CFX13-03-计算域及边界条件设置

1、Chapter 3计算域及边界条件的设置CFX条件设置 DomainsDomains are regions of space in which the equations of fluid flow or heat transfer are solvedOnly the mesh components which are included in a domain are included in the simulatione.g. A simulation of a copper heating coil in water will require a fluid domain and a

2、solid domain.e.g. To account for rotational motion, the rotor is placed in a rotating domain.RotorStatorHow to Create a Domain (as shown earlier)Define Domain PropertiesRight-click on the domain and pick EditOr right-click on Flow Analysis 1 to insert a new domainWhen editing an item a new tab panel

3、 opens containing the properties. You can switch between open tabs.Sub-tabs contain various different propertiesComplete the required fields on each sub-tab to define the domainOptional fields are activated by enabling a check boxDomain CreationGeneral Options panel: Basic SettingsLocation: Only ass

4、emblies and 3D primitivesDomain Type: Fluid, Solid, or PorousCoordinate Frame: select coordinate frame from which all domain inputs will be referenced toNot to be confused with the reference frame, which can be stationary or rotatingThe default Coord 0 frame is usually usedFluids and Particles Defin

5、itions: select the participating materialsEx. 2: Preference= 100,000 PaDomain Creation Reference PressureGeneral Options panel: Domain ModelsReference Pressure Represents the absolute pressure datum from which all relative pressures are measuredPabs = Preference + PrelativePressures specified at bou

6、ndary and initial conditions are relative to the Reference PressureUsed to avoid problems with round-off errors which occur when the local pressure differences in a fluid are small compared to the absolute pressure levelPressurePressureEx. 1: Preference= 0 PaPrefPrel,max=100,001 PaPrel,min=99,999 Pa

7、Prel,max=1 PaPrel,min=-1 PaPrefDomain Creation - BuoyancyGeneral Options panel: BuoyancyWhen gravity acts on fluid regions with different densities a buoyancy force arisesWhen buoyancy is included, a source term is added to the momentum equations based on the difference between the fluid density and

8、 a reference densitySM,buoy=( ref)g ref is the reference density. This is just the datum from which all densities are evaluated. Fluid with density other than ref will have either a positive or negative buoyancy force applied.See below for more on the reference densityThe ( ref) term is evaluated di

9、fferently depending on your chosen fluid:Domain Creation - BuoyancyFull Buoyancy ModelEvaluates the density differences directlyUsed when modeling ideal gases, real fluids, or multicomponent fluidsA Reference Density is requiredUse an approximate value of the expected domain density Boussinesq Model

10、Used when modeling constant density fluidsBuoyancy is driven by temperature differences ( ref) = - ref (T Tref)A Reference Temperature is requiredUse an approximate value of the average expected domain temperatureDomain Creation - BuoyancyBuoyancy Ref. DensityThe Buoyancy Reference Density is used t

11、o avoid round-off errors by solving at an offset levelThe Reference Pressure is used to offset the operating pressure of the domain, while the Buoyancy Reference Density should be used to offset the hydrostatic pressure in the domainThe pressure solution is relative to rref g h, where h is relative

12、to the Reference LocationIf rref = the fluid density (r), then the solution becomes relative to the hydrostatic pressure, so when visualizing Pressure you only see the pressure that is driving the flowAbsolute Pressure always includes both the hydrostatic and reference pressuresPabs = Preference + P

13、relative + rref g hFor a non-buoyant flow a hydrostatic pressure does not existPressure and Buoyancy ExampleConsider the case of flow through a tankThe inlet is at 30 psi absoluteBuoyancy is included, therefore a hydrostatic pressure gradient existsThe outlet pressure will be approximately30 psi plu

14、s the hydrostatic pressure given by r g hThe flow field is driven by small dynamic pressure changesNOT by the large hydrostatic pressure or the large operating pressureTo accurately resolve the small dynamic pressure changes, we use the Reference Pressure to offset the operating pressure and the Buo

15、yancy Reference Density to offset the hydrostatic pressure30 psih30 psi + r ghGravity, gSmall pressure changes drive the flow field in the tankDomain CreationGeneral Options panel: Domain MotionYou can specify a domain that is rotating about an axisWhen a domain with a rotating frame is specified, t

16、he CFX-Solver computes the appropriate Coriolis and centrifugal momentum terms, and solves a rotating frame total energy equationMesh DeformationUsed for problems involving moving boundaries or moving subdomainsMesh motion could be imposed or arise as an implicit part of the solutionDomain TypesThe

