CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -1
Solver Control
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -2
Solver Control
Settings for the behavior of the CFX-5 solver
> Maximum number of timesteps
> Timestep selection
> Transient timestep control for results output
> Convergence criteria
> Order of differencing scheme
> Advanced Options
> Mesh Adaption
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -3
Flow Solver Parameters Form
Convergence Control
Convergence criteria
Advection scheme
Options
Mesh Adaption
Also Timestep results selection for
transient simulations
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -4
Transient Timesteps
? Specify creation of transient results files
? Set time to every for intervals or at for specific
time values
? Set Minimal Results File with specific
variables to reduce file sizes (if not full results
files get written at each timestep)
? Only accessible if transient was selected in the
Domain Options of the Domains form
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -5
Flow Solver Parameters Form
Convergence Control
> Max,Number of Timesteps
* Sets the maximum timesteps
for the current run
> Equation Class Scope
> Timestep Selection
* Auto Timestep
* Physical Timestep
* Auto Timestep User Max
* Local Timestep Factor
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -6
Solver Parameters
? Auto timestep.
Default setting for timestep control.
Use internally calculated fixed physical timestep size based on
boundary conditions,initial guess and geometry of the domain.
? Auto timestep,user maximum.
Use an internally calculated timestep,but the value is capped by a
user specified maximum.
If the internally calculated timestep is less than the specified
maximum,the solver uses this value for the first five timesteps,
After five timesteps,the timestep is doubled for each successive
timestep and checked against the specified maximum value,When
this value is reached,it is used for the remaining timesteps.
If the internally calculated value is greater than or equal to the
specified maximum,the maximum value is used for all timesteps.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -7
Solver Parameters
? Physical timestep
Permits true transient simulation
Value near an L/V scale usually gives good
convergence rate
Not obvious sometimes
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -8
Solver Parameters
? Local timestep factor,E.
Different time scales are used in different regions of the domain.
Smaller time steps for regions of fast flow and larger time steps
for regions of slow flow.
Can produce poor asymptotic convergence if mesh has large
range in element size.
Similar concept to false time stepping in CFX-4 or etfact in
TASCflow.
Useful on uniform mesh when L/V scale not obvious or vastly
different local velocity scales are present in problem.
Only suitable for starting a run,not accurate for a fully
converged solution,Large physical timestep needed.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -9
Timestep Selection
Too large - bouncy convergence or divergence
Too small - slow convergence and not fully accurate
Initial timesteps may need to be very small (transonic
flows,large regions of separated flows,openings with
simultaneous inflow and outflow …)
A physical timestep of 1/3 the smallest local L/V scale
is usually optimal
Once the solver has got going (e.g,5-10 iterations)
then the asymptotically optimal physical timestep can
be used
Typically,a local timestep factor of 5.0 is small
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -10
Timestep Selection
For advection dominated flows,residence time for the
flow is a good approximation for the timestep,This is
the time taken for a fluid particle to make its way
through the flow domain
For rotating components,timestep ~ 1/? (? in rad/s)
For buoyancy dominated flows,the following timestep
is more appropriate:
Tg
lt
?
??
?
