Fluent Software Training
TRN-99-003
Boundary Conditions
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Outline
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Overview
Inlet and Outlet Boundaries
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Velocity
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Pressure Boundaries and others...
Wall, Symmetry, Periodic and Axis Boundaries
Internal Cell Zones
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Fluid
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Profiles
Turbulence Parameters
Porous Media
Moving Cell Zones
Solid
Internal Face Boundaries
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Overview
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Boundary Conditions:
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orifice_plate and
orifice_plate-shadow
outlet
e.g., mass, momentum, and energy
Fluid/Solid regions represented by cell
zones.
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Boundaries direct motion of flow.
Boundary Conditions are a required
component of mathematical model.
Specify fluxes into computational domain.
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orifice
(interior)
Material and Source terms are assigned to
cell zones.
Boundaries and internal surfaces are
represented by face zones.
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Boundary data are assigned to face zones.
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wall
inlet
fluid
Example: Face and Cell zones
associated with Pipe Flow
through orifice plate
© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Setting Boundary Conditions
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Zones and zone types are initially defined in
pre-processor.
To change zone type for a particular zone:
Define Õ Boundary Conditions...
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Choose the zone in Zone list.
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Select new zone type in Type list.
To set boundary conditions for particular zone:
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Can also select boundary zone using right
mouse button in Display Grid window.
Choose the zone in Zone list.
Click Set... button
Boundary condition data can be copied from one zone to another.
Boundary condition data can be stored and retrieved from file.
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file ® write-bc
file ® read-bc
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Flow Inlets and Outlets
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Wide range of boundary conditions types permit flow to enter and exit
solution domain:
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General
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Pressure inlet
Pressure outlet
Compressible flows
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Incompressible
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Velocity inlet
Outflow
Mass flow inlet
Pressure far-field
Special
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Inlet vent, outlet vent,
intake fan, exhaust fan
Boundary data required depends on physical models selected.
General guidelines:
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Select boundary location and shape such that flow either goes in or out.
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Should not observe large gradients in direction normal to boundary.
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Not necessary, but will typically observe better convergence.
Indicates incorrect set-up.
Minimize grid skewness near boundary.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Velocity Inlets
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Defines velocity vector and scalar
properties of flow at inlet boundaries.
Useful when velocity profile is
known at inlet.
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uniform profile is default
Intended for incompressible flows.
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Total (stagnation) properties of flow
are not fixed.
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Stagnation properties vary to accommodate prescribed velocity distribution.
Using in compressible flows can lead to non-physical results.
Avoid placing velocity inlet too close to a solid obstruction.
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Can force the solution to be non-physical, e.g., imposes velocity field, etc., at
boundary that may not be intended.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Using Profiles
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Alternative to UDF’s for defining
boundary profiles.
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Profiles can be generated by:
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Writing a profile from another CFD
simulation
Creating an appropriately formatted text
file with location information and
boundary condition data.
Profiles can be manipulated through:
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Profiles can define spatial and time
varying boundary conditions.
Define à Profiles
Profiles data applied to boundary
through ‘hooks’.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Determining Turbulence Parameters
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When turbulent flow enters domain at inlet, outlet, or at a far-field
boundary, FLUENT 5 requires boundary values for:
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Turbulence dissipation rate ε
Four methods available for specifying turbulence parameters:
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Turbulent kinetic energy k
Set k and ε explicitly
Set turbulence intensity and turbulence length scale
Set turbulence intensity and turbulent viscosity ratio
Set turbulence intensity and hydraulic diameter
Intensity and length scale depend on conditions upstream, e.g.:
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Exhaust of a turbine
Intensity = 20 %
Length scale = 1 - 10 % of blade span
Downstream of perforated plate or screen
Intensity = 10 %
Length scale = screen/hole size
Fully-developed flow in a duct or pipe
Intensity = 5 %
Length scale = hydraulic diameter
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Pressure Boundary Conditions
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pressure
level
Pressure boundary conditions require
gauge pressure inputs:
gauge
pressure
p absolute = p gauge + p operating
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Operating pressure input is set under:
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Define → Operating Conditions
Useful when:
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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.
