BoundaryCondiionsIntroductoryFLUENTTraining
ANSYS, Inc. Proprietary Boundary Conditions Introductory FLUENT Training
Defining Boundary ConditionsTo define a problem that results in a unique solution, you must specifyinformation on the dependent(flow) variables at the domainboundariesSpecifyfluxes ofmass,momentum, energy,etc.into thedomainDefining boundary conditions involves:Identifying the location of the boundaries (e.g., inlets, walls, symmetry)Supplying information at the boundariesThe data required at a boundary depends upon the boundary conditiontype and the physical models employed.You must be aware of the information that is required of the boundaryconditionandlocatetheboundarieswheretheinformationontheflowvariablesareknownorcanbereasonablyapproximatedPoorly defined boundary conditions can have a significant impact on yoursolution
4-2 ANSYS, Inc. Proprietary Introductory FLUENT Notes Fluent User Services Center FLUENT v6.3 December 2006 Defining Boundary Conditions ◆ To define a problem that results in a unique solution, you must specify information on the dependent (flow) variables at the domain boundaries ⚫ Specify fluxes 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 models employed. ◆ You 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 ⚫ Poorly defined boundary conditions can have a significant impact on your solution
Locating Boundaries: ExampleAirThree possible approaches inCombustorWalllocating inlet boundaries:1Upstream ofmanifoldCan use uniform profileProperly accounts formixingNon-premixedreactionmodelsRequires more cells2 Nozzle inlet planeNon-premixed reactionmodelsRequires accurateprofileNozzledata3NozzleoutletplaneManifold boxPremixedreactionmodelRequiresaccurateprofileFuel
4-3 ANSYS, Inc. Proprietary Introductory FLUENT Notes Fluent User Services Center FLUENT v6.3 December 2006 Fuel Air Combustor Wall Manifold box Nozzle Locating Boundaries: Example ◆ Three possible approaches in locating inlet boundaries: 1 Upstream of manifold ◼ Can use uniform profile ◼ Properly accounts for mixing ◼ Non-premixed reaction models ◼ Requires more cells 2 Nozzle inlet plane ◼ Non-premixed reaction models ◼ Requires accurate profile data 3 Nozzle outlet plane ◼ Premixed reaction model ◼ Requires accurate profile
General GuidelinesGeneral guidelines:Upperpressure boundarymodified to ensurethatflowalwaysentersdomainIfpossible, selectboundarylocation and shape such thatflow either goes in or out.Not necessary, but willtypically observe betterconvergence.Should not observe largegradients in direction normal toboundaryIndicates incorrect set-up.Minimize grid skewness nearthe boundary.Otherwiseitwouldintroduceerrorearlyin calculation
4-4 ANSYS, Inc. Proprietary Introductory FLUENT Notes Fluent User Services Center FLUENT v6.3 December 2006 General Guidelines ◆ General 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. ◼ Otherwise it would introduce error early in calculation. Upper pressure boundary modified to ensure that flow always enters domain
Available Boundary Condition TypesExternalfacesGeneral-pressureinlet,pressureoutletoutletIncompressible -velocity inlet, outfloworificeCompressible-massflow inlet,pressurefarwallfield,massflowoutletOther -wall, symmetry,axis, periodicSpecial -inletvent, outlet vent, intakefan.exhaustfanCell zonesFluidplateSolidinletplate-shadowPorousmediaHeat exchangerInternalfacesFan,interior,porous jump,radiator,wal
4-5 ANSYS, Inc. Proprietary Introductory FLUENT Notes Fluent User Services Center FLUENT v6.3 December 2006 Available Boundary Condition Types ◆ External faces ⚫ General – pressure inlet, pressure outlet ⚫ Incompressible – velocity inlet, outflow ⚫ Compressible – mass flow inlet, pressure farfield, mass flow outlet ⚫ Other – wall, symmetry, axis, periodic ⚫ Special – inlet vent, outlet vent, intake fan, exhaust fan ◆ Cell zones ⚫ Fluid ⚫ Solid ⚫ Porous media ⚫ Heat exchanger ◆ Internal faces ⚫ Fan, interior, porous jump, radiator, wall orifice plate plate-shadow outlet inlet wall