ma x
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -11
Flow Solver Parameters Form
Convergence Criteria
> Convergence Criteria
* RMS or MAX
residuals
> Set Target Residual
> Target Convergence
Imbalance
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -12
Solver Parameters
? Target residuals
Residuals based on normalized satisfaction of discrete
conservation equations
MAX residuals should be 1.0E-03 for incompressible
flows,1.0E-04 for transonic flows
RMS residuals should be approximately an order of
magnitude smaller
? Global balance
Normalized total conservation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -13
Flow Solver Parameters Form
Advection Scheme
> Equation Class Scope
> Advection Scheme
* First Order
* Second order
bounded
* Specified blend of first
and second order
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -14
Solver Parameters
? Discretization
1st Order - pure first-order accurate
Upwind Difference Scheme (UDS)
2nd Order High Resolution - Bounded second-order
accurate (NAC with MUSCL)
Advanced Control - Specify bounded 2nd order or
blend of 2nd order and 1st order for both
mass/momentum and turbulence
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -15
Options Panel
? Smart start
Choose ‘Yes’ to freeze the
Turbulence modeling for the
first 5 iterations
? Solver Backup Frequency
Saves a,res file automaticaly
every ‘x’ iterations
? Output Equation Flows
Choose ‘Yes’ to output extra
force information to the,res file
> Necessary for Torque
calculations in the Post
processor
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -16
Mesh Adaption Controls
? Adaption variables
Density,Mach number,pressure,
total pressure,turbulence eddy
dissipation,turbulence energy,
velocity,viscosity,additional
variable,mass fraction,
temperature,static enthalpy,
total enthalpy,total
temperature
One or more adaption variables can
be chosen
? Number of adaption steps
recommended between 1 and 5
? Number of nodes in (final) adapted
mesh
Final number of nodes (less than 5
times)
Multiple of initial mesh (greater than 1.2)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -17
Mesh Adaption Controls
? Adaption criteria
Solution variation
> The adaption criteria is calculated using only the
variation of a solution variable across an edge
> Specify a minimum edge length to prevent
refining too much near a discontinuity
Solution variation * edge length
> The adaption criteria is calculated using the
variation of a solution variable across an edge
and its edge length
If more than one adaption variables are
selected,then the adaption criteria are
calculated for each variable at each edge,and
the highest value at each edge is used
? Save intermediate files
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -18
Advanced Mesh Adaption Parameters
? Node allocation parameters
Control over how much the mesh is refined at
each adaption step
When the node allocation parameter is set to
0,then the same number of nodes is added for
each mesh adaption step
When the node allocation parameter is
negative,more nodes are added in to later
adaption steps
When the node allocation parameter is
positive,more nodes are added in to earlier
adaption steps
Recommend in the range of -2 to 2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -19
Advanced Parameters
? Maximum number of adaption levels
the maximum number of times that any
element of the original mesh can be
subdivided
It must not exceed the maximum
number of adaption steps
? Adaption to geometry
? Intermediate convergence criteria
It is used for the original mesh and any
intermediate meshes
The convergence criteria set in the
solver parameters form is only used for
the final adapted mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -20
Mesh Adaption Process
? Mesh adaption
consists of:
An appropriate
adaption criteria is
calculated for each
mesh edge
The appropriate
number of nodes are
added to the existing
mesh according to
the length calculated
The solution already
calculated on the
older mesh is
linearly interpolated
onto the new mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -21
Limitations of Mesh Adaption
? You can’t specify that mesh adaption should take
place until the mesh stops changing or until a
particular solution criterion is satisfied
? The method is not well-suited to meshes with high-
aspect ratio elements,since it can only refine
elements in an isotropic manner
? Mesh adaption can’t improve the quality of the
existing mesh
? Mesh adaption can’t be controlled using CCL
? Mesh adaption can’t be used with a transient
calculation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -22
Definition File Form
Set the file name to write CFD model
information
Option to:
> Automatically start the CFX-5 solver manager
after file is written,or start in batch mode
> Turn off meshing and inflation (for use with
restart files)
> Shutdown CFX-Build after the file is written,Free
memory for solver and license for other users
> Start the Solver manager or start the run in Batch
mode
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -23
Summary
Solver parameters for controlling time steps while
considering convergence and accuracy
> Time step
> Number of time steps
> Local time step factor
> Mesh Adaption
> Discretization scheme
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -24
Importing Geometry/Mesh into CFX-5
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -25
Data Import
? Neutral geometry imports supported -
IGES (Initial Graphics Exchange Specification)
Parasolid v11.0
ACIS (SAT) v5.3 (Optional Feature)
Patran
? Direct CAD imports supported -
Pro/ENGINEER v2000i2 (Optional Feature)
Unigraphics v 16 (Optional Feature)
CATIA v 4.2.1 (Optional Feature)
CADDS5 v8.1 (Optional Feature)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -26
IGES
? Surface,lines,points and solids supported
? Must be imported in the units created in (found in header)
? Suitable to come from Pro/E
? Some problems because:
Unclear standards set up
Some systems output poor files - try ‘round tripping’
CATIA does 2 translations at low tolerance !
