Fluent Bcs

© Fluent Inc. 2/20/01C1

Fluent Software TrainingTRN-99-003

Boundary Conditions



Outlineu Overviewu Inlet and Outlet Boundaries

l Velocityn Profilesn Turbulence Parameters

l Pressure Boundaries and others...

u Wall, Symmetry, Periodic and Axis Boundariesu Internal Cell Zones

l Fluidn Porous Median Moving Cell Zones

l Solid

u Internal Face Boundaries



Overview

u Boundary Conditions:l Boundaries direct motion of flow.l Boundary Conditions are a required

component of mathematical model.

u Specify fluxes into computational domain.l e.g., mass, momentum, and energy

u Fluid/Solid regions represented by cellzones.l Material and Source terms are assigned to

cell zones.

u Boundaries and internal surfaces arerepresented by face zones.l Boundary data are assigned to face zones.

Example: Face and Cell zonesassociated with Pipe Flowthrough orifice plate

inlet

outlet

wall

orifice(interior)

orifice_plate andorifice_plate-shadow

fluid



Setting Boundary Conditionsu Zones and zone types are initially defined in

pre-processor.u To change zone type for a particular zone:

Define Õ Boundary Conditions...

l Choose the zone in Zone list.n Can also select boundary zone using right

mouse button in Display Grid window.

l Select new zone type in Type list.

u To set boundary conditions for particular zone:l Choose the zone in Zone list.l Click Set... button

u Boundary condition data can be copied from one zone to another.u Boundary condition data can be stored and retrieved from file.

l file ® write-bcl file ® read-bc



Flow Inlets and Outletsu Wide range of boundary conditions types permit flow to enter and exit

solution domain:l General

n Pressure inletn Pressure outlet

l Incompressiblen Velocity inletn Outflow

u Boundary data required depends on physical models selected.u General guidelines:

l Select boundary location and shape such that flow either goes in or out.n Not necessary, but will typically observe better convergence.

l Should not observe large gradients in direction normal to boundary.n Indicates incorrect set-up.

l Minimize grid skewness near boundary.

l Compressible flowsn Mass flow inletn Pressure far-field

l Specialn Inlet vent, outlet vent,

intake fan, exhaust fan



Velocity Inlets

u Defines velocity vector and scalarproperties of flow at inlet boundaries.

u Useful when velocity profile isknown at inlet.l uniform profile is default

u Intended for incompressible flows.l Total (stagnation) properties of flow

are not fixed.n Stagnation properties vary to accommodate prescribed velocity distribution.

l Using in compressible flows can lead to non-physical results.

u Avoid placing velocity inlet too close to a solid obstruction.l Can force the solution to be non-physical, e.g., imposes velocity field, etc., at

boundary that may not be intended.



Using Profiles

u Alternative to UDF’s for definingboundary profiles.l Profiles can define spatial and time

varying boundary conditions.

u Profiles can be generated by:l Writing a profile from another CFD

simulationl Creating an appropriately formatted text

file with location information andboundary condition data.

u Profiles can be manipulated through:l Define à Profiles

u Profiles data applied to boundarythrough ‘hooks’.



Determining Turbulence Parametersu When turbulent flow enters domain at inlet, outlet, or at a far-field

boundary, FLUENT 5 requires boundary values for:l Turbulent kinetic energy k l Turbulence dissipation rate ε

u Four methods available for specifying turbulence parameters:l Set k and ε explicitlyl Set turbulence intensity and turbulence length scalel Set turbulence intensity and turbulent viscosity ratiol Set turbulence intensity and hydraulic diameter

u Intensity and length scale depend on conditions upstream, e.g.:l Exhaust of a turbine

Intensity = 20 % Length scale = 1 - 10 % of blade spanl Downstream of perforated plate or screen

Intensity = 10 % Length scale = screen/hole sizel Fully-developed flow in a duct or pipe

Intensity = 5 % Length scale = hydraulic diameter



Pressure Boundary Conditions

u Pressure boundary conditions requiregauge pressure inputs:

u Operating pressure input is set under:l Define → Operating Conditions

u Useful when:l flow rate and/or velocity is not

known (e.g., buoyancy-driven flows).

l “free” boundary in an external orunconfined flow needs to be defined.

operatinggaugeabsolute ppp += gaugepressure

operatingpressure

pressurelevel

operatingpressure

absolutepressure

vacuum



Pressure Inlet Boundary (1)u Defines total pressure, temperature,

and other scalar quantities at flowinlets.

u Supersonic/Initial Gauge Pressure:l Defines static pressure at boundary

for locally supersonic flows.l Used, if necessary, to initialize flow field for incompressible flows.

u Total temperature:l must be defined for compressible flows.l is used, if necessary, to set static temperature for incompressible flows.

