Chapter 4
BOUNDARY CONDITIONS
Boundary Condition Overview
Well Posed Problems
•
Categorization of boundaries
– Flow Boundary Conditions (loads)
– Thermal Boundary Conditions (loads)
•
Application
– Commands
CFD ANALYSIS 5.7
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Training Manual
– Graphical User Interface
– Nodal or Solid Model
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General Comments
Training Manual
In CFD analyses, almost every boundary must be accounted for in
some fashion.
•
If a boundary condition is not specified for a dependent variable,
the assumption is that the derivative of the variable normal to the
surface is zero.
•
Boundary conditions consist of conditions applied to any of the
velocity degrees of freedom (VX,VY,VZ), or pressure (PRES), the
turbulence quantities (ENKE,ENDS) or the temperature (TEMP).
Heat fluxes, film coefficients, and volumetric heat sources may
also be applied.
•
Boundary conditions are set on nodes and elements with the D
and SF commands.
•
Solid model boundary conditions are applied with the DL, DA,
SFL, and SFA commands… (NOT the DK command)
CFD ANALYSIS 5.7
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Boundary Condition Types
Inlet/Outlet
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Symmetry Condition
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Stationary Wall, Moving Wall
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Periodic Boundaries
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Fixed Temperature
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Heat Flux
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Volumetric Heat Source
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Heat Transfer (Film) Coefficient
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Radiation
CFD ANALYSIS 5.7
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Training Manual
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Inlets/Outlets
Problems are generally velocity driven or pressure driven.
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Velocity into or out of the problem domain may be specified
CFD ANALYSIS 5.7
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Training Manual
– Constant value
Profile constructed with function or table
•
Pressure may be specified across a boundary
– Flow may enter or leave.
– Locate pressure boundaries away from significant geometry changes.
– Typically, Pressure set to 0 at outlet.
•
Turbulence Boundary conditions
– Specification optional
FLOTRAN will define defaults (generally, turbulent boundary
conditions need not be known in detail as the flow conditions
themselves generate most of the turbulence.)
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Symmetry Boundary
Training Manual
Used to represent either plane of symmetry or axisymmetric
centerline.
•
Assumption is that flow patterns are symmetric.
•
Velocity component normal to symmetry boundary is set to
zero.
•
Prevents mass or heat transfer across boundary.
•
Symmetry boundary must be aligned with a global coordinate
axis.
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The User must determine whether or not transient effects might
result in asymmetric flow patterns. Obviously these effects will
not be modeled with a symmetric geometry.
CFD ANALYSIS 5.7
•
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Stationary Wall
Apply no-slip boundary condition.
– All velocity components set to zero.
•
Turbulence model boundary conditions automatically applied.
– Law of the Wall, Log-Law of the Wall ( methods of treating conditions
very close to the wall)
– They are a part of the FLOTRAN turbulence modeling scheme. No
user action is required to implement wall turbulence conditions
•
If no thermal boundary conditions are specified, wall is treated as
adiabatic boundary.
•
No pressure specification necessary.
CFD ANALYSIS 5.7
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Training Manual
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Moving Wall
CFD ANALYSIS 5.7
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Training Manual
Moving wall - Stationary boundary.
– Problem domain cannot change shape.
– Examples:
• Flow between rotating cylinders
• Flow past cars: movement of the ground
•
Wall “drags” flow or moves with it.
•
Specify velocity normal to wall as zero.
•
Specify velocity tangent to wall as wall speed.
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Tell FLOTRAN that this is not an inlet!
– Flag is to set turbulent kinetic energy (ENKE) to -1.
•
Specified as moving wall via GUI in Velocity BC Dialog Box under
loads
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Internal Flows
Training Manual
CFD ANALYSIS 5.7
Tcold
Thot
Forced Flow
Gravity
Natural Convection
•
Internal flow problems are bounded by walls, symmetry planes,
and inlets/outlets.
•
There need not be any inlets/outlets.
•
Unspecified boundaries are permitted, but can be unstable.
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External Flows
Training Manual
CFD ANALYSIS 5.7
VX, VY
P=0
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Notes on External Flows
Training Manual
The boundary must be located far from the body.
