CFX 5.5 V5.5.1H
? 2001AEA Technology
6-1
Boundary and Initial Conditions
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-2
Introduction
? Boundary conditions are required to set on all external
domain boundaries
? Initial values/guess are required to set for all
calculations
? Boundary conditions and initial values/guess can be
either constants or expressions
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-3
Boundary Conditions
? Default external boundary condition,adiabatic,no-slip
wall
? Available boundary conditions:
> Inlets
> Outlets
> Openings
> Walls
> Symmetry Planes
> Periodic Pairs (GGI Periodic not support with Multi-phase
flows)
? It is generally possible to specify different type options
for different fluids on the same boundary condition
with the same general boundary condition type
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-4
Boundary Condition Forms (single-phase)
Specify name (will be referred to
in post-processor)
Select boundary condition type
Select surfaces on which to
apply the boundary condition
Specify boundary condition values
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-5
Boundary Conditions
? A sample values form
? Values change according to BC
type
? Inlet - speed/velocity,
pressure,mass flow
? Wall - No slip,free slip,
moving,counter rotating,
wall roughness
? Opening - velocity,pressure
? Also specify additional variables
and MCF/2-Phase settings
? Some options governed by
domain settings
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-6
Boundary Conditions (multi-phase)
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-7
Inlet/Outlet
? Inlets/Outlets
Flow is allowed into domain for
inlet or out of domain for outlet
Artificial walls are built to
prevent reverse flow
Need to specify some of the
following quantities at the inlet,
velocity,total/static pressure,
mass flow rate,temperature,
turbulent quantities,additional
scalars,mass fractions and
volume fractions
For rotating frame of reference
all velocities are in rotating
frame (except for the case when
total pressure in stationary
frame is specified along with
velocity component direction)
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-8
Degassing Outlet for Multi-phase
? Mixed boundary conditions are
required to account for simulations
where one fluid may flow out of or
into the domain through the
boundary but other fluids may not
? This is a typical requirement of
bubble column applications where
gas extraction occurs at a boundary
? Some settling and filtration
applications require addition or
removal of a continuous phase from
a dispersed phase
? In practice,the boundary may be
treated as an inlet or outlet for
those fluids may move across the
boundary,and a no flow boundary
for those that may not
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-9
Opening
? Openings
Mixed flow permitted
Specified pressure deemed
total when flow is in
Specified pressure deemed
static when flow is out
Can also specify Cartesian
velocity components
Useful when the exact flow
conditions are unknown
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-10
Wall
? Wall type
External surface
Internal thin surface
? Wall influence on flow
no-slip (stationary)
no-slip (moving)
free slip
? Wall Roughness
Smooth wall
Rough wall
> Roughness height
? Conditions on temperature field
adiabatic
fixed temperature
fixed heat flux
convection coefficient and ambient temperature
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-11
Wall
? Wall Contact Model
(Multiphase)
Use Volume Fraction
Specify Area Fraction
? Conditions on additional
variables
Zero flux
Fixed value
Fixed Flux
? Specify Fluid Boundary
Condition Values (Multiphase)
this form allows the user to
specify the Wall Influence on
Flow for each fluid
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-12
Thin Surface
? Thin surface is used to model a two-dimensional wall boundary
within a fluid region
? Thin surfaces are useful to model features whose thickness can be
safely neglected such as guide vanes within ducts
? Creating thin surfaces in this way simplifies problem setup,
reduces the number of mesh elements required and saves
computation time
? Thin surfaces are created by defining sub-domains and using
their exterior faces to describe the location of a thin surface
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-13
Thin Surface Example
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-14
Solid Boundary
? For conjugate heat transfer
calculations,the user can set
solid boundary conditions
Wall
> adiabatic
> fixed temperature
> fixed heat flux
> convection coefficient and
ambient temperature
Symmetry
Periodic pair
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-15
Fluid -Solid Boundary
? For conjugate heat
transfer calculations,the
user can set the fluid-solid
interface conditions
? Only Walls are available
? Values can be set for
Influence on Flow
Wall Roughness
Heat Transfer
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-16
Symmetry Plane
? Symmetry
Flat plane required
Symmetry1
Sy
mm
etr
y2
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-17
Periodic Pair
? Periodic Pair
Specify rotating or translating
periodicity
Select respective surfaces
Be sure to select the Lists in
the same order so that item 1 in
Surface List 1 is periodic to
item 1 in Surface List 2
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-18
Initial Guess/Value
? For a steady state flow simulation,the user has to set
an initial guess
? For a transient flow simulation,the user has to set an
initial value
? Initial guess/value can be constant or expression
? Initial guess/value can be set on per fluid and/or per
equation class basis
