CFX-FSI 14.5 Lect-05 Two Way Solving Post

8/13/2019 CFX-FSI 14.5 Lect-05 Two Way Solving Post

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© 2011 ANSYS, Inc. July 26, 20131 Release 14.5

14. 5 Release

Solving FSI Applications Using

ANSYS Mechanical and ANSYS CFX

Lecture 5

Two-way FSI Solving and PostProcessing




Outline

Solution ProcessHere we discuss starting and monitoring the FSI solution in serial and

parallel. The various log and output files are also discussed.

Post Processing

This section will discuss common post-processing in CFD-Post and how

to post-process FSI results in the Mechanical post-processor.

Appendix A: Restarting FSI Simulations

This section covers restarting FSI simulations from completed runs and

backup points. Making physics changes to the CFD and Mechanical

setups when restarting is also discussed.

Appendix B: Multifield APDL Commands

This appendix provides some useful APDL commands relevant to MFX

simulations.




Starting the Run

Write the Definition file and proceedto the CFX Solver Manager

• WB writes the def file in the background

Run Definition panel:

• Use Double Precision for FSI runs, since

the mesh deformations are often very

small compared to the size of the

domain




Starting the Run

MultiField panel:

• MFX Run Mode:• Start ANSYS and CFX

• Both solvers started automatically

• Start ANSYS only / Start CFX only

• If running on different machine – see

parallel discussion later

• Process Input File only

• Produces a .mf file

• Used for physics changes; discussed later

• Mechanical Input File

• As set in CFX-Pre, but can be changed here

• Process ANSYS Input File

• Uncheck if the Mechanical Input File is a .mf

file (i.e. has already been processed)

• See restarts discussion later

… then Start Run




Running in Parallel

CFX can use all the usual parallel methods

• Set up as for any non-FSI run

MAPDL solver is limited to local parallel

To run MAPDL in local parallel specify,for example, -np 4 in the Additional

Arguments field

• Will run on the local machine

• MAPDL can run on two cores without any

additional licenses

From the Command Line:

> cfx5solve -def CFX_Setup.def -double -mfx-run-mode "Start ANSYSand CFX" -ansys-input ds.dat -ansys-arguments "-np 2"

Start both solvers using the files specified, run CFX in double precision, run MAPDL solver

on two partitions. Note that on some systems spaces in the argument strings will need tobe escaped, e.g. “Start\ ANSYS\ and\ CFX” and “-np\ 2”




Running on Two Different Hosts

To run on different machines:• Start ANSYS first on machine A

• MFX Run Mode = Start ANSYS only

• CFX out file will show a port and

hostname that ANSYS is listening on

• Start CFX on machine B• Other machines can be used as slaves

• Provide the port and hostname output

by ANSYS

...

...+--------------------------------------+| || Starting ANSYS Solver || |+--------------------------------------+ ANSYS solver listening on 1971@MACHINE1


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Running on Two Different Hosts

From the command line:

> cfx5solve -def CFX_Setup.def -mfx-run- mode “Start ANSYS only" -ansys-input ds.dat

• Starts the ANSYS solver using the input file specified

> cfx5solve -def CFX_Setup.def -double -mfx-run-mode"Start CFX only" -cplg-host 2249@MACHINE1

• Starts the CFX solver using the specified def file in double precision,

connects to the ANSYS solver already running on MACHINE1 using

the specified port

Run ‘cfx5solve –help’ or see doc for all command options



Running in Parallel

Slave processes/threads must be on different cores• Otherwise things will run VERY slow

• The codes are (usually) running sequentially, it would seem logical

that they can share cores

• However, a slave process pulls 100% of the available CPU while it’s in

‘waiting’ mode

• The other code is starved of CPU

• OK for CFX and MAPDL to share a master processor

Example: single quad-core machine

• CFX: 4 partitions, MAPDL serial

• OK – only the master is shared

• CFX: 3 partitions, MAPDL 2 partitions (cores)

