?；九嘤?xùn)講義文件fluent.04.boundary

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 boundary.Intro

6、duces error early in calculation.21Available Boundary Condition TypesBoundary Condition Types of External FacesGeneral: Pressure inlet, Pressure outlet pressible: Velocity inlet, OutflowCompressible flows: Mass flow inlet, Pressure far-fieldSpecial: Inlet vent, outlet vent, intake fan, exhaust fanOt

7、her: Wall, Symmetry, Periodic, AxisBoundary Condition Types of Cell BoundariesFluid and SolidBoundary Condition Types of Double-Sided Face BoundariesFan, Interior, Porous Jump, Radiator, WallsinletoutletwallinteriorOrifice_plate and orifice_plate-shadowChanging Boundary Condition TypesZones and zone

8、 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 select boundary zone using right mouse button in Display Grid window.Select new zone type in Type list.Setting Boundary Condition DataExplicitly as

9、sign 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 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

10、be defined by UDFs and Profiles.Profiles can be generated by:Writing a profile from another CFD simulationCreating an appropriately formatted text file with boundary condition data.Velocity InletSpecify Velocity by:Magnitude, Normal to BoundaryComponentsMagnitude and DirectionVelocity profile is uni

11、form by defaultIntended for pressible flows.Static pressure adjusts to modate prescribed velocity distribution.Total (stagnation) properties of flow also 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

12、mass conservation is satisfied if multiple inlets are used.Pressure Inlet (1)Specify:Total Gauge PressureDefines energy to drive flow.Doubles as back pressure (static gauge) for cases where back flow occurs.Direction of back flow determined from interior solution.Static Gauge PressureStatic pressure

13、 where flow is locally supersonic; ignored if subsonicWill be used if flow field is initialized from this boundary.Total TemperatureUsed as static temperature for pressible flow.Inlet Flow Direction pressible flows:Compressible flows:Pressure Inlet (2)Note: Gauge pressure inputs are required. Operat

14、ing pressure input 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

15、on interior solution 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 locallysup

16、ersonic; ignored if 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 c

17、onverge than with 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 pre

18、ssure outlet during iterations or as part of final solution.Backflow direction is assumed to be normal to the boundary.Backflow boundary data must be set for all transport variables.Convergence difficulties minimized by realistic values for backflow quantities.Suitable for compressible and pressible

19、 flowsPressure is ignored if flow is locally supersonic.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 is applied at boundary.Flow exiting Outflow bound

20、ary exhibits zero normal diffusive flux for all flow variables.Appropriate where exit flow is close to fully developed condition.Intended for pressible flows.Cannot be used with a Pressure Inlet; must use velocity inlet.Combination does not uniquely set pressure gradient over whole domain. Cannot be

21、 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 Pressure Outlet or Outflow boundaries.Pressure OutletsOutflow

22、: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-outlet (ps)2velocity-inlet (v,T0)pressure-outlet (ps)1orFRW2velo

23、city 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 specified.Exhaust Fan/Outlet VentModel external exhaust fan/ou

24、tlet 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) pressure and temperature.Wall BoundariesUsed to bound

25、 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 material and thickness can be defined for 1-D or shel

26、l 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.Symmetry and Axis BoundariesSymmetry BoundaryUsed to reduce computational effort in pro

27、blem.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 slip walls in viscous flowAxis BoundaryUsed at centerlin

28、e for 2D axisymmetric problems.No 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 boundaries:p = 0 across periodic planes.Axis of rotation must b

29、e 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 translational.Translationally periodic planes2D tube heat exchange

30、rflowRotationally 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, etc.Define fluid zone as laminar flow region if modeling t

31、ransitional 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 Fluid panel.Pressure loss in flow determined via user inputs

32、of 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 solve