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1、 Fluent Inc. 10/17/20214-1 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CBoundary Conditions Fluent Inc. 10/17/20214-2 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CDefining Boundary ConditionsuTo define a problem that results in a unique solution, you mus

2、t specify information on the dependent (flow) variables at the domain boundaries.lSpecifying fluxes of mass, momentum, energy, etc. into domain.uDefining boundary conditions involves:lidentifying the location of the boundaries (e.g., inlets, walls, symmetry)lsupplying information at the boundariesuT

3、he data required at a boundary depends upon the boundary condition type and the physical models employed.uYou must be aware of the information that is required of the boundary condition and locate the boundaries where the information on the flow variables are known or can be reasonably approximated.

4、lPoorly defined boundary conditions can have a significant impact on your solution. Fluent Inc. 10/17/20214-3 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CFuelAirCombustor WallManifold box1123NozzleLocating Boundaries: ExampleuThree possible approaches in locating inlet bounda

5、ries:l1. Upstream of manifoldnCan use uniform profilenProperly accounts for mixingnNon-premixed reaction modelsnRequires more cellsl2. Nozzle inlet planenNon-premixed reaction modelsnRequires accurate profile datal3. Nozzle outlet planenPremixed reaction modelnRequires accurate profile Fluent Inc. 1

6、0/17/20214-4 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CGeneral GuidelinesuGeneral guidelines:lIf possible, select boundary location and shape such that flow either goes in or out.nNot necessary, but will typically observe better convergence.lShould not observe large gradien

7、ts in direction normal to boundary.nIndicates incorrect set-up.lMinimize grid skewness near the boundary.nOtherwise it would introduce error early in calculation.21Upper pressure boundary modified to ensure that flow always enters domain. Fluent Inc. 10/17/20214-5 Introductory FLUENT Notes FLUENT v6

8、.1 Feb 2003Fluent User Services CAvailable Boundary Condition TypesuBoundary Condition Types of External Faces:lGeneral: Pressure inlet, pressure outletlIncompressible: Velocity inlet, outflowlCompressible flows: Mass flow inlet, pressure far-field, mass flow outletlSpecial: Inlet vent, outlet vent,

9、 intake fan, exhaust fanlOther: Wall, symmetry, axis, periodicuBoundary Condition Types of cell zones:lFluid, solid, porous media and heat exchanger modelsuBoundary Condition Types of double-sided internal faceslFan, interior, porous Jump, radiator, wallsinletoutletwallinteriororifice_plate and orif

10、ice_plate-shadow Fluent Inc. 10/17/20214-6 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CChanging Boundary Condition TypesuZones and zone types are initially defined in pre-processor.uTo change zone type for a particular zone:Define Boundary Conditions.lChoose the zone in Zone

11、list.nCan also select boundary zone using right mouse button in Display Grid window.lSelect new zone type in Type list. Fluent Inc. 10/17/20214-7 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CSetting Boundary Condition DatauExplicitly assign data in BC panels.lTo set boundary c

12、onditions for particular zone:nChoose the zone in Zone list.nClick Set. buttonlBoundary condition data can be copied from one zone to another.uBoundary condition data can be stored and retrieved from file.lfile write-bc and file read-bcuBoundary conditions can also be defined by UDFs and Profiles.uP

13、rofiles can be generated by:lWriting a profile from another CFD simulationlCreating an appropriately formatted text file with boundary condition data. Fluent Inc. 10/17/20214-8 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CVelocity InletuSpecify Velocity by:lMagnitude, Normal t

14、o BoundarylComponentslMagnitude and DirectionuVelocity profile is uniform by defaultuIntended for incompressible flows.lStatic pressure adjusts to accommodate prescribed velocity distribution.lTotal (stagnation) properties of flow also varies.lUsing in compressible flows can lead to non-physical res

15、ults.uCan be used as an outlet by specifying negative velocity.lYou must ensure that mass conservation is satisfied if multiple inlets are used. Fluent Inc. 10/17/20214-9 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CPressure Inlet (1)uSpecify:lTotal Gauge PressurenDefines ener