• OK – when MAPDL is ‘waiting’ 1 core will be in use, leaving 3 for CFX



Another Look at the Workflow

MAPDL is the master process• MAPDL controls the transient

timestep, interface mapping,

interface convergence checks,

load transfer, etc

But all these settings are defined in

CFX-Pre

We have an input file from

Mechanical that contains the

structural model with the FSI

surfaces tagged

• Workbench writes this input file

into the project directory

CFX-Pre

Definition FileContains CCL definition of

fluid model and interface /

coupling settings

Mechanical

Input FileContains APDL definition

of structural model

MAPDL SolverInput File + appended APDL

commands from CFX

Process Input FileCFX adds APDL commands for

interface / coupling settings

CFX SolverSettings from the Definition

file



Another Look at the Workflow

CFX-Pre writes a Definition File(.def) containing the CFD model

and the interface / MFX controls

When the run is started, CFX

creates APDL commands for the

interface / coupling settings andcombines them with the contents

of the input file from Mechanical

• Important to understand what’s

happening here when

performing restarts

• See Appendix A for restart

details

CFX-Pre


fluid model and interface /

coupling settings

Mechanical


of structural model

MAPDL SolverInput File + appended APDL

commands from CFX

Process Input FileCFX adds APDL commands for

interface / coupling settings


file




MAPDL Commands Inserted by CFX

You can view the commandsgenerated by CFX in the

MAPDL Solver output,

following the end of the Input

File

CFX generated these

commands

End of Input File from

Mechanical




“Process Input File”

If you disable Process ANSYS Input

File then CFX does not generateany APDL commands

• Therefore the Input File for

MAPDL must already contain the

interface and coupling settings

• Occurs in some restart workflow

Also see CFX documentation:

• ANSYS CFX Solver Manager User’s

Guide >> Working with Solver

Manager >> ANSYS Multi-field Run

>> Processing the ANSYS Input File

CFX-Pre


fluid model

Mechanical


of structural model and

interface / coupling

settings

ANSYS SolverSettings from Input File


file




Solver Files Generated

CFX

• The working directory will contain the CFX Outputand Results files as usual

• The transient (.trn) and backup (.bak) results files

are stored in the <name>_001 directory

MAPDL Solver

• The subdirectory <name>_001.ansys is created andcontains all the MAPDL Solver files

• All results are stored in the .rst file

• The auto-generated ANSYS.mf file can be viewed inthis subdirectory

• ANSYS.stdout is the ANSYS Solver output asdisplayed in the CFX Solver Manager

Workbench

• The working directory is:…/<ProjectName>_files/dp0/CFX/CFX




CFX Solver OutputStart of Time Step

First Coupling Iteration

Solve Mesh Displacement

- Should be fully converged

CFX Coefficient Iterations

- Should be converging

Second Coupling Iteration

Solve Mesh Displacement

- Should be fully converged

CFX Coefficient Iterations

- Should be converged by the

end of the last Coupling Iteration




CFX Solver Output

ANSYS Field Solver and Interface Loads plot also shown

L2 norm al ized values should b e less than

correspo nding conv ergence cr i ter ia (CRIT) Interface loads sho uld be negat ive

at the end of each t imestep

X-axis values do not

correspon d to each

other or to CFX

Cumulat ive

ANSYS

Iterat ions

Cumulat ive

Coupl ing

Iterat ion




MAPDL Output – Check Interface Mapping

In the MAPDL output, ALWAYS check the interface mapping

Scroll to the top of the output file (ANSYS.stdout)

Right-click in the window and select Find…

Search for the following – make sure Match case is enabled:

This section provides information on the mapping

between the CFX and ANSYS meshes. If the

Non-Matching Area Fraction or the No. of Un-Mapped Nodes is non-zero, then some parts of

the FSI interface will not be passing loads

between the field solvers. In this case check that

the two side physically match and completely

overlap.




MAPDL Solver OutputTime Step and Coupling Iteration #

Start and End Time for this Time Step

Actual Force Loads Received

(and/or Thermal Loads)

Start and End Time for this Load Step – 1:1 correspondence to Time Step

ANSYS Field Solver

convergence and iterations

Check if CFX & ANSYS FieldSolvers are Converged

Check Interface Load

Convergence




Summary

• The CFX Solver Manager or CFX Solver command line is used to

launch 2-way FSI simulations

• Mapping is performed by the MAPDL solver. Mapping

diagnostics are provided in the MAPDL solver output file and

should always be checked.

• FSI settings defined in CFX-Pre are passed to MAPDL at run-time by generating APDL commands

• Understanding this workflow is important for restarts

• Restarts can be made from completed runs, interrupted runs

and from backup points. Physics changes can be made when

restarting. See Appendix A.




Outline

Solution ProcessHere we discuss starting and monitoring the FSI solution in serial and


Post Processing









simulations.