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-19
Initial Guess
? Initial guess of variables must
be set
All Automatic
> Read values for the initial
guess/values from the existing
definition or results file for all
variables
> If a variable is not defined in
the existing file,default values
are used
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-20
Initial Guess
Set Individually
> Automatic - the solver
automatically reads an initial value
from an initial file if it can find one,
otherwise uses a solver default
value
> Automatic with values - the solver
automatically reads an initial value
from an initial file if it can find one,
otherwise uses the specified value
> Default - Always use the solver
default value as the initial value
> Value - always use the specified
value for the initial value
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-21
Initial Guess for Multi-phase
? For Multi-phase flows,the
initial guess form allows the
user to set values for each fluid
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-22
Initial Values for Transient Simulations
? For transient flows,the
initial values form
allows the user
All Automatic
Set Individually
> Automatic
> Automatic with Value
> Value
? The user can set Initial
Time for the simulation
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-23
Initial Guess for Conjugate Heat Transfer
? For conjugate heat transfer
simulations,solid initial
conditions must be set
Use Domain Setting
All Automatic
Set Individually
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-24
Initial Guess/Values
? Initial guess/values can be defined with expressions,via CEL,in
order to make them functions of the spatial coordinates (x,y,z)
? For rotating frame of reference calculations,initial guess/values
are in RFR
? Set initial guess for individual variables
Set T to an average of the values at all temperature
specified boundaries
Set P to the average of the highest value of pressures
specified on any of outlets and the lowest value of
pressures specified on any of inlets
Set U,V,W to inlet values or below (k and epsilon
initialized from velocity scales internally)
Set Additional Variables to inlet values
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-25
Restarting Calculation
? Restart from a simpler problem using All automatic on
the Initial Guess form
1st order -> 2nd order
Laminar -> zero-equation -> k-e -> Reynolds stress
Incompressible -> compressible
Isothermal -> thermal energy -> total energy
Velocity/mass specified BCs -> pressure specified BCs
Pressure specified openings -> static pressure outlets
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-26
Summary
? Boundary conditions for single-phase and multi-phase
Inlet
Outlet
Opening
Wall
Symmetry plane
Periodic pair
? Initial guess for steady state flows and initial values for
transient flows
All automatic
Set individually
? 2001AEA Technology
6-1
Boundary and Initial Conditions
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-2
Introduction
? Boundary conditions are required to set on all external
domain boundaries
? Initial values/guess are required to set for all
calculations
? Boundary conditions and initial values/guess can be
either constants or expressions
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-3
Boundary Conditions
? Default external boundary condition,adiabatic,no-slip
wall
? Available boundary conditions:
> Inlets
> Outlets
> Openings
> Walls
> Symmetry Planes
> Periodic Pairs (GGI Periodic not support with Multi-phase
flows)
? It is generally possible to specify different type options
for different fluids on the same boundary condition
with the same general boundary condition type
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-4
Boundary Condition Forms (single-phase)
Specify name (will be referred to
in post-processor)
Select boundary condition type
Select surfaces on which to
apply the boundary condition
Specify boundary condition values
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-5
Boundary Conditions
? A sample values form
? Values change according to BC
type
? Inlet - speed/velocity,
pressure,mass flow
? Wall - No slip,free slip,
moving,counter rotating,
wall roughness
? Opening - velocity,pressure
? Also specify additional variables
and MCF/2-Phase settings
? Some options governed by
domain settings
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-6
Boundary Conditions (multi-phase)
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-7
Inlet/Outlet
? Inlets/Outlets
Flow is allowed into domain for
inlet or out of domain for outlet
Artificial walls are built to
prevent reverse flow
Need to specify some of the
following quantities at the inlet,
velocity,total/static pressure,
mass flow rate,temperature,
turbulent quantities,additional
scalars,mass fractions and
volume fractions
For rotating frame of reference
all velocities are in rotating
frame (except for the case when
total pressure in stationary
frame is specified along with
velocity component direction)
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-8
Degassing Outlet for Multi-phase
? Mixed boundary conditions are
required to account for simulations
where one fluid may flow out of or
into the domain through the
boundary but other fluids may not
? This is a typical requirement of
bubble column applications where
gas extraction occurs at a boundary
? Some settling and filtration
applications require addition or
removal of a continuous phase from
a dispersed phase
? In practice,the boundary may be
treated as an inlet or outlet for
those fluids may move across the
boundary,and a no flow boundary
for those that may not
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-9
Opening
? Openings
