonicsoundatf=1kHz.©FluentInc.1/13/20207/45FluentUserServicesCenter(dB):02030405060708090100110120p(Pa):0.00010.0010.010.1110©FluentInc.1/13/20208/45FluentUserServicesCenter•Aero/hydro-acousticsreferstosoundproducedbyorintheprocessoffluidflow.ThefieldofAero/hydro-acousticscanbedividedintothreegroups.•Free-spaceproblems:Noisefromturbulence,jetnoise.•Free-spaceproblemswithsolidsurfaces:Wingsandrotors.•Boundedproblems:Ductedfans,Pipingsystems.©FluentInc.1/13/20209/45FluentUserServicesCenter©FluentInc.1/13/202010/45FluentUserServicesCenter–Whatdoweexpect?SourceInformationStrength,contributionfromdifferentsources,sourceclassificationPropagationAccuratetrackingofacousticwavesReceiverReceiversignal(acousticpressure)Spectra,shapeofspectra(broadband,tones)Sound,AcousticRadiationFlowAcousticMediumReceiverSource,PseudoSound©FluentInc.1/13/202011/45FluentUserServicesCenter=m(t)AeroacousticSourceClassificationpsurface=psurface(t)MonopoleOnesourceDipoleTwoMonopolesourcesQuadrupoleTwodipolesourcesUnsteadymassinjectionAcoustic~U3MPowerUnsteadyexternalforcesAcoustic~U3M3PowerUnsteadyshearstressesAcoustic~U3M5Powert=t(t)MonopoleanddipolesourcesdominantatlowMachnumbers©FluentInc.1/13/202012/45FluentUserServicesCenterSoundisasequenceofwavesofpressurewhichpropagatesthroughcompressiblemediasuchasairorwater.(Soundcanpropagatethroughsolidsaswell,butthereareadditionalmodesofpropagation).Duringtheirpropagation,wavescanbereflected,refracted,orattenuatedbythemedium.Allmediahavethreepropertieswhichaffectthebehaviourofsoundpropagation:Arelationshipbetweendensityandpressure.Thisrelationship,affectedbytemperature,determinesthespeedofsoundwithinthemedium:Themotionofthemediumitself,e.g.,wind.Independentofthemotionofsoundthroughthemedium,ifthemediumismoving,thesoundisfurthertransported.Theviscosityofthemedium.Thisdeterminestherateatwhichsoundisattenuated.Formanymedia,suchasairorwater,attenuationduetoviscosityisnegligible.Acousticpropagation343/0.6*(20)/vmsTms©FluentInc.1/13/202013/45FluentUserServicesCenter=planesDirectionofpropagationPlanewaves©FluentInc.1/13/202014/45FluentUserServicesCenter=spheresSphericalwaves©FluentI