17、additional domain tabs/settings depend on the Domain Type selectedDomain Type: Fluid ModelsHeat TransferSpecify whether a heat transfer model is used to predict the temperature throughout the flowDiscussed in Heat Transfer LectureTurbulence Specify whether a turbulence model is used to predict the e

18、ffects of turbulence in fluid flow Discussed in Turbulence LectureDomain Type: Fluid ModelsReaction or Combustion ModelsCFX includes combustion models to allow the simulation of flows in which combustion reactions occurAvailable only if Option = Material Definition on the Basic Settings tabNot cover

19、ed in detail in this courseDomain Type: Fluid ModelsRadiation ModelsFor simulations when thermal radiation is significantSee the Heat Transfer chapter for more details Domain Type: Solid ModelsSolid domains are used to model regions that contain no fluid or porous flow (for example, the walls of a h

20、eat exchanger)Heat Transfer (Conjugate Heat Transfer)Discussed in Heat Transfer LectureRadiation Only the Monte Carlo radiation model is available in solidsTheres no radiation in solid domains if it is opaque!Solid MotionUsed only when you need to account for advection of heat in the solid domainSol

21、id motion must be tangential to its surface everywhere (for example, an object being extruded or rotated) Tubular heat exchangerImages Courtesy of Babcock and Wilcox, USA Domain Type: Porous DomainsUsed to model flows where the geometry is too complex to resolve with a gridInstead of including the g

22、eometric details, their effects are accounted for numericallyDomain Type: Porous DomainsArea PorosityThe area porosity (the fraction of physical area that is available for the flow to go through) is assumed isotropicVolume PorosityThe local ratio of the volume of fluid to the total physical volume (

23、can vary spatially)By default, the velocity solved by the code is the superficial fluid velocity. In a porous region, the true fluid velocity of the fluid will be larger because of the flow volume reduction Superficial Velocity = Volume Porosity * True VelocityThis setting should be consistent with

24、the velocity used when the Loss Coefficients (next slide) were calculatedDomain Type: Porous DomainsLoss ModelIsotropic: Losses equal in all directionsDirectional Loss: For many applications, different losses are induced in the streamwise and transverse directions. (Examples: Honeycombs and Porous p

25、lates)Losses are applied using Darcys LawPermeability and Loss CoefficientsLinear and Quadratic Resistance CoefficientsMaterialsCreate a name for the fluid to be usedSelect the material to be used in the domainCurrently loaded materials are available in the drop down listAdditional Materials are ava

26、ilable by clickingMaterialsA Material can be created/edited by right clicking “Materials” in the Outline TreeMulticomponent/Multiphase FlowANSYS CFX has the capability to model fluid mixtures (multicomponent) and multiple phases Multicomponent (more details on next slide) One flow field for the mixt

27、ure Variations in the mixture accounted for by variable mass fractions Applicable when components are mixed at the molecular level Multiphase Each fluid may possess its own flow field (not available in “CFD-Flo” product) or all fluids may share a common flow fieldApplicable when fluids are mixed on

28、a macroscopic scale, with a discernible interface between the fluids. Creating multiple fluids will allow you to specify fluid specific and fluid pair models Multicomponent FlowEach component fluid may have a distinct set of physical propertiesThe ANSYS CFX-Solver will calculate appropriate average

29、values of the properties for each control volume in the flow domain, for use in calculating the fluid flowThese average values will depend both on component property values and on the proportion of each component present in the control volumeIn multicomponent flow, the various components of a fluid

30、share the same mean velocity, pressure and temperature fields, and mass transfer takes place by convection and diffusionCompressible Flow Modelling Activated by selecting an Ideal Gas, Real Fluid, or a General Fluid whose density is a function of pressureCan solve for subsonic, supersonic and transo

31、nic flowsSupersonic/Transonic flow problems Set the heat transfer option to Total EnergyGenerally more difficult to solve than subsonic/incompressible flow problems, especially when shocks are presentClick to load a real gas libraryBoundary ConditionsDefining Boundary ConditionsYou must specify info

32、rmation on the dependent (flow) variables at the domain boundariesSpecify fluxes of mass, momentum, energy, etc. into the domain.Defining boundary conditions involves:Identifying the location of the boundaries (e.g., inlets, walls, symmetry)Supplying information at the boundariesThe data required at

33、 a boundary depends upon the boundary condition type and the physical models employedYou must be aware of types of the boundary condition available and locate the boundaries where the flow variables have known values or can be reasonably approximatedPoorly defined boundary conditions can have a sign

34、ificant impact on your solutionAvailable Boundary Condition TypesInletVelocity Components-Static Temperature (Heat Transfer)Normal Speed-Total Temperature (Heat Transfer)Mass Flow Rate-Total Enthalpy (Heat Transfer)Total Pressure (stable)-Relative Static Pressure (Supersonic)Static Pressure-Inlet Tu