)1/(2)2

11( −−

+= kkstatictotal M

kpp

2

21

vpp statictotal ρ+= incompressible flows

compressible flows



Pressure Inlet Boundary (2)

u Flow Direction must be defined.l Can get non-physical results if you don’t specify a reasonable direction.

u Suitable for compressible and incompressible flows.l Pressure inlet boundary is treated as loss-free transition from stagnation

to inlet conditions.l Mass flux through boundary varies depending on interior solution and

specified flow direction.

u Outflow can occur at pressure inlet boundaries.l Flow direction taken from interior solution.l Exhaust static pressure is defined by value specified for gauge total

pressure wherever outflow occurs.



Pressure Outlet Boundary (1)u Defines static (gauge) pressure at

the outlet boundary.l Interpreted as static pressure of

environment into which flowexhausts.

u Radial equilibrium pressuredistribution option available.

u Backflow can occur at pressureoutlet boundaries:l during solution process or as part of solution.l Backflow is assumed to be normal to the boundary.l Convergence difficulties minimized by realistic values for backflow quantities.l Value specified for static pressure used as total pressure wherever backflow

occurs.



Pressure Outlet Boundary (2)

u For incompressible flows:l The static pressure input defines the boundary pressurel All other flow quantities are extrapolated from the interior.

u For compressible flows:l The static pressure input is ignored if locally supersonic.l All flow quantities are extrapolated from interior.

u Pressure Outlet must be used when problem is set up with PressureInlet.



Outflow Boundary

u Flow exiting domain at Outflow boundary has zero normalgradients for all flow variables except pressure.

u FLUENT extrapolates required information from interior.u Useful when:

l Details of flow velocity and pressure not known prior to solution offlow problem.

l Appropriate where exit flow is close to fully developed condition.

u Note: Use of Pressure Outlet (instead of Outflow) often results inbetter rate of convergence when backflow occurs duringiteration.



Restrictions on Outflow Boundariesu Outflow Boundaries cannot be used:

l with compressible flows.l with the Pressure Inlet boundary condition (use Velocity Inlet instead):

n Combination does not uniquely set a pressure gradient over the whole domain.

l in unsteady flows with variable density.

u Do not use outflowboundaries where:l Flow enters domainl Gradients in flow

direction are significantl Conditions downstream

of exit plane impactflow in domain

outflowconditionill-posed

outflow conditionnot obeyed

outflowconditionobeyed

outflowconditioncloselyobeyed



Modeling Multiple Exitsu Using Outflow boundary condition:

l Mass flow divided equally among alloutflow boundaries by default.

l Flow Rate Weighting (FRW) set to 1 bydefault.

l For uneven flow distribution:n specify Flow Rate Weighting for each

outflow boundary: mi=FRWi/ΣFRWi.n static pressure varies among exits to

accommodate flow distribution.

u Can also use Pressure Outlet boundariesto define exits.

pressure-inlet (p0,T0) pressure-outlet(ps)2

velocity-inlet (v,T0)pressure-outlet(ps)1

or

FRW2

velocityinlet

FRW1



Other Inlet/Outlet Boundary Conditionsu Mass Flow Inlet

l Used in compressible flows to prescribe mass flow rate at inlet.l Not required for incompressible flows.

u Pressure Far Fieldl Available when density is calculated from the ideal gas law.l Used to model free-stream compressible flow at infinity, with free-stream

Mach number and static conditions specified.

u Exhaust Fan/Outlet Ventl Model external exhaust fan/outlet vent with specified pressure jump/loss

coefficient and ambient (discharge) pressure and temperature.

u Inlet Vent/Intake Fanl Model inlet vent/external intake fan with specified loss coefficient/

pressure jump, flow direction, and ambient (inlet) pressure andtemperature.