Mixed flow permitted
Specified pressure deemed
total when flow is in
Specified pressure deemed
static when flow is out
Can also specify Cartesian
velocity components
Useful when the exact flow
conditions are unknown
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-10
Wall
? Wall type
External surface
Internal thin surface
? Wall influence on flow
no-slip (stationary)
no-slip (moving)
free slip
? Wall Roughness
Smooth wall
Rough wall
> Roughness height
? Conditions on temperature field
adiabatic
fixed temperature
fixed heat flux
convection coefficient and ambient temperature
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-11
Wall
? Wall Contact Model
(Multiphase)
Use Volume Fraction
Specify Area Fraction
? Conditions on additional
variables
Zero flux
Fixed value
Fixed Flux
? Specify Fluid Boundary
Condition Values (Multiphase)
this form allows the user to
specify the Wall Influence on
Flow for each fluid
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-12
Thin Surface
? Thin surface is used to model a two-dimensional wall boundary
within a fluid region
? Thin surfaces are useful to model features whose thickness can be
safely neglected such as guide vanes within ducts
? Creating thin surfaces in this way simplifies problem setup,
reduces the number of mesh elements required and saves
computation time
? Thin surfaces are created by defining sub-domains and using
their exterior faces to describe the location of a thin surface
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-13
Thin Surface Example
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-14
Solid Boundary
? For conjugate heat transfer
calculations,the user can set
solid boundary conditions
Wall
> adiabatic
> fixed temperature
> fixed heat flux
> convection coefficient and
ambient temperature
Symmetry
Periodic pair
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-15
Fluid -Solid Boundary
? For conjugate heat
transfer calculations,the
user can set the fluid-solid
interface conditions
? Only Walls are available
? Values can be set for
Influence on Flow
Wall Roughness
Heat Transfer
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-16
Symmetry Plane
? Symmetry
Flat plane required
Symmetry1
Sy
mm
etr
y2
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-17
Periodic Pair
? Periodic Pair
Specify rotating or translating
periodicity
Select respective surfaces
Be sure to select the Lists in
the same order so that item 1 in
Surface List 1 is periodic to
item 1 in Surface List 2
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-18
Initial Guess/Value
? For a steady state flow simulation,the user has to set
an initial guess
? For a transient flow simulation,the user has to set an
initial value
? Initial guess/value can be constant or expression
? Initial guess/value can be set on per fluid and/or per
equation class basis
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-19
Initial Guess
? Initial guess of variables must
be set
All Automatic
> Read values for the initial
guess/values from the existing
definition or results file for all
variables
> If a variable is not defined in
the existing file,default values
are used
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-20
Initial Guess
Set Individually
> Automatic - the solver
automatically reads an initial value
from an initial file if it can find one,
otherwise uses a solver default
value
> Automatic with values - the solver
automatically reads an initial value
from an initial file if it can find one,
otherwise uses the specified value
> Default - Always use the solver
default value as the initial value
> Value - always use the specified
value for the initial value
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-21
Initial Guess for Multi-phase
? For Multi-phase flows,the
initial guess form allows the
user to set values for each fluid
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-22
Initial Values for Transient Simulations
? For transient flows,the
initial values form
allows the user
All Automatic
Set Individually
> Automatic
> Automatic with Value
> Value
? The user can set Initial
Time for the simulation
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-23
Initial Guess for Conjugate Heat Transfer
? For conjugate heat transfer
simulations,solid initial
conditions must be set
Use Domain Setting
All Automatic
Set Individually
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-24
Initial Guess/Values
? Initial guess/values can be defined with expressions,via CEL,in
order to make them functions of the spatial coordinates (x,y,z)
? For rotating frame of reference calculations,initial guess/values
are in RFR
? Set initial guess for individual variables
Set T to an average of the values at all temperature
specified boundaries
Set P to the average of the highest value of pressures
specified on any of outlets and the lowest value of
pressures specified on any of inlets
Set U,V,W to inlet values or below (k and epsilon
initialized from velocity scales internally)
Set Additional Variables to inlet values
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-25
Restarting Calculation
? Restart from a simpler problem using All automatic on
the Initial Guess form
1st order -> 2nd order
Laminar -> zero-equation -> k-e -> Reynolds stress
Incompressible -> compressible
Isothermal -> thermal energy -> total energy
Velocity/mass specified BCs -> pressure specified BCs
Pressure specified openings -> static pressure outlets
CFX 5.5 V5.5.1H
? 2001AEA Technology
6-26
Summary
? Boundary conditions for single-phase and multi-phase
Inlet
Outlet
Opening
Wall
Symmetry plane
Periodic pair
? Initial guess for steady state flows and initial values for
transient flows
All automatic
Set individually
