2 Outline Purpose of Boundary Conditions Setting Boundary Conditions Flow Inlets and ExitsWall, Repeating, and Pole BoundariesInternal Cell ZonesInternal Face Boundaries
3 Purpose of Boundary Conditions Boundaries direct and constrain motion of flow.Boundary Conditions are a required component of mathematical model.Specify fluxes into computational domain of:MassMomentumEnergyBoundary Conditions are assigned to Zones.Zones are a collection of cells (fluid or solid continuum) or cell faces (boundaries, interior surfaces).Surfaces are used for post-processing.Surfaces can correspond to Zones:Surfaces are automatically generated from cell face Zones.Surfaces corresponding to fluid and solid zones are not.outwallsinK.E. of turbulenceDissipation rate of turbulenceSpecies mass fractions
4 Setting Boundary Conditions To set boundary conditions for particular zone:Define Boundary Conditions...Choose the zone in Zone list.Click on selected zone type in Type listClick Set.. buttonCan also select boundary zone in graphics window using right mouse button.Useful if:Setting up problem for first timeTwo or more zones of same type in problem.
5 Flow Inlets and Exits - Introduction Wide range of boundary conditions permit flow to enter and exit solution domain.Types of boundary conditions for specification of flow inlets and exits:GeneralPressure inletPressure outletIncompressibleVelocity inletOutflowCompressible flowsMass flow inletPressure far-fieldSpecialInlet vent, outlet vent, intake fan, exhaust fan
6 Inlet and Outlet Boundaries Inlet and Outlet boundary conditions are available to specify fluxes for:Internal Flows: jet engine, reactorExternal Flows: aircraft in flight, natural convection flowsGeneral guidelines:Select inlet and outlet 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.
7 Velocity Inlets (1)Defines velocity and scalar properties of flow at inlet boundaries.Useful when velocity or velocity profile is known at inlet.Intended for incompressible flows only.Total (or stagnation) properties of flow are not fixed.Stagnation properties vary to accommodate prescribed velocity distribution.Use in compressible flows non-physical result
8 Velocity Inlets (2)User-defined functions (UDF) can be used to define spatial- and time- varying velocity profiles (magnitude and direction).If upstream flow comes from region of constant total energy and there are no losses (upstream), it may be easier to use the Pressure Inlet condition.Still need to specify direction of velocity vector.Don’t place velocity inlet too close to a solid obstruction.Can force the solution to be non-physical, e.g., imposes velocity field, etc., at boundary that may not be intended.
9 Determining Turbulence Parameters When turbulent flow enters domain at inlet, outlet, or at a far-field boundary, FLUENT 5 requires boundary values for:Turbulent kinetic energy kFour methods available for specifying turbulence parameters:Set k and explicitlySet turbulence intensity and turbulence length scaleSet turbulence intensity and turbulent viscosity ratioSet turbulence intensity and hydraulic diameterIntensity and length scale depend on conditions upstream, e.g.:Exhaust of a turbineIntensity = 20 % Length scale = % of blade spanDownstream of perforated plate or screenIntensity = 10 % Length scale = screen/hole sizeFully-developed flow in a duct or pipeIntensity = 5 % Length scale = hydraulic diameterTurbulence dissipation rate
10 Pressure Preliminaries gauge pressureoperating pressurepressure levelabsolute pressurevacuumAbsolute pressure is referenced to a vacuum.Can be expressed relative to operating pressure as the gauge pressure:Static pressure is thermodynamic pressure (Stoke’s Hypothesis).Expressible as absolute or gauge pressureBoundary conditions require gauge pressure inputs.
11 Calculations Using Pressure Total (stagnation) pressure is defined as:Pressure at thermodynamic state which would exist if fluid were brought to rest (zero velocity) isentropically.1/2 v2 is referred to as the dynamic pressure.Density can be calculated from the ideal gas law:For incompressible flow:For compressible flow:ptotal = pstatic + 1/2 v2
12 Setting Operating Pressure For compressible flows:Set operating pressure = 0Treat gauge pressures as absolute pressuresFor incompressible, constant-density flows, operating pressure not used.For incompressible flows using ideal-gas law to determine densitySelect incompressible-ideal-gas in Define Materials...Set operating pressure close to mean pressure in problem.
13 Pressure Inlet Boundary (1) Defines total pressure, temperature, and other scalar quantities at flow inlets.Also requires direction of velocity vector to be defined.Can get non-physical results if you don’t specify reasonable direction for velocity vector.Useful when:flow rate and/or velocity is not known (e.g., buoyancy-driven flows).“free” boundary in an external or unconfined flow needs to be defined.Suitable for compressible and incompressible flows.
14 Pressure Inlet Boundary (2) Inflow stagnation properties are prescribed:Mechanical head of pressure/total pressure drives flow into computational domain.Mass flux varies depending on interior solution and direction specified for velocity vector.Note:Value specified for total pressure used as static pressure wherever outflow occurs.Total temperature set to static temperature for incompressible flows.
15 Pressure Outlet Boundary (1) Flow exits computational domain at fixed static pressure.Requires specification of static (gauge) pressure at outlet boundary.All other flow quantities at the pressure outlet boundary are extrapolated from the interior.Value of specified static pressure:used only while exit flow is subsonic.ignored for supersonic flow (pressure is extrapolated from flow in interior).
16 Pressure Outlet Boundary (2) “Backflow” conditions must be specified.Required for calculations if flow reverses direction at Pressure Outlet boundary during solution process.When backflow occurs, it is assumed to be normal to the boundary.Cannot specify the direction of the flow entering the domain, in contrast to pressure inlet boundary condition.Convergence difficulties minimized by realistic values for backflow quantities.Value specified for static pressure used as total pressure wherever backflow occurs.Pressure Outlet must be used when problem is set up with Pressure Inlet.
