fluent教程海基公司培训-vboundary

1、Boundary ConditionsDefining Boundary ConditionsTo define a problem that results in a unique solution, you must specify information on the dependent (flow) variables at the domain boundaries.Specifying fluxes of mass, momentum, energy, etc. into domain.Defining boundary conditions involves:identifyin

2、g the location of the boundaries (e.g., inlets, walls, symmetry)supplying information at the boundariesThe data required at a boundary depends upon the boundary condition type and the physical models employed.You must be aware of the information that is required of the boundary condition and locate

3、the boundaries where the information on the flow variables are known or can be reasonably approximated.Poorly defined boundary conditions can have a significant impact on your solution.FuelAirCombustor WallManifold box1123NozzleLocating Boundaries: ExampleThree possible approaches in locating inlet

4、boundaries:1. Upstream of manifoldCan use uniform profileProperly accounts for mixingNon-premixed reaction modelsRequires more cells2. Nozzle inlet planeNon-premixed reaction modelsRequires accurate profile data3. Nozzle outlet planePremixed reaction modelRequires accurate profileGeneral GuidelinesG

5、eneral guidelines: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 set-up.Minimize grid skewness near the boundary.O

6、therwise it would introduce error early in calculation.21Upper pressure boundary modified to ensure that flow always enters domain.Available Boundary Condition TypesBoundary Condition Types of External Faces:General: Pressure inlet, pressure outlet pressible: Velocity inlet, outflowCompressible flow

7、s: Mass flow inlet, pressure far-field, mass flow outletSpecial: Inlet vent, outlet vent, intake fan, exhaust fanOther: Wall, symmetry, axis, periodicBoundary Condition Types of cell zones:Fluid, solid, porous media and heat exchanger modelsBoundary Condition Types of double-sided internal facesFan,

8、 interior, porous Jump, radiator, wallsinletoutletwallinteriororifice_plate and orifice_plate-shadowChanging Boundary Condition TypesZones and zone types are initially defined in pre-processor.To change zone type for a particular zone:Define Boundary Conditions.Choose the zone in Zone list.Can also

9、select boundary zone using right mouse button in Display Grid window.Select new zone type in Type list.Setting Boundary Condition DataExplicitly assign data in BC panels.To set boundary conditions for particular zone:Choose the zone in Zone list.Click Set. buttonBoundary condition data can be copied

10、 from one zone to another.Boundary condition data can be stored and retrieved from file.file write-bc and file read-bcBoundary conditions can also be defined by UDFs and Profiles.Profiles can be generated by:Writing a profile from another CFD simulationCreating an appropriately formatted text file w

11、ith boundary condition data.Velocity InletSpecify Velocity by:Magnitude, Normal to BoundaryComponentsMagnitude and DirectionVelocity profile is uniform by defaultIntended for pressible flows.Static pressure adjusts to modate prescribed velocity distribution.Total (stagnation) properties of flow also

12、 varies.Using in compressible flows can lead to non-physical results.Can be used as an outlet by specifying negative velocity.You must ensure that mass conservation is satisfied if multiple inlets are used.Pressure Inlet (1)Specify:Total Gauge PressureDefines energy to drive flow.Doubles as back pre

13、ssure (static gauge) for cases where back flow occurs.Direction of back flow determined from interior solution.Static Gauge PressureStatic pressure where flow is locally supersonic; ignored if subsonicWill be used if flow field is initialized from this boundary.Total TemperatureUsed as static temper

14、ature for pressible flow.Inlet Flow Direction pressible flows:From 1-D Compressible flow relationship:Pressure Inlet (2)Note: Gauge pressure inputs are required. Operating pressure level sometimes may affect solution accuracy (when pressure fluctuations are relatively small).Operating pressure input

15、 is set under: Define Operating ConditionsSuitable for compressible and pressible flows.Pressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions.Fluent calculates static pressure and velocity at inletMass flux through boundary varies depending on interior soluti

16、on and specified flow direction. Can be used as a “free” boundary in an external or unconfined flow.Mass Flow InletSpecify:(a) Mass Flow Rate or (b) Mass Flux(a) implies uniform mass flux(b) can be defined by profiles/UDFStatic Gauge PressureStatic pressure where flow is locallysupersonic; ignored i

17、f subsonicWill be used if flow field is initialized from this boundary.Total TemperatureUsed as static temperature for pressible flow.Inlet Flow DirectionIntended for compressible; can be used for pressible flows.Total pressure adjusts to modate mass flow inputs.More difficult to converge than with

18、pressure inlet.Pressure OutletSpecify static gauge pressureInterpreted as static pressure of environment into which flow exhausts.Radial equilibrium pressuredistribution option available.Doubles as inlet pressure (total gauge)for cases where backflow occurs.BackflowCan occur at pressure outlet durin

