CFX13_03_计算域及边界条件设置

1、3-1ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Chapter 3计算域及边界条件的设置计算域及边界条件的设置CFX条件设置条件设置 Boundary Conditions3-2ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomains Domains are regions of space in which

2、 the equations of fluid flow or heat transfer are solved Only 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 solid domain.e.g. To account for rotational motion, the rotor is placed

3、 in a rotating domain.RotorStatorBoundary Conditions3-3ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualHow to Create a Domain (as shown earlier)Define Domain PropertiesRight-click on the domain and pick EditOr right-click on Flow Analysis 1 to ins

4、ert a new domainWhen editing an item a new tab panel 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 boxBoundary Condition

5、s3-4ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Creation General Options panel: Basic Settings Location: Only assemblies and 3D primitives Domain Type: Fluid, Solid, or Porous Coordinate Frame: select coordinate frame from which all dom

6、ain inputs will be referenced to Not to be confused with the reference frame, which can be stationary or rotating The default Coord 0 frame is usually used Fluids and Particles Definitions: select the participating materialsBoundary Conditions3-5ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights re

7、served.Release 13.0December 2010Training ManualEx. 2: Preference= 100,000 PaDomain Creation Reference Pressure General Options panel: Domain Models Reference Pressure Represents the absolute pressure datum from which all relative pressures are measuredPabs = Preference + Prelative Pressures specifie

8、d at boundary and initial conditions are relative to the Reference Pressure Used 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=

9、99,999 PaPrel,max=1 PaPrel,min=-1 PaPrefBoundary Conditions3-6ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Creation - Buoyancy General Options panel: Buoyancy When gravity acts on fluid regions with different densities a buoyancy force a

10、rises When buoyancy is included, a source term is added to the momentum equations based on the difference between the fluid density and 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 wi

11、ll have either a positive or negative buoyancy force applied. See below for more on the reference density The ( ref) term is evaluated differently depending on your chosen fluid:Boundary Conditions3-7ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training Manua

12、lDomain Creation - Buoyancy Full Buoyancy Model Evaluates the density differences directly Used when modeling ideal gases, real fluids, or multicomponent fluids A Reference Density is required Use an approximate value of the expected domain density Boussinesq Model Used when modeling constant densit

13、y fluids Buoyancy is driven by temperature differences ( ref) = - ref (T Tref) A Reference Temperature is required Use an approximate value of the average expected domain temperatureBoundary Conditions3-8ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training M

14、anualDomain Creation - Buoyancy Buoyancy Ref. Density The Buoyancy Reference Density is used to avoid round-off errors by solving at an offset level The Reference Pressure is used to offset the operating pressure of the domain, while the Buoyancy Reference Density should be used to offset the hydros

15、tatic pressure in the domain The pressure solution is relative to rref g h, where h is relative to the Reference Location If 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 flow A

16、bsolute Pressure always includes both the hydrostatic and reference pressuresPabs = Preference + Prelative + rref g h For a non-buoyant flow a hydrostatic pressure does not existBoundary Conditions3-9ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training Manua

17、lPressure and Buoyancy Example Consider the case of flow through a tank The inlet is at 30 psi absolute Buoyancy is included, therefore a hydrostatic pressure gradient exists The outlet pressure will be approximately30 psi plus the hydrostatic pressure given by r r g h The flow field is driven by sm

18、all dynamic pressure changes NOT by the large hydrostatic pressure or the large operating pressure To accurately resolve the small dynamic pressure changes, we use the Reference Pressure to offset the operating pressure and the Buoyancy Reference Density to offset the hydrostatic pressure30 psih30 p

19、si + r r ghGravity, gSmall pressure changes drive the flow field in the tankBoundary Conditions3-10ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Creation General Options panel: Domain Motion You can specify a domain that is rotating about

20、 an axis When a domain with a rotating frame is specified, the CFX-Solver computes the appropriate Coriolis and centrifugal momentum terms, and solves a rotating frame total energy equation Mesh Deformation Used for problems involving moving boundaries or moving subdomains Mesh motion could be impos