•
Typically, the back half of the boundary has a zero relative
pressure applied.
•
In supersonic problems, pressure and velocity may be specified in
the forward section, with the back half left as an unspecified
boundary.
•
Make sure that any shock waves do not extend all the way to a
specified boundary. Remove the specified condition if it does.
•
It is sometimes desirable to specify the velocity condition all
around the problem domain. (Usually requires use of initial
conditions)
•
Considering a characteristic dimension of the body as a chord
length, the boundaries of the problem domain should be 30-100
chord lengths from the body.
CFD ANALYSIS 5.7
•
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Periodic Boundaries
Training Manual
Flow at corresponding nodes of two boundaries is unknown but
identical.
•
Boundaries must have identical node patterns.
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All DOF's are coupled for each periodic node pair.
•
Apply with peri.mac macro, supplied with FLOTRAN.
CFD ANALYSIS 5.7
•
– Select nodes on one boundary.
– Specify spatial offset between the boundaries (the selected set is the
base).
– Not accessed in GUI
•
Not available as solid model load
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Periodic Boundaries
Training Manual
CFD ANALYSIS 5.7
FLOTRAN Polar Coordinates
The PERI macro checks the keyopt
to determine the FLOTRAN
coordinate system being used and
interprets the offsets accordingly.
Select These Nodes
Peri,0,90,0
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Well Posed Problems
Make sure enough boundary conditions are applied.
•
Problems with two flow boundaries are straightforward:
– Known pressure drop > calculate flow
– Known inlet flow rate > calculate pressure drop
•
Three Inlets/Outlets
– Suppose two flows are specified….
CFD ANALYSIS 5.7
•
Training Manual
• Specify pressure at third boundary
– One flow is specified..
• Specify pressure at two boundaries
– Try to determine from boundary conditions and geometry whether
multiple solutions are physically possible...
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More on Velocity Conditions
Training Manual
For 2D problems, the user must determine if the nodes at the endpoints of the
line are to be set. Generally, they are.. The same is true of 3D problems and
areas. Note the choice of Keypoints is not allowed for FLOTRAN loadings.
The best procedure for the edges of areas is to use the boundary lines option,
rather than the “apply velocity on lines”.
•
If the inlet nodes have been previously selected, click on “Pick all in the Picker
and then OK”. If they haven't, use the picker to select the desired nodes
before OK.
•
Note that for walls, simply select VX,VY and if necessary VZ in the Apply V on
Nodes menu.
•
It is important to note that the priority of solid model velocity boundary
conditions is correctly observed by FLOTRAN automatically. That is, the zero
condition of a wall will overwrite an inlet velocity, but an inlet velocity will not
overwrite a wall condition. In other words, the numbering scheme of the
areas or the order in which the solid model boundary conditions are applied
do not matter.
•
ANSYS may, however, produce some warning messages regarding overwriting
conditions.
CFD ANALYSIS 5.7
•
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Thermal Boundary Conditions
Specified Temperature
– FLOTRAN calculates the heat transfer required to maintain the wall at
the specified temperature.
•
Specified Heat Flux
– FLOTRAN calculates the wall temperature associated with the
specified heat flux and the flow conditions.
•
CFD ANALYSIS 5.7
•
Training Manual
Specified Heat Transfer (Film) Coefficient
– Specify Ambient (Bulk) Temperature . FLOTRAN iterates to calculate
the heat flux and the surface temperature.
•
Volumetric Heat Sources
– Can be specified in fluid or non-fluid elements.
•
Radiation (external or surface-to-surface)
– Specify surface emissivity and ambient temperature.
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More on Thermal Boundary Conditions
Training Manual
CFD ANALYSIS 5.7
•
The heat transfer coefficient can be calculated for output at all
thermal boundaries where it is not specified. It will be based on
the calculated or input heat flux, the calculated or specified
surface temperature, and the specified ambient temperature.
•
Note that surface boundary conditions cannot be prescribed on
internal faces.
