成果2021-06学习fluent燃烧

1、Advanced ANSYS CFD AeroacousticsLecture 3Acoustic Analogy MethodsAeroacoustics and CFD What Is the Problem?Magnitude of acoustic pressure fluctuations are very small compared to hydrodynamic pressure fluctuationsAcoustic radiation contains only tiny fraction of energy of the primary flowThe classica

2、l CFD methods are not suited to cope well with the needed level of accuracy Lets try to decouple acoustics from flow dynamicsSPL (dB)p rms (Pa)Intensity (W/m2)6002140200102Separate Acoustic & Hydrodynamic FluctuationsAcoustic pressure fluctuations in a turbulent flowFor a Ma = 0.9 jetD

3、ecay of acoustic pressure fluctuations is much slower than hydrodynamic pressure fluctuationsLighthills EquationWave equation derived from conservation of mass and Navier-Stokes equations (1)(2)with(Lighthills tensor)Lighthills Theory of Aerodynamic NoiseUniform medium, r0, p0, a0Heterogeneous wave

4、equation in turbulent source fieldUniform medium, r0, p0, a0Complex fluid process can be represented by equivalent acoustic sources (2Tij / (xi xj ) thought to be in uniform medium at rest.This is Lighthills analogy (1952)Acoustic Analogy ModelingOn the basis of Lighthills analogyNoise sources are a

5、ssumed in a uniform fluid at restAcoustic field at observer is described by wave equationAcoustic and hydrodynamic fields are decoupledA two step approach can be appliedSimulate transient flow field accurately using CFD to get the acoustic sources Propagate noise from sources to receiver by solving

6、wave equation analyticallyCFD domainAcoustic receiverWave equation Acoustic sourcesElementary Acoustic PropagationImpulse response to a point sourceSolution represents the spherical wave which arrives at the receiver at t + r/a0 and decays with 1/rAcoustic receiver at (x, t)Wave equationAcoustic sou

7、rce at (y, t)rwhereGreens function- receiver location- source location- receiver time- emission timewhereDistributed Acoustic SourcesConsidering a general acoustic source S(y,t), the radiated field is the convolution of this source with the Greens functionCompact sources approximationNo approximatio

8、n is used in ANSYS FLUENTReceiverSource distributionxyr0Lighthill SolutionConvolution of Greens function and Lighthills source term This solution represents the propagation of sound generated by fluid motion in a free spaceNo account is made for walls Curles EquationExtension of Lighthill to include

9、 the effects of walls (Curle 1955)Noise generated in the fluid volume (Quadrupole sources)Noise generated at the walls (Dipole sources)Walls are not movingFW-Hs EquationFfowcs Williams-Hawkings (FW-H) derived a generalized Lighthill equation valid for moving surfaces with where : ui - fluid velocity

10、 componentun - fluid velocity component normal to the surfacevi - surface velocity componentvn - surface velocity component normal to the surfaced - Dirac delta functionH - Heaviside functionandWallsMoving WallsIntegral FW-H FormulationThe solution contains surface integrals over source surfaces and

11、 a volume integral over the volume outside the source surfacesIn its most general formulation, source surfaces do not have to be walls, but can be porous or permeable surfacesMost general acoustic integral formulation, valid for moving bodies enclosed by moving permeable source surfacesLess sensitiv

12、e to proper placement of permeable source surfaces than other integral methods (e.g. Kirchhoff)Noise generated in the fluid volume (Quadrupole sources)Loading noise (Dipole sources)Thickness noise (Monopole sources)FW-H Formulation in ANSYS FLUENTMost general acoustic integral formulation1, valid fo

13、r moving bodies enclosed by permeable source surfacesRdVdS (y,t)VPermeable integration surface, f=0Body1Brentner, K.S., and Farassat, F., “An Analytical Comparison of the Acoustic Analogy and Kirchhoff Formulations for Moving Surfaces,” AIAA Journal, 36(8), 1998. Monopole (thickness) noise, , due to

14、 unsteady volume displacement of fluid, mass injectionDipole (loading) noise, , due to interaction of flow with moving bodies, external forcesQuadrupole (vortex) noise, , due to unsteady stresses, turbulenceFW-H Formulation in ANSYS FLUENTFor low Mach number flows: Monopoles and dipoles are dominant

15、 sources Quadrupoles tend to be negligible if dipoles or monopoles are presentQuadrupole (volume integral) term is dropped in FW-H implementation in ANSYS FLUENTFW-H Formulation in ANSYS FLUENTImpermeable (solid walls) and permeable (interior) integration surfaces permissibleStationary and rotating/