Combined Post-Processing

Editing the CFX Results cell for a2-way FSI case will automatically

load both the CFX and structural

results into CFD-Post

Can also use File > Load Results

and browse for .rst (structural)or .rth (thermal) results

• Enable Keep Current Case

Loaded to post-process

multiple sets of results

• Disable Open in new view to

have CFX and Mechanical

results in a single window




In CFD-Post CFX and structural resultsentries and regions are shown in the

outline tree

When creating objects they can be scopedto all or selected domains





Use the Timestep Selector to loaddifferent time steps as usual

• Tab panels show the availableresults from each solver

• Sync Cases allows both sets ofdata to be updated when a timestep from one set is selected

Animations can show both fluid and

structural results simultaneously





Combined Post-Processing Animations

• Transient blood flow through athree-leaf valve

• Symmetry used with 1/6th

of fluid domain

• Non-Newtonian fluid (i.e.

blood) defined in Fluent• Non-Linear contact in Transient

Structural to cater for closure of

valve leaflets

• Blood vessel wall has

combination of isotropic andanisotropic (non-linear)

materials




Combined Post-Processing Animations

• Transient free surface flow ina liquid storage tank with

baffles

• Tank is excited by a time

varying gravitational load for

a duration of 10s• Baffles are “non-metallic”

and fixed to the tank with an

adhesive

• Forces are transferred (one-

way only) via co-simulationto determine the stresses

acting on the baffles

• Determine the integrityand viability of the

adhesive bonding




When selecting Variables to plot,the variables list shows all variables

from CFD and structural results

• If a variable is not available (e.g.

Pressure on a slice plane in a structural

region) is it plotted in the specifiedUndefined Color

Some variables apply to both fluidand structural regions, e.g.:

• Total Mesh Displacement (X/Y/Z)

• Velocity (u/v/w)

• Mesh metrics (Area, Edge Length Ratio,

X, Y, Z, etc)





CFD-Post read components from a .cm filewritten out by the MAPDL solver

• This is created by default for FSI cases using the

command CMWRITE,file,cm

• You’ll see a Mesh Regions branch in CFD-Post

containing the components

When load structural results containingshell elements, CFD-Post reads and

displays data from the top side of the

shells

• To read and display results from the bottom

side set the system environment variable

CFDPOST_RST_READ_BOTTOM_SHELL_DATA

to any value





Post-Processing in Mechanical

Structural results are associated with theCFX Solution and Results cells

To post-process in Mechanical:

• Edit the Mechanical Solution cell

• Select the Solution entry from the Outlinetree

• Select Tools > Read Results Files… and

browse for the .rst/.rth file

associated with the CFX FSI

system




Outline

Solution Process

Here we discuss starting and monitoring the FSI solution in serial and


Post Processing









simulations.




Restarts from a Completed Run

This includes runs that were manually

stopped before reaching the End Time

On the CFX side restart as usual

• If manually specifying a previous res file,

Continue History From must be enabled for

FSI restarts

MAPDL always resumes from a previous

database

• Select the db file from the previous *.ansys

run directory

• The Mechanical Input File must bespecified but is not used in this case

• Enable Process ANSYS Input File:

– APDL commands are created again from the

CFX Solver Input File and a RESUME command

is also inserted




Restarts from a Completed Run

Example of the commands generated by CFX

shown

Note that there’s no reference to the

Mechanical Input File

• Any changes made there are not used

The MF commands are recreated from theCFX Solver Input File

• The MF commands stored in the db file are not

used

If Process ANSYS Input File is disabled, the

Mechanical Input File is used and is run “as is”

• The Restart ANSYS Run toggle is ignored because

CFX doesn’t generate a RESUME command

• No MF commands generated

resume,ANSYS,db/SOLU

MFAN,ON

MFCLEAR,MFLC

MFRS,-1,SING

MFTI,1

MFDT,1,1,1,OFF

MFLC,SURF,ANSYS,1, …

MFLC,SURF,CFX,'Wall', …MFIT,20,1,20

MFCO,ALL,1e-3

MFRE,ALL,0.75,RELX

MFPS,group1,ANSYS

MFPS,group2,CFX

MFSO,group2,group1

CMWRITE,ANSYS,cmKBC,1

/GST,ON,ON

SOLVE

SAVE

CMWRITE,ANSYS,cm

FINISH

/EXIT

Typical Restar t Command s

Generated b y CFX




Restarts from a Backup Point

A failed run can be restarted using a CFX

backup file and MAPDL files producedusing the MFRC command

• Must be from a consistent time point

Pick the CFX .def file and use the CFX

backup file as the Initial Values File, withContinue History From enabled

Same MultiField settings as restartingfrom a completed run, except the rdb

(restart database) file is used

• This file is created by the MFRC command

Again, the Mechanical Input File is notused and CFX re-creates the MF

commands




Restarts from a Backup Point

Only the first FSI run will contain a rdb file. For multiple restartscopy the rdb file to the most recent run directory