17 Outflow BoundaryFlow exiting domain at Outflow boundary has zero normal gradients for all flow variables except pressure.FLUENT extrapolates required information from interior.Useful when:Details of flow velocity and pressure not known prior to solution of flow problem.Appropriate where exit flow is close to fully developed condition.Note: Use of Pressure Outlet (instead of Outflow) often results in better rate of convergence when backflow occurs during iteration.
18 Restrictions on Outflow Boundaries Outflow Boundaries cannot be used:with compressible flows.with the Pressure Inlet boundary condition (use Velocity Inlet instead):Combination does not uniquely set a pressure gradient over the whole domain.in unsteady flows with variable density.outflow condition ill-posedoutflow condition not obeyedoutflow condition obeyedoutflow condition closely obeyedDo not use outflow boundaries where:Flow enters domainGradients in flow direction are significantConditions downstream of exit plane impact flow in domain
19 Modeling Multiple Exits Using Outflow boundary condition:Mass flow divided equally among all outflow boundaries by default.Flow Rate Weighting (FRW) set to 1 by default.For uneven flow distribution:specify Flow Rate Weighting for each outflow boundary: mi=FRWi/FRWi.static pressure varies among exits to accommodate flow distribution.Can also use Pressure Outlet boundaries to define exits.FRW2velocity inletFRW1pressure-inlet (p0,T0)pressure-outlet (ps)2velocity-inlet (v,T0)pressure-outlet (ps)1or
20 Other Inlet/Outlet Boundary Conditions Mass Flow InletUsed in compressible flows to prescribe mass flow rate at inlet.Not required for incompressible flows.Pressure 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/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) pressure and temperature.
21 Wall, Repeating, and Pole Boundaries Purpose of Boundary ConditionsSetting Boundary ConditionsFlow Inlets and ExitsWall, Repeating, and Pole BoundariesWallSymmetryPeriodicAxisInternal Cell ZonesInternal Face Boundaries
22 Wall Boundaries Used to bound fluid and solid regions. In viscous flows, no-slip condition enforced at wallsTangential velocity component specified in terms of translational or rotational motion of wall boundary.Wall shear stress and heat transfer based on local flow field.Assumed to be rigid and impermeableNormal velocity component = 0For accurate predictions of wall shear stress, be sure to resolve boundary layers in viscous flows.
23 Symmetry Boundaries Used to reduce computational effort in problem. Flow field and geometry must be symmetric:Zero normal velocity at symmetry planeZero normal gradients of all variables at symmetry planeNo inputs required.Must take care to correctly define symmetry boundary locations.Also used to model slip walls in viscous flowsymmetry planes
24 Periodic Boundaries4 tangential inletscyclic boundariesUsed when physical geometry of interest and expected pattern of flow/thermal solution have periodically repeating nature.Reduces computational effort in problem.Two types available in FLUENT 5.Type 1: Does not allow pressure drop across periodic planes.Type 2: Periodic boundaries with pressure drop.Rotationally periodicIJPeriodic at I=NIPeriodic at I=1Translationally periodic
25 Periodic Boundaries with Pressure Drop - Type 2 MUST be translationally periodicDesigned to model fully-developed conditionsFully-developed flow in pipes and ductsTube banksPeriodic heat transfer also possibleSpecify either:Mean pressure gradient per periodNet mass flow ratecomputational domainStreamlines in a 2D tube heat exchangerflow direction
26 Axis Boundaries Used: Specify: At centerline (y=0) of an axisymmetric gridWhere multiple grid lines meet at a point in a 3D O-type gridSpecify:No inputs requiredAXIS boundary
27 Internal Cell Zones Purpose of Boundary Conditions Setting Boundary ConditionsFlow Inlets and ExitsWall, Repeating, and Pole BoundariesInternal Cell ZonesFluidPorousType of fluid zoneSolidInternal Face Boundaries
28 Fluid ConditionsFluid zone = group of cells for which all active equations are solved.Only required input is type of fluid materialSo appropriate material properties usedOptional inputs allow setting of source terms:HeatMassMomentumCan define motion for fluid zoneIf rotationally periodic boundaries adjacent to fluid zone, use rotation axis.Define fluid zone as laminar flow region if modeling transitional flow.TurbulenceSpecies
29 Porous Media Conditions Porous zone modeled as special type of fluid zone.Enable Porous Zone option in Fluid panel.Pressure loss in flow determined via user inputs.Used to model flow through porous media and other “distributed” resistances:Packed bedsFilter papersPerforated platesFlow distributorsTube banks
30 Solid Conditions“Solid” zone = group of cells for which only heat conduction problem solved.No flow equations solvedMaterial being treated as solid may actually be fluid, but it is assumed that no convection takes place.Only required input is material typeSo appropriate material properties used.Optional inputs allow you to set volumetric heat generation rate (heat source).Can define motion for solid zoneNeed to specify rotation axis if rotationally periodic boundaries adjacent to solid zone.
31 Internal Face Boundaries Defined on cell facesDo not have finite thicknessProvide means of introducing step change in flow properties.Used to implement physical models representing:FansRadiatorsPorous jumpThin porous membranesInterior wall
32 Summary Zones are used to assign boundary conditions. Wide range of boundary conditions permit flow to enter and exit solution domain.Wall boundary conditions used to bound fluid and solid regions.Repeating boundaries used to reduce computational effort.Internal cell zones used to specify fluid, solid, and porous regions.Internal face boundaries provide way to introduce step change in flow properties.