CFX13-03-計算域及邊界條件設(shè)置.ppt

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1、Chapter 3計算域及邊界條件的設(shè)置,CFX條件設(shè)置,Domains,Domains are regions of space in which the equations of fluid flow or heat transfer are solved Only the mesh components which are included in a domain are included in the simulation,e.g. A simulation of a copper heating coil in water will require a fluid domain

2、and a solid domain.,e.g. To account for rotational motion, the rotor is placed in a rotating domain.,Rotor,Stator,How to Create a Domain (as shown earlier),Define Domain Properties Right-click on the domain and pick Edit Or right-click on Flow Analysis 1 to insert a new domain,,When editing an item

3、a new tab panel opens containing the properties. You can switch between open tabs.,,Sub-tabs contain various different properties Complete the required fields on each sub-tab to define the domain Optional fields are activated by enabling a check box,,Domain Creation,General Options panel: Basic Set

4、tings Location: Only assemblies and 3D primitives Domain Type: Fluid, Solid, or Porous Coordinate Frame: select coordinate frame from which all domain 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 us

5、ed Fluids and Particles Definitions: select the participating materials,,Ex. 2: Preference= 100,000 Pa,Domain Creation Reference Pressure,General Options panel: Domain Models Reference Pressure Represents the absolute pressure datum from which all relative pressures are measured Pabs = Preference +

6、Prelative Pressures specified 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 level,,,,,Pressure,,,,,Pressure,Ex. 1: Preferenc

7、e= 0 Pa,Pref,Prel,max=100,001 Pa,Prel,min=99,999 Pa,Prel,max=1 Pa,Prel,min=-1 Pa,,,Pref,Domain Creation - Buoyancy,General Options panel: Buoyancy When gravity acts on fluid regions with different densities a buoyancy force arises When buoyancy is included, a source term is added to the momentum equ

8、ations based on the difference between the fluid density and a reference density SM,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 fo

9、r more on the reference density The ( ref) term is evaluated differently depending on your chosen fluid:,,Domain 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 Us

10、e an approximate value of the expected domain density Boussinesq Model Used when modeling constant density 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 temperature,,Domain Crea

11、tion - 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 hydrostatic pressure i

12、n 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 Absolute Pressure

13、 always includes both the hydrostatic and reference pressures Pabs = Preference + Prelative + rref g h For a non-buoyant flow a hydrostatic pressure does not exist,,Pressure and Buoyancy Example,Consider the case of flow through a tank The inlet is at 30 psi absolute Buoyancy is included, therefore

14、a hydrostatic pressure gradient exists The outlet pressure will be approximately30 psi plus the hydrostatic pressure given by r g h The flow field is driven by small dynamic pressure changes NOT by the large hydrostatic pressure or the large operating pressure To accurately resolve the small dynamic

15、 pressure changes, we use the Reference Pressure to offset the operating pressure and the Buoyancy Reference Density to offset the hydrostatic pressure,30 psi,30 psi + r gh,,Gravity, g,,Small pressure changes drive the flow field in the tank,Domain Creation,General Options panel: Domain Motion You c

16、an specify a domain that is rotating about 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 mo

17、ving subdomains Mesh motion could be imposed or arise as an implicit part of the solution,,,,,,,Domain Types,The additional domain tabs/settings depend on the Domain Type selected,,,,,Domain Type: Fluid Models,Heat Transfer Specify whether a heat transfer model is used to predict the temperature thr

18、oughout the flow Discussed in Heat Transfer Lecture Turbulence Specify whether a turbulence model is used to predict the effects of turbulence in fluid flow Discussed in Turbulence Lecture,,Domain Type: Fluid Models,Reaction or Combustion Models CFX includes combustion models to allow the simulation

19、 of flows in which combustion reactions occur Available only if Option = Material Definition on the Basic Settings tab Not covered in detail in this course,,Domain Type: Fluid Models,Radiation Models For simulations when thermal radiation is significant See the Heat Transfer chapter for more details

20、,,,Domain Type: Solid Models,Solid domains 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 Ther

21、es no radiation in solid domains if it is 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 exchanger,Images Courtesy of Babcock a

22、nd 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 numerically,,Domain Type: Porous Domains,Area Porosity The area porosity (the fraction of physical area

23、that is available for the flow to go through) is assumed isotropic Volume Porosity The local ratio 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 th

24、e fluid will be larger because of the flow volume reduction Superficial Velocity = Volume Porosity * True Velocity,,,,,,,,,,,,,,,,,,,,This setting should be consistent with the velocity used when the Loss Coefficients (next slide) were calculated,,,,,Domain Type: Porous Domains,Loss Model Isotropic:

25、 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 Coefficie

26、nts,,Materials,Create a name for the fluid to be used Select the material to be used in the domain Currently loaded materials are available in the drop down list Additional Materials are available by clicking,,,,,Materials,A Material can be created/edited by right clicking “Materials” in the Outline

27、 Tree,Multicomponent/Multiphase Flow,ANSYS CFX has the capability to model fluid mixtures (multicomponent) and multiple phases,,Multicomponent (more details on next slide) One flow field for the mixture Variations in the mixture accounted for by variable mass fractions Applicable when components are

28、 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 field Applicable when fluids are mixed on a macroscopic scale, with a discernible interface between the fluids.,,,Creating multiple fluids wil