• Any previous changes you have manually made in a .mf file must be

re-applied to every restart from a rdb file, since the rdb file

represents the structural setup from the first FSI run

• The correct nodal displacements, velocities, etc are still used, since

the rdb file will read the .r00# file corresponding to the restart time




Restarts with Physics Changes

Changes to CFX physics can be made as usual

• In Workbench edit the Setup cell

• Or create a new .def file and use the previous res file as the Initial Values

File, with Continue History From enabled

Changes to coupling, interface or time settings specified inCFX can use the same procedure as a CFX physics change

• Since CFX re-creates the MF commands each time, these changes will be

passed to MAPDL

• Example: Extending a run by increasing the Coupling Time Duration

Changes to Mechanical settings are less straight-forward

since the Mechanical Input File is not used when restarting

• Where should be changes be made?





First, create the .mf file

• This is what CFX is doing in the

background when Process ANSYS Input

File is checked

• Use Process Input File only option

• Select the Mechanical Input File and the

previous db file

• Provide the CFX def file and Start Run

Run will complete in a few seconds

Inside the *.ansys/ run directory an ANSYS.mf file is created

…/RunName_002.ansys/ANSYS.mf





Manually edit the ANSYS.mf file, inserting

APDL commands to make any changes

Start the run using the edited .mf file as theMechanical Input File

Disable Process ANSYS Input File

Select the previous db file

Can also use this approach to change

interface / coupling settings (MF commands)instead of making a new .def file in CFX-Pre

You need to use this approach to restart a

steady-state 2-way FSI case as transient

• Add TIMINT,ON to the .mf file to switch

Mechanical from steady-state to transient




Restarts - General

MAPDL results are appended to the rst file• Old rst is copied into new run directory

• Can be a very large file!

• You can move the old rst file to a new directory before restarting to

prevent it being copied

• A new rst file will be created for the new run

For all restarts also see the CFX documentation:

• ANSYS CFX Solver Manager User’s Guide >> Working with Solver

Manager >> ANSYS Multi-field Run >> Restart Procedure for ANSYSMulti-field Run




Outline

Solution Process

Here we discuss starting and monitoring the FSI solution in serial and


Post Processing









simulations.




Multi-Field Commands - Background

• Note: the Command Reference section in the Mechanical APDL

documentation provide details on all MAPDL commands

• MF commands are APDL commands that control Multifield parameters

• They are created by CFX-Pre and appended to the MAPDL input at run-time




Basic MF Commands

MFRS

• Multifield restart time (for restarts only)

• Usually has a value of -1, meaning restart from end of previous solution

MFAN

• Turns on Multifield solver

MFTI

• End time for the Multifield analysis

MFIT

• Controls the number of Coupling Iterations

MFDT

• Control the transient time step size




Basic MF Commands

MFOU

• Controls how often results are stored

MFRC

• Control how often restart files are written

MFCO

• Controls Field Convergence Criteria

MFRE

• Field Relaxation Factors

MFLC,SURF,CFX,'Interface','Total Force', ANSYS, 1,FORC,CPP• Load transfer command

MFPS and MFSO

• Which field goes first, sequential or parallel




Other Relevant APDL Commands

BCSOPTION,,INCORE

• For the sparse solver tries to retain the entire factorized matrix in memory

• Runs faster, but significantly increases memory usage

KBC,1

• Sets stepped loads - CFX will always append this command to the ANSYS input

• If using Prep7 make sure you set it manually

OUTRES

• Minimum data you can write and still restart

• Note that you may need more data for post-processing (e.g Stress data)

OUTRES, ERASEOUTRES, ALL, NONE

OUTRES, NSOL, LAST

OUTRES, ERASEOUTRES, ALL, NONE

OUTRES, NSOL, LAST

OUTRES, ESOL, LAST

OR

Structural Solution OnlyStructural + Thermal

or Thermal Solution



Other Relevant APDL Commands

NEQIT

• Sets the maximum number of equilibrium iterations

NCNV

• The ANSYS solver will continue when convergence has not been achieved

Note that MAPDL runs until convergence or max. number of equilibrium

iterations, but the calculation will stop if max. number of equilibrium iterations is

reached. You can set NCNV so that MAPDL will continue if convergence is not

achieved and this can be combined with NEQIT to limit the number of

equilibrium iterations. However, you need to be careful when doing this – you

must make sure MAPDL is converged by the end of the last Coupling Iteration.

If you are reaching the maximum number of Coupling Iterations then

something hasn’t converged – CFX, interface loads, or MAPDL – this needs to

be corrected. If one of the field solvers is very poorly converged, it may pass

spurious loads to the other which could cause failure.

Documents

CFX-FSI 14.5 Lect-05 Two Way Solving Post