文本2021-06學(xué)習(xí)fluent燃燒14intro

1、Advanced ANSYS CFD AeroacousticsLecture 01-IntroductionCourse OutlineAcoustics BackgroundBasicsSimulation ApproachesAcoustics Tools in ANSYS CFD R14.5Direct Computational Aeroacoustics (CAA)Ffowcs Williams-Hawkings Model Broadband Noise ModelsFFT Signal PostprocessingCoupling with Acoustics CodesSum

2、marySound is a disturbance of the atmosphere that the human ear can hearAcoustics is the study of soundNoise is the unwanted soundSound and AcousticsSound (e.g. pressure) waves propagate through compressible media such as air or water (Sound can propagate through solids as well, but there are additi

3、onal modes of propagation)Sound PropagationWaves can be reflected, refracted, attenuated and transported (convected) by the medium The media properties affect the sound propagation:State equation (relationship between density and pressure and temperature) determines the speed of soundThe viscosity o

4、f the medium determines the attenuation. For many media, such as air or water, attenuation due to viscosity is negligible at the frequencies of interestWave EquationAcoustics deals with perturbations traveling at the speed of sound Wave equation governing the perturbations is derived from conservati

5、on of mass and Navier-Stokes equationsLighthill tensorfor ideal gasLighthill wave equation for density perturbation Speed of soundAcoustic PerturbationsRelationships for plane weak (isentropic) acoustic wavesIntensity (Power/Area) of acoustic waveIntensity Level (IL) dBCharacteristic Impedance Note:

6、 Increase of 3 dB is equivalent to doubling the power, or intensityReference pressure, pref , is Threshold of Hearing at 1kHzSound Pressure LevelSound Pressure Level (SPL) dBNote: Increase of 6 dB is equivalent to doubling prmsSPL (dB):0 20 30 40 50 60 70 80 90 100 110 120p (Pa) : 0.0001 0.001 0.01

7、0.1 1 10I (W/m2) : 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 Sound Pressure LevelFor multiple uncorrelated sources, decibel values cannot be added up directly Therefore, SPL=IL if the reference values above are usedAudible Frequency RangeGrowing frequency20 Hertz20kHzAudible range3000 Hz

8、best hearingInfra soundUltra soundGrowing Wave lengthEarthquakesMedical imaging, dog whistlesWeighted Sound Pressure LevelsThreshold of hearing changes with frequencyBest hearing around 3kHzLow frequency sound perceived to be less loud than high frequency soundWeighted sound pressure levels (dB(A),

9、dB(B), dB(C) to account for the ears non-uniform perception of sound pressure levels across the frequency spectrumAcoustics ClassificationAeroacoustics Sound generated aerodynamicallyFree-space problem, no solid surfaces: Sound generated from turbulence, jet noiseFree-space problem, with solid surfa

10、ces: Fan noise, airframe noise, rotor noise, boundary layer noise, cavity noiseInterior problem: Duct noise, mufflers, ducted fan noiseVibroacoustics - Sound generated structurallyDifficult when fluid and structure are closely coupledDoable in simple uncoupled casesUnderwater acoustics - Sound gener

11、ated hydrodynamically Sound reflection off water surface and seabed Sound propagation through thermocline regionsSonars Aeroacoustics What do we expect from CFD ?ReceiverReceiver signal (acoustic pressure), directivitySpectra (broadband, tones)Overall Sound Pressure Level (OASP)PropagationAccurate t

12、racking of acoustic wavesSource informationStrengthContribution from different sourcesSource classification Acoustic MediumReceiverSource,Pseudo SoundSound, Acoustic RadiationFlowAeroacoustic Source ClassificationMonopoleSimple sourceDipoleLimit of two monopole sourcesQuadrupoleLimit of two dipole s

13、ourcesUnsteady mass injectionAcoustic U 3M PowerUnsteady external forcesAcoustic U 3M 3PowerUnsteady shear stressesAcoustic U 3M 5PowerFlowFlowFlowScaling valid for acoustically compact sources, l LMonopole and dipole sources dominant at low Mach numberst=t(t)Challenges in AeroacousticsAeroacoustic

14、problems are inherently unsteadyNeed to be able to describe the unsteadiness preciselyAcoustic radiation contains only tiny fraction of energy of primary flowMost unsteadiness in flow is pseudo sound and doesnt radiateMagnitude of acoustic pressure very small compared to aerodynamic pressure SPL=80

15、dB, prms=0.2Pa, patm 105PaAcoustic energy generated by Boeing 747 during take-off is not enough to boil an eggRequires high fidelity simulationsChallenges in Aeroacoustics (continued)Frequency range of interest is large, 20Hz 20kHzTemporal resolution for acoustics may be orders of magnitude larger t

16、han the dynamically interesting time scales in the flowSmall eddies need to be captured, requires spatial resolution Length scale separation at low Mach numbersL=uT, l=cT l/L 1/M, i.e. acoustic wavelength es much larger than the associated eddy length scaleNote that the time scales are the same, i.e

17、. the acoustic period is the same as the time scale of the eddy motionFlow es frozen compared to acoustic fieldCFD Approaches to AeroacousticsDirect calculation - Computational Aeroacoustics (CAA)Resolve the acoustic pressure fluctuations as part of the CFD solutionCouple CFD with specialized acoust

18、ics codes, Boundary Element Methods (BEM), Hybrid zonal methodsAcoustic waves are not tracked with CFD solutionUse special acoustics codes to calculate the wave propagationAcoustic Analogy modelingUse CFD to calculate source fieldUse analytical solution to propagate sound from source to receiver locat