29、l allow you to specify fluid specific and fluid pair models,,,,,Multicomponent 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

30、the fluid flow These average values will depend both on component 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 pla

31、ce by convection and diffusion,Compressible 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/Transonic flow problems Set the heat transfer option to Total Energ

32、y Generally more difficult to solve than subsonic/incompressible flow problems, especially when shocks are present,,,Click to load a real gas library,,,,,Boundary Conditions,Defining Boundary Conditions,You must specify information on the dependent (flow) variables at the domain boundaries Specify f

33、luxes 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 boundaries The data required at a boundary depends upon the boundary condition type and the physical m

34、odels employed You 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 approximated Poorly defined boundary conditions can have a significant impact on your solution,Available Boundary Condition Types,In

35、let Velocity 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 Turbulent conditions Outlet Average Static Pressure-Normal Speed

36、 Velocity Components-Mass Flow Rate Static Pressure Opening Opening Pressure and Dirn-Opening Temperature (Heat Transfer) Entrainment-Opening Static Temperature (Heat Transfer) Static Pressure and Direction-Inflow Turbulent conditions Velocity Components Wall No Slip / Free Slip-Adiabatic (Heat Tran

37、sfer) Roughness Parameters-Fixed Temperature (Heat Transfer) Heat Flux (Heat Transfer)-Heat Transfer Coefficient (Heat Transfer) Wall Velocity (for tangential motion only) Symmetry No details (only specify region which corresponds to the symmetry plane,Inlet,Opening,Outlet,Wall,Symmetry,,,,,,Right-c

38、lick on the domain to insert BCs,How to Create a Boundary Condition,,,,,After completing the boundary condition, it appears in the Outline tree below its domain,,Inlets and Outlets,Inlets are used predominantly for regions where inflow is expected; however, inlets also support outflow as a result of

39、 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

40、ets,Openings,Artificial walls are not erected with the opening type 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 examp

41、les,Symmetry,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 gradients of all variables at symmetry plane Must take care to correctly define symmetry boundary locations Can be used t

42、o model slip walls in viscous flow,symmetry planes,,Specifying Well Posed Boundary Conditions,1 Upstream of manifold Can use uniform profiles since natural profiles will develop in the supply pipes Requires more elements Nozzle inlet plane Requires accurate velocity profile data for the air and fuel

43、 Nozzle outlet plane Requires accurate velocity 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:,Specifying Well Posed Boundary Condi

44、tions,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 boundary Indicates incorrect boundary condition location,Upper pressure boundary modified

45、to ensure that flow always enters domain.,,,This outlet is poorly located. It should be moved further downstream,Boundaries placed over recirculation zones Poor Location: Apply an opening to allow inflow Better Location: Apply an outlet with an accurate velocity/pressure profile (difficult) Ideal

46、 Location: Apply an outlet downstream of the recirculation zone to allow the flow to develop. This will make it easier to specify accurate flow conditions,Specifying Well Posed Boundary Conditions,Opening,,Outlet,,Outlet,,Turbulence at the Inlet Nominal turbulence intensities range from 1% to 5% but

47、 will depend on your specific application. The default turbulence intensity value of 0.037 (that is, 3.7%) is sufficient for nominal 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, conside

48、r extending the domain upstream to allow the walls to generate turbulence and the flow to become developed,Specifying Well Posed Boundary Conditions,,External Flow In 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 upst

49、ream of the building and 10 H downstream of the building. You would want to verify that there are no significant pressure gradients normal 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 Cond

50、itions,,,,,,,,w,h,,,,,5h,10H,,At least 2H,,,,,,,,10w,Concentrate mesh in regions of high gradients,Symmetry Plane and the Coanda Effect Symmetric geometry does not necessarily mean symmetric flow Example: The coanda effect. A jet entering at the center of a symmetrical duct will tend to flow along o

51、ne side above a certain Reynolds number,Specifying Well Posed Boundary Conditions,When there is 1 Inlet and 1 Outlet Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet. The Inlet total pressure is an implicit result of the prediction. Robust: Total Pressure at an Inlet; Veloci

52、ty/Mass Flow at an Outlet. The static pressure at the Outlet and the velocity at the Inlet are part of the solution. Sensitive to Initial Guess: Total Pressure at an Inlet; Static Pressure at an Outlet. The system mass flow is part of the solution Very Unreliable: Static Pressure at an Inlet; Static

53、 Pressure at an Outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combination is a very weak constraint on the system).,Specifying Well Posed Boundary Conditions,Specifying Well Posed

54、 Boundary Conditions,At least one boundary should specify Pressure (either Total or Static) Unless its a closed system Using a combination of Velocity and Mass Flow conditions at all boundaries over constrains the system Total Pressure cannot be set at an Outlet It is unconditionally unstable Outlet

55、s that vent to the atmosphere typically use a Static Pressure = 0 boundary condition With a domain Reference Pressure of 1 atm Inlets that draw flow in from the atmosphere often use a Total Pressure = 0 boundary condition (e.g. an open window) With a domain Reference Pressure of 1 atm,Specifying Wel

56、l Posed Boundary Conditions,Mass flow inlets result in a uniform velocity profile over the inlet Fully developed flow is not achieved You cannot specify a mass flow profile Mass flow outlets allow a natural velocity profile to develop based on the upstream conditions Pressure specified boundary conditions allow a natural velocity profile to develop,