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absolute
pressure
operating
pressure
operating
pressure
vacuum
© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Pressure Inlet Boundary (1)
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Defines total pressure, temperature,
and other scalar quantities at flow
inlets.
ptotal = pstatic +
1 2
ρv
2
ptotal = pstatic (1 +
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k − 1 2 k /( k −1)
M )
2
compressible flows
Supersonic/Initial Gauge Pressure:
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incompressible flows
Defines static pressure at boundary
for locally supersonic flows.
Used, if necessary, to initialize flow field for incompressible flows.
Total temperature:
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must be defined for compressible flows.
is used, if necessary, to set static temperature for incompressible flows.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Pressure Inlet Boundary (2)
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Flow Direction must be defined.
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Suitable for compressible and incompressible flows.
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Can get non-physical results if you don’t specify a reasonable direction.
Pressure inlet boundary is treated as loss-free transition from stagnation
to inlet conditions.
Mass flux through boundary varies depending on interior solution and
specified flow direction.
Outflow can occur at pressure inlet boundaries.
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Flow direction taken from interior solution.
Exhaust static pressure is defined by value specified for gauge total
pressure wherever outflow occurs.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Pressure Outlet Boundary (1)
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Defines static (gauge) pressure at
the outlet boundary.
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Interpreted as static pressure of
environment into which flow
exhausts.
Radial equilibrium pressure
distribution option available.
Backflow can occur at pressure
outlet boundaries:
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during solution process or as part of solution.
Backflow is assumed to be normal to the boundary.
Convergence difficulties minimized by realistic values for backflow quantities.
Value specified for static pressure used as total pressure wherever backflow
occurs.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Pressure Outlet Boundary (2)
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For incompressible flows:
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For compressible flows:
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The static pressure input defines the boundary pressure
All other flow quantities are extrapolated from the interior.
The static pressure input is ignored if locally supersonic.
All flow quantities are extrapolated from interior.
Pressure Outlet must be used when problem is set up with Pressure
Inlet.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Outflow Boundary
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Flow exiting domain at Outflow boundary has zero normal
gradients for all flow variables except pressure.
FLUENT extrapolates required information from interior.
Useful when:
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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.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Restrictions on Outflow Boundaries
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Outflow Boundaries cannot be used:
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with compressible flows.
with the Pressure Inlet boundary condition (use Velocity Inlet instead):
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Combination does not uniquely set a pressure gradient over the whole domain.
in unsteady flows with variable density.
Do not use outflow
boundaries where:
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Flow enters domain
Gradients in flow
direction are significant
Conditions downstream
of exit plane impact
flow in domain
outflow
condition
ill-posed
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outflow
condition
not obeyed
outflow
condition
obeyed
outflow
condition
closely
obeyed
© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Modeling Multiple Exits
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Using Outflow boundary condition:
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outflow boundaries by default.
l Flow Rate Weighting (FRW) set to 1 by
default.
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specify Flow Rate Weighting for each
outflow boundary: mi=FRWi/ΣFRWi.
static pressure varies among exits to
accommodate flow distribution.
Can also use Pressure Outlet boundaries
to define exits.
velocity-inlet (v,T0)
or
pressure-inlet (p0,T0)
FRW1
velocity
inlet
FRW2
pressure-outlet
(ps)1
pressure-outlet
(ps)2
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Other Inlet/Outlet Boundary Conditions
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Mass Flow Inlet
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Pressure Far Field
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Available 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 Vent
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Used in compressible flows to prescribe mass flow rate at inlet.
Not required for incompressible flows.
Model external exhaust fan/outlet vent with specified pressure jump/loss
coefficient and ambient (discharge) pressure and temperature.
Inlet Vent/Intake Fan
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Model inlet vent/external intake fan with specified loss coefficient/
pressure jump, flow direction, and ambient (inlet) pressure and
temperature.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Wall Boundaries
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Used to bound fluid and solid regions.