•
In a compressible adiabatic problem, specified inlet temperatures
are ignored, as FLOTRAN uses the specified total temperature and
the velocity to calculate the static temperature everywhere.
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Summary of Boundary Conditions
Symmetry
Known Temperature
Adiabatic
Heat Flux
Film Coefficient
Volumetric Heat
Source
Periodic
Required Data
Specify all velocity components or specify pressure
Specify pressure (usually zero)
Specify all velocity components as zero
Specify all velocity components and turbulent
kinetic energy as -1.0
Specify the normal velocity as zero
Specify nodal temperature values
Natural boundary condition, no action required
Specify heat flux on boundary nodes
Specify film coefficient on boundary nodes
Specify heat generation per unit volume for
elements
Specify geometry data to locate periodic nodes
The negative kinetic energy specification at a moving wall is used as a flag to
activate the wall turbulence calculation. It is done under VELOCITY in the GUI.
CFD ANALYSIS 5.7
Boundary Type
Inflow
Outflow
Wall
Moving Wall
Training Manual
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Boundary Condition Priority
at Intersection Boundaries
Intersection
Boundary 2
Inflow
VX = Value
VY = Value
VZ = Value
Inflow
VX = Value
VY = Value
VZ = Value
Inflow
VX = Value
VY = Value
VZ = Value
Symmetry
VX = 0 or
VY = 0 or
VZ = 0
Symmetry
VX = 0 or
VY = 0 or
VZ = 0
Wall
VX = 0
VY = 0
VZ = 0
Wall
VX = 0
VY = 0
VZ = 0
Symmetry
VX = 0 or
VY = 0 or
VZ = 0
Outflow
P=0
Outflow
P=0
Wall
VX = 0
VY = 0
VZ = 0
Outflow
P=0
Action
Overwrite the inflow with the wall
condition
Combine the symmetry with the inflow
condition
Overwrite the inflow with the outflow
condition
CFD ANALYSIS 5.7
Intersecting
Boundary 1
Training Manual
Combine the symmetry with outflow
condition
Combine the symmetry with the wall
condition
Combine the wall with outflow condition
Note that one boundary condition can have a priority over another
even if the degrees of freedom do not conflict.
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Boundary Condition Priority
at Intersecting Boundaries
Inlet VX = 10
VY = 0
Apply Inlet
Condition
CFD ANALYSIS 5.7
Wall conditions
Prevail
Wall
VX = 0, VY = 0
Condition
Training Manual
Combine wall
and outflow
P = Outlet
Symmetry, VY = 0
Combine
& Outflow
Symmetry
The solid model boundary condition transfer ensures that a zero velocity
condition at a boundary is not overridden by a non-zero value. This
means that the wall condition will prevail at the intersection of a wall and
an inlet.
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The GUI and Boundary Conditions
Preprocessor or Solution
Phase
•
Specification of the Element
type provides filtering so that
only the FLOTRAN CFD
conditions are presented.
CFD ANALYSIS 5.7
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Training Manual
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Velocity
Training Manual
CFD ANALYSIS 5.7
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Velocity on Areas
Training Manual
CFD ANALYSIS 5.7
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Pressure DOF
Training Manual
CFD ANALYSIS 5.7
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Turbulent DOF
Training Manual
CFD ANALYSIS 5.7
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Volumetric Heat Sources
Training Manual
CFD ANALYSIS 5.7
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Heat Flux
Training Manual
CFD ANALYSIS 5.7
The SF or SFE command is
used. Application is to nodes
or element faces. Apply with
a constant value. (Ensure
Face Number is correct!)
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Convection
Training Manual
CFD ANALYSIS 5.7
• The SF or SFE command is
used.
• The bulk temperature
specification is used to
evaluate heat transfer at the
surface.
• If the tapered option is used,
the settings on all but the
Note that the bulk temperature
first node will be ignored.
may vary with table input
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Radiation : Ambient
Training Manual
CFD ANALYSIS 5.7
Make sure Stefan-Boltzmann constant is in the correct Units! ANSYS
default value is in (BTU/hr/in2/R4)
5.67x10-8 (Watts/m2/K4)
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Radiation : Surface-to-Surface
Training Manual
CFD ANALYSIS 5.7
•
Up to 10 enclosures may be defined.