Wall Boundaries

u Used to bound fluid and solid regions.u In viscous flows, no-slip condition

enforced at walls:l Tangential fluid velocity equal

to wall velocity.l Normal velocity component = 0

u Thermal boundary conditions:l several types available.l Wall material and thickness can be defined for 1-D or in-plane thin plate heat

transfer calculations.

u Wall roughness can be defined for turbulent flows.l Wall shear stress and heat transfer based on local flow field.

u Translational or rotational velocity can be assigned to wall.l Shear stress can also be specified.



Symmetry Boundaries

u Used to reduce computational effort in problem.u Flow field and geometry must be symmetric:

n Zero normal velocity at symmetry planen Zero normal gradients of all variables at symmetry plane

u No inputs required.l Must take care to correctly define symmetry boundary locations.

u Also used to model slip walls in viscous flow

symmetryplanes



Periodic Boundaries

u Used when physical geometry of interest and expected pattern offlow/thermal solution have periodically repeating nature.l Reduces computational effort in problem.

u Two types available in FLUENT 5.l ∆p = 0 across periodic planes.

n Rotationally or translationally periodic.s Rotationally periodic boundaries require axis of rotation be defined in

fluid zone.

l ∆p is finite across periodic planes.n Translationally periodic only.n Models fully developed conditions.n Specify either mean ∆p per period or net mass flow rate.

l By default, periodic boundaries defined in Gambit are assumed to betranslational in FLUENT 5.



Periodic Boundaries: Examples

computationaldomain

Streamlines ina 2D tube heatexchanger

flowdirection

Translationally periodic boundaries

4 tangentialinlets

Rotationally periodic boundaries

l ∆p = 0: l ∆p > 0:



Axis Boundaries

u Used:l At centerline (y=0) of an

axisymmetric gridl Where multiple grid lines meet

at a point in a 3D O-type grid

u Specify:l No inputs required

AXISboundary



Cell Zones: Fluidu Fluid zone = group of cells for

which all active equations aresolved.

u Fluid material input required.l Single species, phase.

u Optional inputs allow settingof source terms:l mass, momentum, energy, etc.

u Define fluid zone as laminar flowregion if modeling transitional flow.

u Can define zone as porous media.u Define axis of rotation for rotationally periodic flows.u Can define motion for fluid zone.



Porous Media Conditions

u Porous zone modeled as special type of fluid zone.l Enable Porous Zone option in Fluid panel.l Pressure loss in flow determined via user inputs

of resistance coefficients to lumped parametermodel.

u Used to model flow through porous mediaand other “distributed” resistances, e.g.,l Packed bedsl Filter papersl Perforated platesl Flow distributorsl Tube banks



Moving Zonesu Single Zone Problems:

l Rotating Reference Frame Modeln define zone as Moving Reference Framen limited applicability

u Multiple Zone Problems:l Each zone defined as moving reference frame:

n Multiple Reference Frame Models least accurate, least demanding on CPU

n Mixing Plane Models field data are averaged at the outlet of one zone

and used as inlet boundary data to adjacent zone.

l Each zone defined as Moving Mesh:n Sliding Mesh Model

s must also define interface.s Mesh positions are calculated; time-accurate simulationss relative motion must be tangential (no normal translation)



Cell Zones: Solidu “Solid” zone = group of cells for which only

heat conduction problem solved.l No flow equations solved

u Material being treated as solid may actually befluid, but it is assumed that no convectiontakes place.

u Only required input is material typel So appropriate material properties used.

u Optional inputs allow you to set volumetricheat generation rate (heat source).

u Need to specify rotation axis if rotationallyperiodic boundaries adjacent to solid zone.

u Can define motion for solid zone



Internal Face Boundaries

u Defined on cell facesl Do not have finite thicknessl Provide means of introducing step change in flow properties.

u Used to implement physical models representing:l Fansl Radiatorsl Porous jump

n Preferable over porous media- exhibits better convergence behavior.

l Interior wall



Summary

u Zones are used to assign boundary conditions.u Wide range of boundary conditions permit flow to enter and exit

solution domain.u Wall boundary conditions used to bound fluid and solid regions.u Repeating boundaries used to reduce computational effort.u Internal cell zones used to specify fluid, solid, and porous regions.u Internal face boundaries provide way to introduce step change in flow

properties.

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