19、g iterations or as part of final solution.Backflow direction can be normal to the boundary, set by direction vector or from neighboring cell .Backflow boundary data must be set for all transport variables.Convergence difficulties are reduced by providing realistic backflow quantities.Suitable for co

20、mpressible and pressible flowsSpecified pressure is ignored if flow is locally supersonic at the outlet.Can be used as a “free” boundary in an external or unconfined flow.OutflowNo pressure or velocity information is required.Data at exit plane is extrapolated from interior.Mass balance correction i

21、s applied at boundary.Flow exiting Outflow boundary exhibits zero normal diffusive flux for all flow variables.Appropriate where the exit flow is “fully-developed.”Intended for pressible flows:Cannot be used with a Pressure-Inlet boundary: must use Velocity-Inlet BC at the inlet.Combination does not

22、 uniquely set pressure gradient over whole domain. Cannot be used for unsteady flows with variable density.Poor rate of convergence when back flow occurs during iteration.Cannot be used if back flow is expected in final solution.Modeling Multiple ExitsFlows with multiple exits can be modeled using P

23、ressure Outlet or Outflow boundaries.Pressure OutletsOutflow:Mass flow rate fraction determined from Flow Rate Weighting by:mi=FRWi/FRWi where 0 FRW 1. FRW set to 1 by default implying equal flow ratesstatic pressure varies among exits to modate flow distribution. pressure-inlet (p0,T0)pressure-outl

24、et (ps)2velocity-inlet (v,T0)pressure-outlet (ps)1orFRW2velocity inletFRW1Other Inlet/Outlet Boundary ConditionsPressure Far FieldAvailable when density is calculated from the ideal gas law.Used to model free-stream compressible flow at infinity, with free-stream Mach number and static conditions sp

25、ecified.Mass-Flow OutletSpecify target mass flow rate (TUI only) at a pressure-outlet boundary First enable and choose method: /define/boundary-conditions /target-mass-flow-rate-settingsThen Select the pressure outlet and set target flow rate: /define/boundary-conditions/select/pressure-outletExhaus

26、t Fan/Outlet VentModel external exhaust fan/outlet vent with specified pressure jump/loss coefficient and ambient (discharge) pressure and temperature.Inlet Vent/Intake FanModel inlet vent/external intake fan with specified loss coefficient/ pressure jump, flow direction, and ambient (inlet) pressur

27、e and temperature.Wall BoundariesUsed to bound fluid and solid regions.In viscous flows, no-slip condition enforced at walls:Tangential fluid velocity equalto wall velocity.Normal velocity component = 0Shear stress can also be specified.Thermal boundary conditions:several types availableWall materia

28、l and thickness can be defined for 1-D or shell conduction heat transfer calculations.Wall roughness can be defined for turbulent flows.Wall shear stress and heat transfer based on local flow field.Translational or rotational velocity can be assigned to wall boundaries.Symmetry and Axis BoundariesSy

29、mmetry BoundaryUsed to reduce computational effort in problem.No inputs required.Flow field and geometry must be symmetric:Zero normal velocity at symmetry planeZero normal gradients of all variables at symmetry planeMust take care to correctly define symmetry boundary locations.Can be used to model

30、 slip walls in viscous flowAxis BoundaryUsed at centerline for axisymmetric problems.No user inputs required.symmetry planesPeriodic BoundariesUsed to reduce computational effort in problem.Flow field and geometry must be either translationally or rotationally periodic.For rotationally periodic boun

31、daries:p = 0 across periodic planes.Axis of rotation must be defined in fluid zone.For translationally periodic boundaries:p can be finite across periodic planes.Models fully developed conditions.Specify either mean p per period or net mass flow rate.Periodic boundaries defined in Gambit are transla

32、tional.Translationally periodic planes2D tube heat exchangerflowRotationally periodic planesCell Zones: FluidFluid zone = group of cells for which all active equations are solved.Fluid material input required.Single species, phase.Optional inputs allow setting of source terms:mass, momentum, energy,

33、 etc.Define fluid zone as laminar flow region if modeling transitional flow.Can define zone as porous media.Define axis of rotation for rotationally periodic flows.Can define motion for fluid zone.Porous Media ConditionsPorous zone modeled as special type of fluid zone.Enable Porous Zone option in F

34、luid panel.Pressure loss in flow determined via user inputsof resistance coefficients to lumped parametermodel.Used to model flow through porous mediaand other “distributed” resistances, e.g.,Packed bedsFilter papersPerforated platesFlow distributorsTube banksCell Zones: Solid“Solid” zone = group of cells for which only heat conduction problem solved.No flow equations solvedMaterial b