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -27
Parasolid
? Modeling kernal of Patran (Build).
? Units in meters.
Import can scale geometry.
? Typically supports solids and is VERY clean.
? Supports version 11.0.
? Suitable from -
Unigraphics.
SolidWorks.
Solid Edge etc.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -28
? Competitor to Parasolid
? File format is SAT (Save As Text)
? Suitable from
AutoCAD (MDT and Inventor)
Cadkey
Others?
? Supports solids in version 2.1/3.0
? Imports with scale factor,but original units
must be known
ACIS
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -29
VMI
Build imports surface mesh only
for visualization
No additional geometry/mesh can
be created
Hex,tet and hybrid meshes
supported
Adaptation can be used,but will
not snap to geometry
Volume mesh is output to def/res
files
Boundary patches must be applied
in sending system for application
of BC’s
Import Mesh
Option
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -30
VMI
? Meshes supported
CFX-TASCflow
CFX-4
ICEM Hexa and
Tetra
PLOT3D
Nastran and Patran
I-DEAS universal
Pointwise Gridgen
Grid-Pro/az3000
User specified
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -31
Groups
Same as Layers or Levels in CAD
systems
Useful for organizing information
Typically used to group and separate
geometric entities such as Solids and
Surfaces
Useful to isolate problem areas and
speed up graphics
Can set colour by group
Obtained from group pull down menu
Common options are create,post,
modify and set current
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -32
Groups
Interactively edit,create and
post groups
Only posted entities can be
mouse selected
None-posted entities can still
be affected
All edits occur to the current
group,By default this is the
posted group
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -33
Customizing Build
Main files:
> CFX.hosts and PVM.hosts - for adding parallel hosts
> settings.pcl - customizing Build environment
> init.pcl - Build setup and removing session files
> CFX.ad - for setting text fonts
> CFXBuild5_template.db - template,db from which all others
are derived
All are in PCL (Patran Command Language) except
the template database
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -34
Summary
? CAD Import
? Mesh Import
? Groups
? Customizing Build
? 2001AEA Technology
8 -1
Solver Control
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -2
Solver Control
Settings for the behavior of the CFX-5 solver
> Maximum number of timesteps
> Timestep selection
> Transient timestep control for results output
> Convergence criteria
> Order of differencing scheme
> Advanced Options
> Mesh Adaption
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -3
Flow Solver Parameters Form
Convergence Control
Convergence criteria
Advection scheme
Options
Mesh Adaption
Also Timestep results selection for
transient simulations
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -4
Transient Timesteps
? Specify creation of transient results files
? Set time to every for intervals or at for specific
time values
? Set Minimal Results File with specific
variables to reduce file sizes (if not full results
files get written at each timestep)
? Only accessible if transient was selected in the
Domain Options of the Domains form
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -5
Flow Solver Parameters Form
Convergence Control
> Max,Number of Timesteps
* Sets the maximum timesteps
for the current run
> Equation Class Scope
> Timestep Selection
* Auto Timestep
* Physical Timestep
* Auto Timestep User Max
* Local Timestep Factor
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -6
Solver Parameters
? Auto timestep.
Default setting for timestep control.
Use internally calculated fixed physical timestep size based on
boundary conditions,initial guess and geometry of the domain.
? Auto timestep,user maximum.
Use an internally calculated timestep,but the value is capped by a
user specified maximum.
If the internally calculated timestep is less than the specified
maximum,the solver uses this value for the first five timesteps,
After five timesteps,the timestep is doubled for each successive
timestep and checked against the specified maximum value,When
this value is reached,it is used for the remaining timesteps.
If the internally calculated value is greater than or equal to the
specified maximum,the maximum value is used for all timesteps.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -7
Solver Parameters
? Physical timestep
Permits true transient simulation
Value near an L/V scale usually gives good
convergence rate
Not obvious sometimes
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -8
Solver Parameters
? Local timestep factor,E.
Different time scales are used in different regions of the domain.
Smaller time steps for regions of fast flow and larger time steps
for regions of slow flow.
Can produce poor asymptotic convergence if mesh has large
range in element size.
Similar concept to false time stepping in CFX-4 or etfact in
TASCflow.