35、rbulent conditionsOutletAverage Static Pressure-Normal SpeedVelocity Components-Mass Flow RateStatic PressureOpeningOpening Pressure and Dirn-Opening Temperature (Heat Transfer)Entrainment-Opening Static Temperature (Heat Transfer)Static Pressure and Direction-Inflow Turbulent conditionsVelocity Com

36、ponentsWallNo Slip / Free Slip-Adiabatic (Heat Transfer)Roughness Parameters-Fixed Temperature (Heat Transfer)Heat Flux (Heat Transfer)-Heat Transfer Coefficient (Heat Transfer)Wall Velocity (for tangential motion only)SymmetryNo details (only specify region which corresponds to the symmetry planeIn

37、letOpeningOutletWallSymmetryRight-click on the domain to insert BCsHow to Create a Boundary ConditionAfter completing the boundary condition, it appears in the Outline tree below its domainInlets and OutletsInlets are used predominantly for regions where inflow is expected; however, inlets also supp

38、ort outflow as a result of velocity specified boundary conditionsVelocity specified inlets are intended for incompressible flowsUsing velocity inlets in compressible flows can lead to non-physical resultsPressure and mass flow inlets are suitable for compressible and incompressible flowsThe same con

39、cept applies to outletsVelocity Specified ConditionPressure or Mass Flow ConditionInletInletInflow allowedInflowallowedOutflowallowedOpeningsArtificial walls are not erected with the opening type boundary, as both inflow and outflow are allowedYou are required to specify information that is used if

40、the flow becomes locally inflow Do not use opening as an excuse for a poorly placed boundarySee the following slides for examplesPressure Specified OpeningInletInflow allowedOutflowallowedSymmetryUsed to reduce computational effort in problem.No inputs are required.Flow field and geometry must be sy

41、mmetric:Zero normal velocity at symmetry planeZero normal gradients of all variables at symmetry planeMust take care to correctly define symmetry boundary locationsCan be used to model slip walls in viscous flowsymmetry planesFuelAirManifold box1Nozzle123Specifying Well Posed Boundary Conditions1 Up

42、stream of manifoldCan use uniform profiles since natural profiles will develop in the supply pipesRequires more elementsNozzle inlet planeRequires accurate velocity profile data for the air and fuelNozzle outlet planeRequires accurate velocity profile data and accurate profile data for the mixture f

43、ractions of air and fuelConsider the following case in which contain separate air and fuel supply pipesThree possible approachesin locating inlet boundaries:Specifying Well Posed Boundary ConditionsIf possible, select boundary location and shape such that flow either goes in or outNot necessary, but

44、 will typically observe better convergenceShould not observe large gradients in direction normal to boundaryIndicates incorrect boundary condition locationUpper pressure boundary modified to ensure that flow always enters domain.This outlet is poorly located. It should be moved further downstreamBou

45、ndaries placed over recirculation zonesPoor Location: Apply an opening to allow inflowBetter Location: Apply an outlet with an accurate velocity/pressure profile (difficult)Ideal Location: Apply an outlet downstream of the recirculation zone to allow the flow to develop. This will make it easier to

46、specify accurate flow conditions Specifying Well Posed Boundary ConditionsOpeningOutletOutletTurbulence at the InletNominal turbulence intensities range from 1% to 5% but will depend on your specific application.The default turbulence intensity value of 0.037 (that is, 3.7%) is sufficient for nomina

47、l turbulence through a circular inlet, and is a good estimate in the absence of experimental data.For situations where turbulence is generated by wall friction, consider extending the domain upstream to allow the walls to generate turbulence and the flow to become developed Specifying Well Posed Bou

48、ndary ConditionsExternal FlowIn general, if the building has height H and width W, you would want your domain to be at least 5H high, 10W wide, with at least 2H upstream of the building and 10 H downstream of the building.You would want to verify that there are no significant pressure gradients norm

49、al to any of the boundaries of the computational domain. If there are, then it would be wise to enlarge the size of your domain. Specifying Well Posed Boundary Conditionswh5h10HAt least 2H10wConcentrate mesh in regions of high gradientsSymmetry Plane and the Coanda EffectSymmetric geometry does not

50、necessarily mean symmetric flowExample: The coanda effect. A jet entering at the center of a symmetrical duct will tend to flow along one side above a certain Reynolds numberSpecifying Well Posed Boundary ConditionsNo Symmetry PlaneSymmetry PlaneCoanda effect not allowedWhen there is 1 Inlet and 1 OutletMost Robust: Velocity/Mass Flow at an Inle