21、ed or arise as an implicit part of the solutionBoundary Conditions3-11ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Types The additional domain tabs/settings depend on the Domain Type selectedBoundary Conditions3-12ANSYS, Inc. Proprietary

22、 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Type: Fluid Models Heat Transfer Specify whether a heat transfer model is used to predict the temperature throughout the flow Discussed in Heat Transfer Lecture Turbulence Specify whether a turbulence model is used

23、to predict the effects of turbulence in fluid flow Discussed in Turbulence LectureBoundary Conditions3-13ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Type: Fluid ModelsReaction or Combustion Models CFX includes combustion models to allow

24、 the simulation of flows in which combustion reactions occur Available only if Option = Material Definition on the Basic Settings tab Not covered in detail in this courseBoundary Conditions3-14ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain

25、 Type: Fluid ModelsRadiation Models For simulations when thermal radiation is significant See the Heat Transfer chapter for more details Boundary Conditions3-15ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Type: Solid Models Solid domains

26、 are used to model regions that contain no fluid or porous flow (for example, the walls of a heat exchanger) Heat Transfer (Conjugate Heat Transfer) Discussed in Heat Transfer Lecture Radiation Only the Monte Carlo radiation model is available in solids Theres no radiation in solid domains if it is

27、opaque! Solid Motion Used only when you need to account for advection of heat in the solid domain Solid motion must be tangential to its surface everywhere (for example, an object being extruded or rotated) Tubular heat exchangerBoundary Conditions3-16ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rig

28、hts reserved.Release 13.0December 2010Training ManualImages Courtesy of Babcock and Wilcox, USA Domain Type: Porous Domains Used to model flows where the geometry is too complex to resolve with a grid Instead of including the geometric details, their effects are accounted for numericallyBoundary Con

29、ditions3-17ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Type: Porous Domains Area Porosity The area porosity (the fraction of physical area that is available for the flow to go through) is assumed isotropic Volume Porosity The local rati

30、o of the volume of fluid to the total physical volume (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 Porosit

31、y * True VelocityThis setting should be consistent with the velocity used when the Loss Coefficients (next slide) were calculatedBoundary Conditions3-18ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDomain Type: Porous Domains Loss Model Isotropi

32、c: Losses equal in all directions Directional Loss: For many applications, different losses are induced in the streamwise and transverse directions. (Examples: Honeycombs and Porous plates) Losses are applied using Darcys Law Permeability and Loss Coefficients Linear and Quadratic Resistance Coeffic

33、ientsilossipermiUKUKdxdp2rilossipermiUKUKdxdp2riRiRiUCUCdxdp21Boundary Conditions3-19ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualMaterials Create a name for the fluid to be used Select the material to be used in the domain Currently loaded mat

34、erials are available in the drop down list Additional Materials are available by clickingBoundary Conditions3-20ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualMaterials A Material can be created/edited by right clicking “Materials” in the Outline

35、 TreeBoundary Conditions3-21ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualMulticomponent/Multiphase Flow ANSYS CFX has the capability to model fluid mixtures (multicomponent) and multiple phases Multicomponent (more details on next slide) One fl

36、ow field for the mixture 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

37、 fluids are mixed on a macroscopic scale, with a discernible interface between the fluids. Creating multiple fluids will allow you to specify fluid specific and fluid pair models Boundary Conditions3-22ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training Man

38、ualMulticomponent Flow Each component fluid may have a distinct set of physical properties The ANSYS CFX-Solver will calculate appropriate average values of the properties for each control volume in the flow domain, for use in calculating the fluid flow These average values will depend both on compo

39、nent property values and on the proportion of each component present in the control volume In multicomponent flow, the various components of a fluid share the same mean velocity, pressure and temperature fields, and mass transfer takes place by convection and diffusionBoundary Conditions3-23ANSYS, I

40、nc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualCompressible Flow Modelling Activated by selecting an Ideal Gas, Real Fluid, or a General Fluid whose density is a function of pressure Can solve for subsonic, supersonic and transonic flows Supersonic/Trans