Useful on uniform mesh when L/V scale not obvious or vastly
different local velocity scales are present in problem.
Only suitable for starting a run,not accurate for a fully
converged solution,Large physical timestep needed.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -9
Timestep Selection
Too large - bouncy convergence or divergence
Too small - slow convergence and not fully accurate
Initial timesteps may need to be very small (transonic
flows,large regions of separated flows,openings with
simultaneous inflow and outflow …)
A physical timestep of 1/3 the smallest local L/V scale
is usually optimal
Once the solver has got going (e.g,5-10 iterations)
then the asymptotically optimal physical timestep can
be used
Typically,a local timestep factor of 5.0 is small
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -10
Timestep Selection
For advection dominated flows,residence time for the
flow is a good approximation for the timestep,This is
the time taken for a fluid particle to make its way
through the flow domain
For rotating components,timestep ~ 1/? (? in rad/s)
For buoyancy dominated flows,the following timestep
is more appropriate:
Tg
lt
?
??
?
ma x
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -11
Flow Solver Parameters Form
Convergence Criteria
> Convergence Criteria
* RMS or MAX
residuals
> Set Target Residual
> Target Convergence
Imbalance
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -12
Solver Parameters
? Target residuals
Residuals based on normalized satisfaction of discrete
conservation equations
MAX residuals should be 1.0E-03 for incompressible
flows,1.0E-04 for transonic flows
RMS residuals should be approximately an order of
magnitude smaller
? Global balance
Normalized total conservation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -13
Flow Solver Parameters Form
Advection Scheme
> Equation Class Scope
> Advection Scheme
* First Order
* Second order
bounded
* Specified blend of first
and second order
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -14
Solver Parameters
? Discretization
1st Order - pure first-order accurate
Upwind Difference Scheme (UDS)
2nd Order High Resolution - Bounded second-order
accurate (NAC with MUSCL)
Advanced Control - Specify bounded 2nd order or
blend of 2nd order and 1st order for both
mass/momentum and turbulence
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -15
Options Panel
? Smart start
Choose ‘Yes’ to freeze the
Turbulence modeling for the
first 5 iterations
? Solver Backup Frequency
Saves a,res file automaticaly
every ‘x’ iterations
? Output Equation Flows
Choose ‘Yes’ to output extra
force information to the,res file
> Necessary for Torque
calculations in the Post
processor
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -16
Mesh Adaption Controls
? Adaption variables
Density,Mach number,pressure,
total pressure,turbulence eddy
dissipation,turbulence energy,
velocity,viscosity,additional
variable,mass fraction,
temperature,static enthalpy,
total enthalpy,total
temperature
One or more adaption variables can
be chosen
? Number of adaption steps
recommended between 1 and 5
? Number of nodes in (final) adapted
mesh
Final number of nodes (less than 5
times)
Multiple of initial mesh (greater than 1.2)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -17
Mesh Adaption Controls
? Adaption criteria
Solution variation
> The adaption criteria is calculated using only the
variation of a solution variable across an edge
> Specify a minimum edge length to prevent
refining too much near a discontinuity
Solution variation * edge length
> The adaption criteria is calculated using the
variation of a solution variable across an edge
and its edge length
If more than one adaption variables are
selected,then the adaption criteria are
calculated for each variable at each edge,and
the highest value at each edge is used
? Save intermediate files
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -18
Advanced Mesh Adaption Parameters
? Node allocation parameters
Control over how much the mesh is refined at
each adaption step
When the node allocation parameter is set to
0,then the same number of nodes is added for
each mesh adaption step
When the node allocation parameter is
negative,more nodes are added in to later
adaption steps
When the node allocation parameter is
positive,more nodes are added in to earlier
adaption steps
Recommend in the range of -2 to 2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -19
Advanced Parameters
? Maximum number of adaption levels
the maximum number of times that any
element of the original mesh can be
subdivided
It must not exceed the maximum
number of adaption steps
? Adaption to geometry
? Intermediate convergence criteria
It is used for the original mesh and any
intermediate meshes
The convergence criteria set in the