16、gy to drive flow.nDoubles as back pressure (static gauge) for cases where back flow occurs.sDirection of back flow determined from interior solution.lStatic Gauge PressurenStatic pressure where flow is locally supersonic; ignored if subsonicnWill be used if flow field is initialized from this bounda

17、ry.lTotal TemperaturenUsed as static temperature for incompressible flow.lInlet Flow Direction21(1)2totalstatickTTM2/(1),1(1)2kktotal absstatic abskppM221vppstatictotalIncompressible flows:From 1-D Compressible flow relationship: Fluent Inc. 10/17/20214-10 Introductory FLUENT Notes FLUENT v6.1 Feb 2

18、003Fluent User Services CPressure Inlet (2)uNote: Gauge pressure inputs are required.l lOperating pressure level sometimes may affect solution accuracy (when pressure fluctuations are relatively small).lOperating pressure input is set under: Define Operating ConditionsuSuitable for compressible and

19、incompressible flows.lPressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions.lFluent calculates static pressure and velocity at inletlMass flux through boundary varies depending on interior solution and specified flow direction.u Can be used as a “free” bounda

20、ry in an external or unconfined flow.operatinggaugeabsoluteppp Fluent Inc. 10/17/20214-11 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CMass Flow InletuSpecify:l(a) Mass Flow Rate or (b) Mass Fluxn(a) implies uniform mass fluxn(b) can be defined by profiles/UDFlStatic Gauge Pre

21、ssurenStatic pressure where flow is locallysupersonic; ignored if subsonicnWill be used if flow field is initialized from this boundary.lTotal TemperaturenUsed as static temperature for incompressible flow.lInlet Flow DirectionuIntended for compressible; can be used for incompressible flows.lTotal p

22、ressure adjusts to accommodate mass flow inputs.lMore difficult to converge than with pressure inlet. Fluent Inc. 10/17/20214-12 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CPressure OutletuSpecify static gauge pressurelInterpreted as static pressure of environment into which

23、flow exhausts.lRadial equilibrium pressuredistribution option available.lDoubles as inlet pressure (total gauge)for cases where backflow occurs.uBackflowlCan occur at pressure outlet during iterations or as part of final solution.lBackflow direction can be normal to the boundary, set by direction ve

24、ctor or from neighboring cell .lBackflow boundary data must be set for all transport variables.lConvergence difficulties are reduced by providing realistic backflow quantities.uSuitable for compressible and incompressible flowslSpecified pressure is ignored if flow is locally supersonic at the outle

25、t.uCan be used as a “free” boundary in an external or unconfined flow. Fluent Inc. 10/17/20214-13 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services COutflowuNo pressure or velocity information is required.lData at exit plane is extrapolated from interior.lMass balance correction is

26、applied at boundary.uFlow exiting Outflow boundary exhibits zero normal diffusive flux for all flow variables.lAppropriate where the exit flow is “fully-developed.”uIntended for incompressible flows:lCannot be used with a Pressure-Inlet boundary: must use Velocity-Inlet BC at the inlet.nCombination

27、does not uniquely set pressure gradient over whole domain. lCannot be used for unsteady flows with variable density.uPoor rate of convergence when back flow occurs during iteration.lCannot be used if back flow is expected in final solution. Fluent Inc. 10/17/20214-14 Introductory FLUENT Notes FLUENT

28、 v6.1 Feb 2003Fluent User Services CModeling Multiple ExitsuFlows with multiple exits can be modeled using Pressure Outlet or Outflow boundaries.lPressure OutletslOutflow:nMass flow rate fraction determined from Flow Rate Weighting by:smi=FRWi/FRWi where 0 FRW 1. sFRW set to 1 by default implying eq

