flueng-acoustics-modeling-fluent-聲學分析教程

1、Fluent Acoustics Modeling,Outline,Acoustics BackgroundBasicsSimulation ApproachesAcoustics Tools in FLUENT 6.2Direct Calculation (CAA)Fluent - SYSNOISE Coupling Ffowcs Williams-Hawkings ModelBroadband Noise Models

2、Summary,Sound is a disturbance of the atmosphere that human beings can hear. The oscillating variations in sound pressure (called the WAVEFORM of the sound) PROPAGATE in the form of a SOUND WAVE.,Acoustics Fundamental

3、s: What is sound,,,,Wave front,,,Wave length,Acoustics Fundamentals:Classification,,Growing frequency,20 Hertz,20 000 Hertz=20kHz,,,Audible range,3000 Hz best hearing,Infra sound,Ultra sound,Growing Wave length,,Earthq

4、uakes, heavy traffic,Medical imaging, dog whistles.,Acoustics Fundamentals: Sound Pressure Level,The term most often used in measuring the magnitude of sound is SOUND PRESSURE LEVEL (SPL). It is a relative quantity in th

5、at it is the ratio between the actual sound pressure variation and a fixed reference pressure.SPL = 10*log (p/pref)2 = 20 log (p/pref)This reference pressure is usually that of the THREASHOLD OF HEARING.,Acoustics

6、Fundamentals: Threshold of hearing,Threshold of hearing changes with the frequency of the sound. In general practice, the value 2e-5 Pascal in gases and 1e-5 Pascal in liquids are used as Threshold of hearing . The ref

7、erence level correspond to the threshold of hearing for harmonic sound at f = 1 kHz.,Acoustics Fundamentals: Noise Examples,,SPL (dB):0 20 30 40 50 60 70 80 90 100 110

8、 120,p (Pa): 0.0001 0.001 0.01 0.1 1 10,Aero/hydro-acoustics,Aero/hydro-acoustics refers to sound produced by or in the process of fluid flo

9、w. The field of Aero/hydro-acoustics can be divided into three groups.Free-space problems: Noise from turbulence, jet noise.Free-space problems with solid surfaces: Wings and rotors.Bounded problems: Ducted fans, Pip

10、ing systems.,Industry Applications,AutomotiveWind NoiseInterior NoiseFan NoiseMufflersCombustionRoad Noise,AerospaceAirframe NoiseFan NoiseJet NoiseCombustor NoiseCompressor NoiseTurbine NoiseBVI - Rotor Noi

11、seDuct AcousticsBL NoiseCavity NoiseSonic BoomArchitectural,HVACFan NoiseJet NoiseDuct AcousticsArchitectural,ChemicalCombustionBubble Noise,PowerTurbine NoiseCombustor Noise,ElectronicsFan NoiseDisk Drive

12、s,Aeroacoustics – What do we expect?,Source InformationStrength, contribution from different sources, source classificationPropagationAccurate tracking of acoustic wavesReceiverReceiver signal (acoustic pressure)

13、Spectra, shape of spectra (broadband, tones),,Sound, Acoustic Radiation,Flow,Acoustic Medium,,Receiver,Source,Pseudo Sound,,,,Aeroacoustic Source Classification,psurface = psurface(t),MonopoleOne source,DipoleTwo Monop

14、ole sources,QuadrupoleTwo dipole sources,Unsteady mass injectionAcoustic ~ U 3M Power,Unsteady external forcesAcoustic ~ U 3M 3Power,Unsteady shear stressesAcoustic ~ U 3M 5Power,t = t(t),Monopole and dipole s

15、ources dominant at low Mach numbers,Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes o

16、f propagation). During their propagation, waves can be reflected, refracted, or attenuated by the medium. All media have three properties which affect the behaviour of sound propagation: A relationship between density

17、and pressure. This relationship, affected by temperature, determines the speed of sound within the medium: The motion of the medium itself, e.g., wind. Independent of the motion of sound through the medium, if the me

18、dium is moving, the sound is further transported.The viscosity of the medium. This determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.,Acou

19、stic propagation,Acoustic propagation,,,,,,,,,,,,,,,,,,,,,,,,,,Surfaces of equal compression = planes,,Direction ofpropagation,,,,Plane waves,Acoustic propagation,,,,,,,,,,,,,Direction ofpropagation,,,,Surfaces of equ

20、al compression = spheres,Spherical waves,Aeroacoustics: Challenges,Acoustic radiation contains only tiny fraction of energy of primary flowMost unsteadiness in flow is ‘pseudo sound’Acoustic energy generated by Boei

21、ng 747 during take-off is not enough to boil an egg!Magnitude of acoustic pressure very small compared to hydrodynamic pressureSPL=80 dB, prms=0.2Pa, patm~ 105PaAeroacoustic problems are inherently unsteadyFrequency

22、range of interest is large, 20Hz – 20,000HzTemporal resolution for acoustics may be orders of magnitude larger than the interesting time scales in the flowSmall eddies need to be captured, requires spatial resolution,C