In viscous flows, no-slip condition
enforced at walls:
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Thermal boundary conditions:
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several types available.
Wall material and thickness can be defined for 1-D or in-plane thin plate heat
transfer calculations.
Wall roughness can be defined for turbulent flows.
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Tangential fluid velocity equal
to wall velocity.
Normal velocity component = 0
Wall shear stress and heat transfer based on local flow field.
Translational or rotational velocity can be assigned to wall.
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Shear stress can also be specified.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Symmetry Boundaries
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Used to reduce computational effort in problem.
Flow field and geometry must be symmetric:
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No inputs required.
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Zero normal velocity at symmetry plane
Zero normal gradients of all variables at symmetry plane
Must take care to correctly define symmetry boundary locations.
Also used to model slip walls in viscous flow
symmetry
planes
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Periodic Boundaries
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Used when physical geometry of interest and expected pattern of
flow/thermal solution have periodically repeating nature.
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Reduces computational effort in problem.
Two types available in FLUENT 5.
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∆p = 0 across periodic planes.
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∆p is finite across periodic planes.
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Rotationally or translationally periodic.
s Rotationally periodic boundaries require axis of rotation be defined in
fluid zone.
Translationally periodic only.
Models fully developed conditions.
Specify either mean ∆p per period or net mass flow rate.
By default, periodic boundaries defined in Gambit are assumed to be
translational in FLUENT 5.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Periodic Boundaries: Examples
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∆p = 0:
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∆p > 0:
4 tangential
inlets
computational
domain
flow
direction
Rotationally periodic boundaries
Streamlines in
a 2D tube heat
exchanger
Translationally periodic boundaries
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Axis Boundaries
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Used:
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At centerline (y=0) of an
axisymmetric grid
Where multiple grid lines meet
at a point in a 3D O-type grid
Specify:
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No inputs required
AXIS
boundary
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Cell Zones: Fluid
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Fluid zone = group of cells for
which all active equations are
solved.
Fluid material input required.
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Optional inputs allow setting
of source terms:
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Single species, phase.
mass, momentum, energy, etc.
Define fluid zone as laminar flow
region if modeling transitional flow.
Can define zone as porous media.
Define axis of rotation for rotationally periodic flows.
Can define motion for fluid zone.
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Porous Media Conditions
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Porous zone modeled as special type of fluid zone.
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Enable Porous Zone option in Fluid panel.
Pressure loss in flow determined via user inputs
of resistance coefficients to lumped parameter
model.
Used to model flow through porous media
and other “distributed” resistances, e.g.,
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Packed beds
Filter papers
Perforated plates
Flow distributors
Tube banks
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Moving Zones
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Single Zone Problems:
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Rotating Reference Frame Model
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define zone as Moving Reference Frame
limited applicability
Multiple Zone Problems:
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Each zone defined as moving reference frame:
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Multiple Reference Frame Model
s least accurate, least demanding on CPU
Mixing Plane Model
s field data are averaged at the outlet of one zone
and used as inlet boundary data to adjacent zone.
Each zone defined as Moving Mesh:
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Sliding Mesh Model
s must also define interface.
s Mesh positions are calculated; time-accurate simulations
s relative motion must be tangential (no normal translation)
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Cell Zones: Solid
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“Solid” zone = group of cells for which only
heat conduction problem solved.
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Material being treated as solid may actually be
fluid, but it is assumed that no convection
takes place.
Only required input is material type
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No flow equations solved
So appropriate material properties used.
Optional inputs allow you to set volumetric
heat generation rate (heat source).
Need to specify rotation axis if rotationally
periodic boundaries adjacent to solid zone.
Can define motion for solid zone
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Internal Face Boundaries
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Defined on cell faces
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Do not have finite thickness
Provide means of introducing step change in flow properties.
Used to implement physical models representing:
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Fans
Radiators
Porous jump
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Preferable over porous media- exhibits better convergence behavior.
Interior wall
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© Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Summary
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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.
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© Fluent Inc. 2/20/01