16、moving surfaces permissibleWith quadrupole term dropped for low Mach numbersSolid source surfacesQuadrupole noise contribution not accounted forPermeable (porous) source surfacesQuadrupole contribution inside the source surface is accounted for if spatial and temporal resolution is sufficient for th

17、e solver to resolve quadrupole noise directly and propagate associated acoustic signal to the permeable integration surfaceContribution from quadrupoles outside the source surface is ignoreddVdS (y,t)VBodyPermeable integration surfaceFW-H Convective EffectsFor the calculation of the aerodynamic soun

18、d caused by external flow around a body, the Convective Effects option must be enabled with the far-field fluid velocity vector to be additionally specified.Useful for situations such as flight tests (microphones mounted on an airplane) or wind tunnel measurements (microphones mounted within the win

19、d tunnel core flow region). FW-H Source CalculationThe spatial and temporal resolution of source data directly determines the fidelity of FW-H predictionsDo we need compressible flow simulations?Can we get away with pressible source data?Unsteady RANS or LES/DES calculation should be usedUnsteady RA

20、NS usually under-predicts SPL and filters high frequenciesDES can be used for detached flowsLES is used in all other casesAdvantages of the Analogy ApproachSmall acoustic fluctuations are separated from large hydrodynamic fluctuationsDisparate length scales at low Mach numbers not an issue any moreT

21、urbulence can possibly be calculated with even pressible NS equationsWave equation deals with long pressure waves analyticallyThe far-field (receiver) is considered only while solving the wave equationNo dispersion and dissipation losses during transmissionComputational domain doesnt have to include

22、 the receiverCFD solution is only necessary in the source region, allowing more accurate and less expensive simulationsCan focus CFD resources on the source regionLess sensitive to BCs, NRBCs not absolutely needed Limitations of the Analogy ApproachLimitations of all acoustic analogy methodsReflecti

23、on and scattering by external (outside source region) solid surfaces not accounted forInhomogeneities in acoustic medium ignoredDoesnt account for effects of sound on the flowNo acoustic coupling inside the flow, e.g. no vortex shedding triggered by acousticsDoesnt account for effects of the flow on

24、 propagating soundE. g. sound refraction by shear layers and boundary layer is ignoredIt general, FW-H analogy is not applicable to internal acoustics, e.g. noise propagation in ducts or wall-enclosed spacesFW-H Acoustics Model in ANSYS FLUENTTargeted applicationsExternal aerodynamic noiseAirframe n

25、oise, landing gear, high-lift devicesCavity noise, sideview mirrors, wipers, antennasJet noiseBlade noise, fan noiseAvailable for3D and 2D planar FW-H model not available for the axisymmetric solverRotationally periodic boundaries allowed All pressure-based and density-based solversFW-H Acoustics Mo

26、del in ANSYS FLUENTRead and computeReceiver signal usingFW-HGeneralFW-H UtilityFan Noise (Gutin)Transient LES, DES, URANSSolution (arbitrary surface motion)Steady RANS solution(rotating reference frame, SRF)Export source surface data(.index and .asd files)during time integration.(Optional quadrupole

27、 export)Calculate receiver signalsimultaneously, “on-the-fly,”using FW-H.Calculate receiver signalusing FW-HExternal CodeFW-H Solution Procedure (Source data export method)Obtain unsteady URANS/LES/DES solutionActivate FW-H acoustics modelSelect Export Acoustic Source Data optionSelect source surfac

28、es from available zones in Acoustic Sources panelSpecify write frequencyIntegrate solution in timeGenerates .index and .asd filesSave FLUENT caseRead FLUENT caseRead .index and .asd filesSpecify receiver locations in Acoustic Receivers panel Compute/write acoustic signalsPost-process acoustic dataSt

29、ep 1Step 2FW-H Acoustics Model InterfaceProblem Setup Models Acoustics Edit.Turn on Ffowcs-Williams & Hawkings under ModelSelect source data export or simultaneous FW-H calculation under OptionsNot mutually exclusive, can be selected togetherSet model parametersFar-field densityFar-field speed of so

30、undReference pressure for SPL calculationsDefine source surfaces under SourcesSpecify receiver locations under ReceiversFW-H Source Surface Selection Problem Setup Models Acoustics Define SourcesSelect all source surfaces under Source ZonesMultiple surfaces may be selected“On the fly” FW-H calculati

31、on requires consistent surface selectionIf source data is exported, redundant surfaces may be selectedAllows identification of contributions from different sourcesFor permeable (interior) source surfaces, FLUENT requires the specification of the inner cell zonewall_lgfwh1fwh2fwh3FW-H Source Surface