solver parameters form is only used for
the final adapted mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -20
Mesh Adaption Process
? Mesh adaption
consists of:
An appropriate
adaption criteria is
calculated for each
mesh edge
The appropriate
number of nodes are
added to the existing
mesh according to
the length calculated
The solution already
calculated on the
older mesh is
linearly interpolated
onto the new mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -21
Limitations of Mesh Adaption
? You can’t specify that mesh adaption should take
place until the mesh stops changing or until a
particular solution criterion is satisfied
? The method is not well-suited to meshes with high-
aspect ratio elements,since it can only refine
elements in an isotropic manner
? Mesh adaption can’t improve the quality of the
existing mesh
? Mesh adaption can’t be controlled using CCL
? Mesh adaption can’t be used with a transient
calculation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -22
Definition File Form
Set the file name to write CFD model
information
Option to:
> Automatically start the CFX-5 solver manager
after file is written,or start in batch mode
> Turn off meshing and inflation (for use with
restart files)
> Shutdown CFX-Build after the file is written,Free
memory for solver and license for other users
> Start the Solver manager or start the run in Batch
mode
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -23
Summary
Solver parameters for controlling time steps while
considering convergence and accuracy
> Time step
> Number of time steps
> Local time step factor
> Mesh Adaption
> Discretization scheme
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -24
Importing Geometry/Mesh into CFX-5
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -25
Data Import
? Neutral geometry imports supported -
IGES (Initial Graphics Exchange Specification)
Parasolid v11.0
ACIS (SAT) v5.3 (Optional Feature)
Patran
? Direct CAD imports supported -
Pro/ENGINEER v2000i2 (Optional Feature)
Unigraphics v 16 (Optional Feature)
CATIA v 4.2.1 (Optional Feature)
CADDS5 v8.1 (Optional Feature)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -26
IGES
? Surface,lines,points and solids supported
? Must be imported in the units created in (found in header)
? Suitable to come from Pro/E
? Some problems because:
Unclear standards set up
Some systems output poor files - try ‘round tripping’
CATIA does 2 translations at low tolerance !
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -27
Parasolid
? Modeling kernal of Patran (Build).
? Units in meters.
Import can scale geometry.
? Typically supports solids and is VERY clean.
? Supports version 11.0.
? Suitable from -
Unigraphics.
SolidWorks.
Solid Edge etc.
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -28
? Competitor to Parasolid
? File format is SAT (Save As Text)
? Suitable from
AutoCAD (MDT and Inventor)
Cadkey
Others?
? Supports solids in version 2.1/3.0
? Imports with scale factor,but original units
must be known
ACIS
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -29
VMI
Build imports surface mesh only
for visualization
No additional geometry/mesh can
be created
Hex,tet and hybrid meshes
supported
Adaptation can be used,but will
not snap to geometry
Volume mesh is output to def/res
files
Boundary patches must be applied
in sending system for application
of BC’s
Import Mesh
Option
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -30
VMI
? Meshes supported
CFX-TASCflow
CFX-4
ICEM Hexa and
Tetra
PLOT3D
Nastran and Patran
I-DEAS universal
Pointwise Gridgen
Grid-Pro/az3000
User specified
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -31
Groups
Same as Layers or Levels in CAD
systems
Useful for organizing information
Typically used to group and separate
geometric entities such as Solids and
Surfaces
Useful to isolate problem areas and
speed up graphics
Can set colour by group
Obtained from group pull down menu
Common options are create,post,
modify and set current
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -32
Groups
Interactively edit,create and
post groups
Only posted entities can be
mouse selected
None-posted entities can still
be affected
All edits occur to the current
group,By default this is the
posted group
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -33
Customizing Build
Main files:
> CFX.hosts and PVM.hosts - for adding parallel hosts
> settings.pcl - customizing Build environment
> init.pcl - Build setup and removing session files
> CFX.ad - for setting text fonts
> CFXBuild5_template.db - template,db from which all others
are derived
All are in PCL (Patran Command Language) except
the template database
CFX 5.5 V 5.5.1H
? 2001AEA Technology
8 -34
Summary
? CAD Import
? Mesh Import
? Groups
? Customizing Build
