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Ansys FAQ

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This article contains answers for Ansys related FAQ. Please feel free to add questions and answers here!




Solver related questions

How to avoid 6000 - 7000 K temperatures using finite rate chemistry model

Edit the def file and add the following to the edited ccl file. (You can do it by simply clicking the EDIT button in cfx5solver or using cfx5cmds command) Add these lines to the EXPERT section:

stiff chemistry = t

If you don't have this section, you can create it and then add a line there (see the manual).


FSI (Fluid Structure Interaction)


Which programms are necessary to solve a FSI simulation?

Beginning with Ansys 11.0, Ansys Workbench with Simulation and CFX are required.

Which Ansys licenses are required for FSI in Ansys?

Working with Ansys 10.0 or older, a Muliphysics and a CFX license are required.

Ansys 11.0 has multiple options. It is possible to run an FSI simulation with a single license now.

What kind of coupling methods are possible?

One-way or two-way FSI coupling.

Is it possible to perform a steady-state FSI simulation?

Yes, Ansys 11.0 enables steady state and transient FSI simulations as well.

What is the general procedure for a FSI simulation in Ansys 11.0?

1. Define the Solid setup in Ansys Simulation (Ansys Workbench). This includes an Fluid-Solid-Interface.

2. Write an Ansys Input File (.inp) of this setup.

3. Define the Fluid setup in Ansys CFX-Pre. A link to the Ansys Input File is required.

4. Write an Ansys Definition File (.def) of this setup.

5. Solve the Definition File with Ansys CFX-Solver.

6. Postprocess the data in CFX-Post.

CFX Setup

Ansys Setup


How do I start an FSI simulation in Ansys 11.0?

Start the CFX Solver Manager and load the .def File. Ansys will load the .inp file automatically.

Is it necessary to define the Ansys Insallation Root in "Define Run" in Ansys 11.0?

On a Windows PC it is not, on Unix it is necessary.

Is it possible to stop a running FSI simulation in Ansys 11.0?

No, the "Stop"-button does not work for FSI simulations.

Is it possible to write back-up files of a FSI simulation in Ansys 11.0?

The solver terminates with the error message "A negative volume appeared". What went wrong?

This error often appears with FSI simulations. Normally it comes together with a large deflection simulation of the solid part. Mostly the CFD-mesh deformation is too big and negative elements appear. Possible solutions might be

  • a better fluid mesh
  • meshsiffness 1/wall distance
  • smaller timesteps


If I open the FSI simulation in CFX-Post 11.0 only the fluid data are available. Where are the solid data?

CFX-Post only opens the .res file by default. If you want to postprocess fluid and solid data together you have to load the solid data additionally: File -> Load Result -> Load the Ansys .rst or similar data an activate the radio button "add data". Now you can postprocess all data in CFX-Post.

Ansys Workbench



How should I ask my question on the CFX forum to get the best possible answer?

The most important point to understand about the CFX forum is that the quality of the answer to your question will depend entirely on the quality of your question!

Most of the questions posed on the CFX forum are so poorly posed that it's impossible to understand what the poster is actually asking. If you want effective help with your problem adhere to the following guidelines:

  1. Make sure your question is as clear, concise and as intelligible as possible. Use punctuation. Other forum readers are not going to spend time trying to decipher a garbled question.
  2. Give a clear general description of what class of problem and/or application you are working on BEFORE you start asking specific questions. This will aid other forum readers to better understand your specific questions.
  3. Describe precisely what you have done yourself to try and solve your problem, giving examples.
  4. Depending on your problem you should always include the following:
    • A copy of your command file as a file attachment. Many simple problems can be spotted in a command file by an expert user.
    • If you are asking a mesh quality related question then include some sample images of your mesh, including the boundary layer.
    • If you are asking a user Fortran and/or user CEL question then include a copy of your existing code file as an attachment.


  • Dont ask over broad questions e.g. "How do I simulate a 4-stroke engine". Nobody is going to type out fifty pages of guidance. They have better things to do.
  • Don't shorten the words. It's a web forum, not a SMS text message. It's very hard to read it. Or R U 2 lazy? People may tend to be lazy to decipher and answer.

Example of a well posed question:

Examples of questions that are unanswerable:

How to upload images:

  • Create the image on your local machine e.g. skew_mesh.jpg
  • Upload the image to Imageshack
  • Copy the link "Thumbnail for websites"
  • Paste the link into your post e.g. <a href="" target="_blank"><img src="" border="0" alt="Free Image Hosting at" /></a>

How to share non-image files as attachments:

  • If you have more than one file, zip them into a single file.
  • Upload the file to Rapidshare Gigasize
  • Scroll down to "I don't want a collector's account right now. Just give me the download-link." and click it.
  • Scroll down until you see the link e.g.

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