ANSYS / CFX (in FSI mode) Tutorial

ANSYS / CFX (in FSI mode)

This is a early release of how to use ANSYS/CFX for FSI applications.

Purpose of this tutorial is to run a basic FSI application that uses both ANSYS and CFX to reach a solution. To run this at VPAC you will need to copy three provided files and launch. In real life of course, you will build your model (if you have not already done so) using the ANSYS GUI and save two of those files, something.def and something.inp, and edit the third, a PBS Script.

Definitions

  • ANSYS - both a code and a company. VPAC has licensed the code in a package that includes CFX. We have one ANSYS license and 8 CFX licenses.
  • CFX - is a CFD code that now belongs to ANSYS
  • FSI - Fluid Structure Interaction, modelling what happens where a fluid meets a solid.
  • MFX
  • PBS - is the scheduler VPAC uses to control access to its supercomputers. More correctly called Torque (and Moab) this system makes sure you get the number of processors you need and that you don't end up sharing them with someone else.

Permissions
ANSYS/CFX is a academically licensed product at VPAC. To be allowed to use it you must agree to use it for only academic purposes, you can do this quite quickly by logging onto the VPAC Users Page, http://www.vpac.org, with your VPAC user name and password and then clicking on add software. Add 'Ansys' and check the license terms. You must agree to these terms befor being allowed to use ANSYS.

Licenses
ANSYS is licensed software with its own setup procedure. Please follow the instructions provided for selecting the appropriate license.

In nearly all cases you will need to use the ANSYS Academic Research CFD license rather than the ANSYS Academic Research license. Please see the attached license comparison file. If you do need to use the Academic Research License rather than the Academic Research CFD license, please set your number of nodes in your PBS script to 4.

Quick Start
This is a quick start that bypasses the building of input files by copying them from two different samples directories. Normally, you would build your own input files using the ANSYS and CFX GUIs but this will demonstrate the scheduling process. This job runs in about 22 minutes on one CPU, a bit less on four on four. A 'real' job should show a much bigger speed up.

mkdir Ansys-demo [enter]
cd Ansys-demo [enter]
cp /common/examples/ANSYS_example/OscillatingPlate.inp . [enter]
cp /common/examples/ANSYS_example/pbs-script . [enter]
cp /common/examples/ANSYS_example/OscillatingPlate.def . [enter]
qsub pbs-script [enter]

Note that the three cp lines have a space and a dot at the end, needed !

That should sit in the queue for a bit, then run for about twenty minutes (check with showq)

Building .def and .inp Files
Clearly, to run your job, you need your input into the job files. You will need to build your own .def and .inp file and, almost certainly, edit the pbs-script.

Please see the ANSYS Tutorial at /usr/local/ansys/v110/CFX/help/pdf/xtutr11.pdf

Make an .inp file
Start the ANSYS Workbench by typing cfx5[enter] and then clicking 'ANSYS' from the menubar and choosing "ANSYS Workbench 11". Now follow the instructions in the ANSYS Tutorial 21 (Page 355). It ends when you save first the .inp file and then the project (page 358).

Make an .def file
Start CFX Pre with the command, cfx5pre [enter] and follow the next stage of the tutorial (page 358).

Follow the tutorial until we get to the heading Obtaining a solution using ANSYS CFX-Solver Manager on page 364. Don't press the "Start Run" button, no matter how tempting it seems. There are two very real reasons why not -

  • The job will run on only one cpu on the trifid head node. That cpu will be shared with lots of other users so your job will run very badly.
  • By doing so, you break a very important VPAC rule, by running a computational intensive process on the head node, you seriously inconvenience all the other users logged on. They will not like that!

Instead, edit your pbs-script to reflect the .inp and .def files you have just created, adjust the number of CPUs (in server places !!), probably adjust the wall time. Launch pretty much as shown above.

Please address any questions and suggestions on how to improve this tutorial to help@vpac.org - your input will be very gratefully received.

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