23、FD Approaches to Aeroacoustics,Direct calculation - Computational Aeroacoustics (CAA)Resolve the acoustic pressure fluctuations as part of the CFD solutionReceiver is located within the computational domainCouple CFD

24、with specialized acoustics codes, Boundary Element Methods (BEM), Hybrid zonal methodsAcoustic waves are not tracked with CFD solutionCan account for external scattering, reflectionAcoustic Analogy modelingUse CFD to

25、 calculate source fieldUse analytical solution to propagate sound from source to receiver locationSteady RANS based noise source modelingUse empirical correlations to estimate acoustic radiation based on mean flow sol

26、utions,,Decreasing computational effort,Direct Calculation (CAA),Extract all acoustic information directly from CFD solutionThe acoustic pressure fluctuations are directly monitored at the microphone locationRequires t

27、hat all acoustic wavelengths of interest are spatially sufficiently resolved in complete domain from source to receiver~ 10-15 grid points per wavelength for second-order schemeUnderresolved waves dissipate quicklyExp

28、ensive in 3D, practical only for nearfield (~ 10l) calculationsRequires nonreflective outflow boundary conditions, NRBCs Artificially reflected sound should not interfere with primary sound fieldSuffers from all known

29、 difficulties in computational acousticsWeak acoustic waves are easily ‘lost’ in hydrodynamic fluctuations in nearfield, double precision numerics is requiredRequires compressible unsteady flow solutionVery difficult

30、for low Mach number flows where flow is ‘frozen’, CFL constraints require very small time steps compared to flow evolutionRequires high order discretization schemes to propagate waves with fidelity into farfield,FLUENT

31、6.2 Coupling with LMS SYSNOISE,Fluent offers coupling with BEM/FEM acoustics tools through partnering with LMS SYSNOISESYSNOISE provides additional analysis tools to solve vibro- and aeroacoustics problems, such asSo

32、und radiation by vibrating structuresVibrations generated by soundSound reflection, scattering, transmissionFan noiseThe Boundary Element Method (BEM)is done in the frequency domain, and implies time-harmonic datai

33、s efficient for exterior problems, only boundaries need to be discretizedis beneficial over FW-H method when scattering and reflection from surfaces is important,,Based on a two step approachSimulate transient flow fie

34、ld accurately near sources using CFDPropagate noise from sources to receiver by solving wave equation analytically‘Analogy’ refers to the notion that the complex fluid process is represented by equivalent acous

35、tic sourcesSources are assumed in a uniform fluid at restAcoustic field at observer is described by wave equation,,Acoustic Analogy Modeling,Source Region,,,,,,,,,,,,,,,Navier-Stokes (NS) solution,Acoustic Receiver,,Wa

36、ve Equation,Acoustic Analogy Modeling,Advantages of the two step ‘a(chǎn)nalogy’ approachSeparate length scales, especially at low Mach numbersTurbulent length scales can be calculated with incompressible NS equationsWave e

37、quation deals with long pressure waves analyticallySmall acoustic fluctuations are separated from large hydrodynamic fluctuationsThe farfield (receiver) is considered only while solving the wave equationNo dispersion

38、and dissipation losses during transmissionComputational domain doesn’t have to include receiverCFD solution only necessary in source region, allowing more accurate and less expensive simulations. Can focus CFD resource

39、sLess sensitive to BCs, NRBC not absolutely needed CompromisesReflection and scattering by external (outside source region) solid surfaces not accounted forAcoustic analogy doesn’t account for effects of sound on flo

40、wOr effects of flow (shear) on propagating sound,Acoustic Analogy Modeling – FW-H,Ffowcs Williams-Hawkings (FW-H) acoustic analogyMost general acoustic integral formulation, valid for moving bodies enclosed by permeabl

41、e source surfacesThe quadrupole term can be dropped for low Mach numbersImplication is that the contribution from quadrupoles outside the integration surface are ignoredQuadrupole contributions inside a permeable inte

42、gration surface are properly accounted forImpermeable (solid walls) and permeable (interior) integration surfaces permissibleStationary and rotating/moving surfaces permissibleSource information from transient (LES, D

43、ES, URANS) CFD calculationsSpatial and temporal resolution of source data directly determines fidelity of FW-H prediction!,Farassat formulation 1A (Brentner, 1986; Farassat and Succi, 1983), di Francescantonio (1997),RA

44、NS Based Noise Source Modeling,Unsteady LES/DES is time consumingSteady RANS results contain some amount of useful information: mean velocity components, mean pressure, Reynolds stresses, turbulent kinetic energy, rate

45、of dissipationCan this information be used to shed some light on broadband (turbulence) noise?Semi-empirical approach - need approximations for unsteady motion, correlationsPotentially useful toScreen ‘noisier’ desig

46、nsIdentify the primary source of the noiseProvide ideas to reduce the noise,CFD Approaches to Aeroacoustics,Outline,Acoustics BackgroundBasicsSimulation ApproachesAcoustics Tools in FLUENT 6.2Direct CalculationFlu