32、Selection (continued)Problem Setup Models Acoustics Define SourcesSpecify Write Frequency N for source data exportSource data does not necessarily need to be exported every time stepFlow (CFL) requirements usually more strict than acoustic requirementsFor example, if CFD requires t = 2.510-6 s fmax

33、= 1/ 2t = 200 kHzSpecify No. of Time Steps per File to cluster source data into source data (.asd) filesSpecifies time interval covered per fileConvenient FW-H processing of subsets of source dataSpecify Source Data Root Filename of source data filesHighest frequency produced by FW-H will be fFW-H =

34、 1/(2NtCFD)Keep a safety margin between fFW-H and desired peak frequencyFW-H Source Surface Selection (continued)Source surfaces need to enclose dominant sources and important scattering surfacesSolid (wall) source surfaces For compressible or pressible source data pressible source data for acoustic

35、ally compact source regions, i.e. LPermeable (interior, sliding mesh) source surfaces For compressible source dataAcoustic pressure field needs to be resolved accurately within the region enclosed by permeable source surfacesQuadrupole sources inside permeable source surfaces are accounted forSolid

36、(wall) source surfacePermeable (interior) source surfacesFW-H Receiver SpecificationProblem Setup Models Acoustics Define ReceiversSpecify number of receivers and receiver locationsEach receiver generates a signal (.ard) fileReceiver location can be inside or outside the CFD domainReceivers do not n

37、eed to be specified if only source data is extractedFW-H Save Source Data FilesSolution Run Calculation Calculate Advance flow solution.Automatically updates .index file and writes .asd filesKnown limitation (same as for all FLUENT monitors): No source data is written if user manually interrupts a t

38、ime step during which source data is supposed to be written FW-H Acoustic Signal CalculationSolution Run Calculation Acoustic Signals . Calculates FW-H integral from saved source dataLoad .index file under Load Index FileAutomatically updates the available source data listSelect the source data (.as

39、d files) to be processed under Source Data FilesCan use subset (in time) of source dataNo gaps allowedChoose source zones (i.e. source surfaces) to be used under Active Source ZonesCompute/Write calculates the acoustic signal for the selected sources and receivers and writes out the receiver (.ard)

40、filesFW-H PostprocessingResults Plots FFTGeneral purpose FFT tool in FLUENTResults Graphics and Animation Contours Surface dipole strength approximated by available under Surface dpdt RMS in Acoustics categorySurface post-processing quantity available subsequent to a FW-H calculation Receiver OASP93

41、.4 dB98.3 dB92.9 dB96.8 dB92.3 dBFW-H Signal Calculation Receiver signal is calculated forward in timeAll sources radiate at emission time tSignals from different sources arrive at the receiver at different times , depending on the source-receiver distanceTails of assembled receiver signal are autom

42、atically trimmed where signal is plete (pruning)Auto-pruning control in TUI (auto-pruning in on by default): /define/models/acoustics/auto-prune yes/no Raw receiver signalAuto-pruned signalFW-H Fan Noise in ANSYS FLUENTFW-H model available for Unsteady Fan NoiseGutin NoiseFan Noise Discrete + Broadb

43、andSteady Rotating Forces(Gutin Noise)DiscreteUnsteady Rotating ForcesDiscrete + BroadbandSteady flow, discrete Unsteady flow, discrete + broadband Secondary flow, discrete + broadband Vortex shedding, narrowband + broadband Turbulent BL, broadbandMonopoleBlade Thickness NoiseDiscreteDipoleBlade Loa

44、ding NoiseDiscrete + BroadbandQuadrupoleTurbulence NoiseBroadbandNeise chart for fan noise (1988)FW-H Unsteady Fan NoiseUsed for unsteady SRF or sliding mesh LES/DES/URANS calculationsCan handle rotationally periodic boundaries (model single blade only)Moving fan blades as source surfacesFor compres

45、sible simulations, can use interior or sliding interface zones as alternative source surfacesInclude duct/shroud surfaces to source zones if surfaces are acoustically importantExample Ventilation Fan4-bladed, 2000 rpm, BPF=133.3Hz LES with sliding meshFW-H Gutin Fan Noise Steady state Gutin noise mo

46、delFW-H available for steady state (RANS) SRF calculationsUse fan blades as source surfacesSource surfaces internally revolved by code as specified in Acoustics Model panelSpecify Number of Time Steps Per RevolutionSpecify Number of RevolutionsPostprocess receiver signals as in unsteady FW-H casesFor many fans Gutin noise component less importantr1r2ReceiverLC=?FW