41、onic flow problems Set the heat transfer option to Total Energy Generally more difficult to solve than subsonic/incompressible flow problems, especially when shocks are presentClick to load a real gas library3-24ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Bo

42、undary ConditionsBoundary Conditions3-25ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualDefining Boundary Conditions You must specify information on the dependent (flow) variables at the domain boundaries Specify fluxes of mass, momentum, energy,

43、etc. into the domain. Defining boundary conditions involves: Identifying the location of the boundaries (e.g., inlets, walls, symmetry) Supplying information at the boundaries The data required at a boundary depends upon the boundary condition type and the physical models employed You must be aware

44、of types of the boundary condition available and locate the boundaries where the flow variables have known values or can be reasonably approximated Poorly defined boundary conditions can have a significant impact on your solutionBoundary Conditions3-26ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rig

45、hts reserved.Release 13.0December 2010Training ManualAvailable Boundary Condition Types InletVelocity 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)S

46、tatic Pressure-Inlet Turbulent conditions OutletAverage Static Pressure-Normal SpeedVelocity Components-Mass Flow RateStatic Pressure OpeningOpening Pressure and Dirn-Opening Temperature (Heat Transfer)Entrainment-Opening Static Temperature (Heat Transfer)Static Pressure and Direction-Inflow Turbule

47、nt conditionsVelocity Components WallNo 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 correspo

48、nds to the symmetry planeInletOpeningOutletWallSymmetryBoundary Conditions3-27ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualRight-click on the domain to insert BCsHow to Create a Boundary ConditionAfter completing the boundary condition, it appe

49、ars in the Outline tree below its domainBoundary Conditions3-28ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualInlets and Outlets Inlets are used predominantly for regions where inflow is expected; however, inlets also support outflow as a result

50、of velocity specified boundary conditions Velocity specified inlets are intended for incompressible flows Using velocity inlets in compressible flows can lead to non-physical results Pressure and mass flow inlets are suitable for compressible and incompressible flows The same concept applies to outl

51、etsVelocity Specified ConditionPressure or Mass Flow ConditionInletInletInflow allowedInflowallowedOutflowallowedBoundary Conditions3-29ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualOpenings Artificial walls are not erected with the opening type

52、 boundary, as both inflow and outflow are allowed You are required to specify information that is used if the flow becomes locally inflow Do not use opening as an excuse for a poorly placed boundary See the following slides for examplesPressure Specified OpeningInletInflow allowedOutflowallowedBound

53、ary Conditions3-30ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualSymmetry Used to reduce computational effort in problem. No inputs are required. Flow field and geometry must be symmetric: Zero normal velocity at symmetry plane Zero normal gradie

54、nts of all variables at symmetry plane Must take care to correctly define symmetry boundary locations Can be used to model slip walls in viscous flowsymmetry planesBoundary Conditions3-31ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualFuelAirManif

55、old box1Nozzle123Specifying Well Posed Boundary Conditions1 Upstream of manifoldCan use uniform profiles since natural profiles will develop in the supply pipesRequires more elements Nozzle inlet planeRequires accurate velocity profile data for the air and fuel Nozzle outlet planeRequires accurate v

56、elocity profile data and accurate profile data for the mixture fractions of air and fuel Consider the following case in which contain separate air and fuel supply pipes Three possible approachesin locating inlet boundaries:Boundary Conditions3-32ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights re

57、served.Release 13.0December 2010Training ManualSpecifying Well Posed Boundary Conditions If possible, select boundary location and shape such that flow either goes in or out Not necessary, but will typically observe better convergence Should not observe large gradients in direction normal to boundar

58、y Indicates incorrect boundary condition locationUpper pressure boundary modified to ensure that flow always enters domain.This outlet is poorly located. It should be moved further downstreamBoundary Conditions3-33ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010

59、Training ManualBoundaries 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

60、make it easier to specify accurate flow conditions Specifying Well Posed Boundary ConditionsOpeningOutletOutletBoundary Conditions3-34ANSYS, Inc. Proprietary 2010 ANSYS, Inc. All rights reserved.Release 13.0December 2010Training ManualTurbulence at the InletNominal turbulence intensities range from