47、ent - SYSNOISE Coupling Ffowcs Williams-Hawkings ModelBroadband Noise ModelsSummary,FLUENT 6.2 – Direct Calculation (CAA),Usable for nearfield problems where all wave lengths of interest can be resolved with ~ 15 gri

48、d pointsExamples:Buffeting (Open sunroof or side-window),Intake/exhaust manifolds,mufflers,HVAC ductsImpractical for large scale problems, such as aircraft noiseDirectly monitor static pressure at microphone locat

49、ion,FLUENT 6.2 – Direct Calculation (CAA),Features in Fluent 6.2 that benefits CAA:Non-iterative time advancement (NITA) PISOFractional-step methodNonreflecting unsteady boundary conditions (NRBC) for coupled solver

50、More accurate spatial discretization schemes 3rd-order MUSCLBounded central differences for LES in segregated solverLow diffusion 2nd-order scheme for LES in coupled solverLES modelingNew dynamic SGS modelsDynamic

51、Smagorinsky modelDynamic Kinetic-Energy Transport modelStochastic velocity inlet BCsTurbulence synthesizer, vortex method,FLUENT 6.2 – FW-H Model,Fluent implements the FW-H integral method in its general formStationa

52、ry and moving/rotating source surfaces permissibleSpecial steady state fan noise (Gutin noise) implementation for rotating bladesPermeable (interior, sliding interface) and impermeable (wall) source surfacesMultiple s

53、ource surfaces permissibleQuadrupole noise contribution outside permeable source surfaces neglected (i.e. no FW-H volume integral)Time-domain implementationForward-time formulation, allows for “on the fly” simultaneou

54、s noise calculationsGenerates time signal of acoustic pressure at multiple receivers simultaneouslyImplemented for segregated and coupled implicit solversCompressible or incompressible source data3D and 2D planar imp

55、lementationFW-H not available for axisymmetric solver,Targeted applicationsExternal aerodynamic noiseLanding gear, high-lift devices, …Cavity noise, sideview mirrors, ….Fan/rotor noiseDuct noiseJet noiseRequire

56、s high quality unsteady LES or DES source dataOnly resolved eddies, in time and space, can generate sound with the FW-H model,FLUENT 6.2 – FW-H Model,FW-H Solution Procedure,FW-H – 3D Landing Gear Noise,Landing gear s

57、cale model M=0.2, ReD=1.23x106Segregated solver, incompressibleLES, Smagorinsky, Cs=0.12nd-order in time, ?t=2.5x10-6 sBounded central-differencing in spaceMesh:5.3M cells173,000 surfaces tri-element

58、s, ?s=0.0135D6 prism layers, h1=1.6x10-3DRun time:3min 40s per time step, 4 nodes9950 time steps for flow to pass throughdomain (18D), ? 25 days run timeFW-H analysis:Source data extraction after one flow passSou

59、rce data sampled during 0.9 flow passes,FW-H – 3D Landing Gear Noise,Cells are clustered in wake and near landing gear,FW-H – 3D Landing Gear Noise,Surface pressure, cp,Vortical structures, (?2 - SjiSij)/2 = (u0/D)2,V

60、orticity iso-surface, ? = u0/D,FW-H – FLUENT 6.2 Interface,Define ? Models ? Acoustics…Select source data export or simultaneous FW-H calculationSet model parametersFar-field densityFar-field speed of soundSet refe

61、rence pressure for SPL calculation,FW-H – Source Surface Selection,Define ? Models ? Acoustics…? Sources…Select all source surfacesMultiple surfaces may be selected“on the fly” FW-H calculation requires consistent su

62、rface selectionIf source data is exported, redundant surfaces may be selectedAllows identification of contributions from different sourcesFor permeable (interior) surfaces, Fluent requires the specification of the ‘in

63、ner’ cell zone,wall_lg,fwh1,fwh2,fwh3,FW-H – Source Surface Selection,Define ? Models ? Acoustics…? Sources… (cont.)Select write frequency for source data exportSource data does not need to be exported every time st

64、epFlow (CFL) requirements usually more strict than acoustic requirementsLanding gear, ?t=2.5x10-6 s ? fmax= 1/ 2?t = 200 kHzSelect clustering of source data into source data (.asd) files Convenient FW-H proce

65、ssing of subsets of source dataSolve ? Iterate… Advance flow solution….Updates .index file and writes .asd files,FW-H – Receiver Specification,Define ? Models ? Acoustics…? Receivers… Specify receiver l

66、ocationsEach receiver generates a signal (.ard) fileReceiver location can be inside or outside the CFD domain,FW-H – Acoustic Signal Calculation,Solve ? Acoustic Signals… Calculates FW-H integral from saved source d

67、ataLoad .index fileautomatically updates the available source data listSelect the source data (.asd files) to be processedcan use subset (in time) of source dataChoose the source zones (i.e source surfaces) to be us

68、ed‘Compute/Write’ calculates FW-H integral for the selected receivers and writes out the receiver (.ard) files,p(t),p(t),cy,1,2,6,4,SPL receivers 1, 3,PSD(cy),receivers 2, 4,receivers 1, 3,Receiver OASP,5,SPL receivers