•
All surfaces in an enclosure look at each other.
•
The effect on the fluid temperature is indirect, being manifested
by changes in surface temperatures….
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Non-Constant Boundary Conditions
Tables - “This Vs. That” (see Basic Procedures, Loading)
– Detailed example in Thermal chapter.
– Independent variables as a function of the load applied are shown on
the following page.
– Tables may be created at any time
CFD ANALYSIS 5.7
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Training Manual
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FLOTRAN Analyses
Training Manual
Independent Variable
Nodal DOF
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
Nodal DOF for ALE formulation
TIME, X, Y, Z, TEMP, VELOCITY, PRES, Xr, Yr, Zr
Heat Flux/Film Coefficient
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
Element Heat Generation
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
Nodal Heat Generation
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
Nodal Body Force
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
Radiation
TIME, X, Y, Z, TEMP, VELOCITY, PRESSURE
CFD ANALYSIS 5.7
Condition
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Displaying Boundary Condition
Symbols
Training Manual
When boundary conditions are applied, the symbol for plotting
them is automatically turned on.
•
For large models, plotting the nodes with the boundary condition
symbols turned on can be time consuming.
– Turn off Symbols after initial verification of model
– Utility Menu: Plot Ctrls > Symbols
CFD ANALYSIS 5.7
•
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Symbols...
Training Manual
CFD ANALYSIS 5.7
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Initial Conditions
Training Manual
Initial conditions are applied at the
beginning of a transient analysis.
•
External flow problems benefit
from using the free stream velocity
as an initial condition for a steady
state analysis.
•
Loads > Apply…
2
CFD ANALYSIS 5.7
•
3
4 - Set a value...
1
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Creation of An “Infinitely Thin” Wall
Training Manual
The goal is to avoid the solid model inconvenience of modeling
the actual thickness of a separating wall.
•
The procedure involves merging nodes rather than solid model
entities such as keypoints.
•
(Why not just set velocities at a line to zero? Because then
pressure would be continuous across the wall…)
•
Example Problem
CFD ANALYSIS 5.7
•
– Re = 100
– Model an obstruction (wall) in the flow path of a duct
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Create Geometry
Create the four areas with obstruction at the end of the first set of
areas…
– Total height of flow path: 1.0
– Distance to obstruction: 3.0
– Distance behind obstruction: 7.0
– Height of obstruction: 0.5
•
CFD ANALYSIS 5.7
•
Training Manual
Do Not Merge Keypoints !!
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Establish line Divisions - flow direction
Training Manual
The lines in the main flow direction of area 1 must be the same as
2, and 3 must match 4.
•
Bias the mesh towards the inlet and walls…..
•
Note that the lines on top of each other at the boundaries must
each be done. Flip the bias of the outlet area lines if necessary...
CFD ANALYSIS 5.7
•
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The Transverse Direction
Training Manual
The two “upper areas” must have the same number of divisions
(e.g. 9)
•
The two “lower areas” must each have different numbers of
elements (e.g. 9 for the inlet, 10 for the outlet)
•
Bias the mesh towards the walls and the tip of the obstruction.
•
Take care, as there are lines on top of each other ….
•
(The picture has the Y distortion factor set to 5)
9
CFD ANALYSIS 5.7
•
10
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Mesh The Areas
No merging of any entities is done before the meshing!
•
Unselect the nodes at the bottom of the wall on both sides!!
•
Then merge the nodes..
CFD ANALYSIS 5.7
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Training Manual
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The Merging of Nodes
Merge the nodes and you will get the following message
•
Take care that a line is preserved on each side of the wall
CFD ANALYSIS 5.7
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Training Manual
– Boundary conditions will subsequently be applied to both sides of the
wall.
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Checking the Model
Select the external nodes and plot them
•
Scale factor has been removed.
CFD ANALYSIS 5.7
•
Training Manual
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Results: Velocity Vectors Pressure
Contours
Training Manual
CFD ANALYSIS 5.7
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