29、ual flow ratesnstatic pressure varies among exits to accommodate flow distribution. pressure-inlet (p0,T0)pressure-outlet (ps)2velocity-inlet (v,T0)pressure-outlet (ps)1orFRW2velocity inletFRW1 Fluent Inc. 10/17/20214-15 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services COther Inlet

30、/Outlet Boundary ConditionsuPressure Far FieldlAvailable when density is calculated from the ideal gas law.lUsed to model free-stream compressible flow at infinity, with free-stream Mach number and static conditions specified.uMass-Flow OutletlSpecify target mass flow rate (TUI only) at a pressure-o

31、utlet boundary nFirst enable and choose method: /define/boundary-conditions /target-mass-flow-rate-settingsnThen Select the pressure outlet and set target flow rate: /define/boundary-conditions/select/pressure-outletuExhaust Fan/Outlet VentlModel external exhaust fan/outlet vent with specified press

32、ure jump/loss coefficient and ambient (discharge) pressure and temperature.uInlet Vent/Intake FanlModel inlet vent/external intake fan with specified loss coefficient/ pressure jump, flow direction, and ambient (inlet) pressure and temperature. Fluent Inc. 10/17/20214-16 Introductory FLUENT Notes FL

33、UENT v6.1 Feb 2003Fluent User Services CWall BoundariesuUsed to bound fluid and solid regions.uIn viscous flows, no-slip condition enforced at walls:lTangential fluid velocity equalto wall velocity.lNormal velocity component = 0lShear stress can also be specified.uThermal boundary conditions:lsevera

34、l types availablelWall material and thickness can be defined for 1-D or shell conduction heat transfer calculations.uWall roughness can be defined for turbulent flows.lWall shear stress and heat transfer based on local flow field.uTranslational or rotational velocity can be assigned to wall boundari

35、es. Fluent Inc. 10/17/20214-17 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CSymmetry and Axis BoundariesuSymmetry BoundarylUsed to reduce computational effort in problem.lNo inputs required.lFlow field and geometry must be symmetric:nZero normal velocity at symmetry planenZero

36、 normal gradients of all variables at symmetry planenMust take care to correctly define symmetry boundary locations.lCan be used to model slip walls in viscous flowuAxis BoundarylUsed at centerline for axisymmetric problems.lNo user inputs required.symmetry planes Fluent Inc. 10/17/20214-18 Introduc

37、tory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CPeriodic BoundariesuUsed to reduce computational effort in problem.uFlow field and geometry must be either translationally or rotationally periodic.uFor rotationally periodic boundaries:lp = 0 across periodic planes.lAxis of rotation must b

38、e defined in fluid zone.uFor translationally periodic boundaries:lp can be finite across periodic planes.nModels fully developed conditions.nSpecify either mean p per period or net mass flow rate.nPeriodic boundaries defined in Gambit are translational.Translationally periodic planes2D tube heat exc

39、hangerflowRotationally periodic planes Fluent Inc. 10/17/20214-19 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CCell Zones: FluiduFluid zone = group of cells for which all active equations are solved.uFluid material input required.lSingle species, phase.uOptional inputs allow s

40、etting of source terms:lmass, momentum, energy, etc.uDefine fluid zone as laminar flow region if modeling transitional flow.uCan define zone as porous media.uDefine axis of rotation for rotationally periodic flows.uCan define motion for fluid zone. Fluent Inc. 10/17/20214-20 Introductory FLUENT Note

41、s FLUENT v6.1 Feb 2003Fluent User Services CPorous Media ConditionsuPorous zone modeled as special type of fluid zone.lEnable Porous Zone option in Fluid panel.lPressure loss in flow determined via user inputsof resistance coefficients to lumped parametermodel.uUsed to model flow through porous mediaand other “distributed” resistances, e.g.,lPacked bedslFilter paperslPerforated plateslFlow distributorslTube banks Fluent Inc. 10/17/20214-21 Introductory FLUENT Notes FLUENT v6.1 Feb 2003Fluent User Services CCell Zones: Solidu“Solid” zone = group of cells for which only heat conduction proble