How To Post Process ANSYS FEA Results in ANSYS CFD

The other day there was a thread on XANSYS about the best way to create animations in ANSYS Mechanical APDL (MAPDL).  If you are an MAPDL user you know that the animation tools found there were actually way ahead of their time… in 2001.  Now they are a bit limited and the AVI file it creates uses a codec that are a bit old.  You can write a macro that creates PNG files that you can then string together into an AVI, WMV, MOOV or animated gif file.  Check out the thread (go to XANSYS and search on animation) and you can see some of the options suggested.

As I was reading the thread I remembered that one of the CFD guys from ANSYS, Inc. once asked if we used CFX Post to post process our FEA results.  I of course pretended that I knew all about it and “just didn’t have time to play with it yet.”  The next time I had time at my desk I looked it up and there it was, under File->Load Results you can set the “Files of Type” drop down to read a whole slew of ANSYS Mechanical and MAPDL files:

 

I then promptly forgot about it till I was reading the thread.  So, I figure the best way to remember is to store it for posterity bouncing through the tubes of the interweb  as a blog posting.

ANSYS CFD Post

CFX Post has been generalized and renamed as CFD Post. As a long time ANSYS MAPDL bigot I have to say, and this is a very difficult admission, CFD Post is a a very nice post processor.  Maybe I think like a Torontonian or something, but no matter how long it has been since I’ve used it, I can jump right in and figure out what needs to be done. 

It is a CFD post processor and you can use it to post CFX, FLUENT, FIDAP, TASKFLOW and the general CGNS files that most CFD tools create.  So the terminology is defiantly oriented towards CFD users.  But because CFX supports moving meshes and conjugate heat transfer, it has most things that you need to read and post process an FEA result.  It is also scriptable and can be run in batch for process automation.

If you are not familiar with the tool I recommend that you go into help and find the CFD-Post manual.  Inside you will find a great user guide, but more importantly there are a set of thorough tutorials. The Mixing Elbow example will teach you the basics you need to know to find your way around the interface.

Before we get into the details, it should be pointed out that you are not going to find some huge, awesome, fantastic feature in CFD Post that is not in ANSYS Mechanical.  It does pretty much the same things, just a little differently.  I find it a bit more intuitive and there are some key things it does better as well as some things that it only does.  What we recommend it for is fancy animations, higher quality graphics, batch post processing, and comparing multiple results in one run.

The Key Things to Know

Most things you want to do in CFD Post occur in the tree to the left of the view window.  It has multiple tabs but you will be using the Outline tab most of the time unless you are doing expressions and stuff (like *get’s in MAPDL, we will leave those as an exercise for the user).  Once you load a result you will see four top level folders:  Cases, User Locations and Plots, Report and Display Properties & Defaults:

 

The file you read in is shown in cases.  The external surface of your model is grouped as the Default Boundary.  You can pretty much ignore Report and Display Properties and Defaults unless you want to get fancy or for some reason you really hate the almost identical report generator in ANSYS Mechanical.

This leaves the User Locations and Plots. This is where you specify what you want to see and how you want to see it. 

The most common thing to do is RMB on User Locations and Plots and insert a Contour Plot.  When you do so you get “Details” dialog below your tree that has all the options. To see the details on an existing menu item double-click on it or RMG->Edit:

 

Most of the options you need are in the first tab, Geometry.  The key option is the Variable. This is the result value you want to display.  The list is limited compared to MAPDL but has all of the values that are explicitly stored in the *.RST file.  You will want to set Range to local (min and max from the domain you have selected).  You can also add or delete contours with # of Contours.  I set this to 256 to get a smooth contour.

At the bottom of the Detail there is an apply button, use this to update your plot as you play with the various options.

By default the contour plot shows color bands and lines between color bands, called Isopleths in most programs. CFD Post calls them Contour lines. If you want them off, go to the Render tab and uncheck Show Contour Lines.  If you just want the lines and no color bands, then check Show Contour Lines and uncheck Show Contour Bands.  Below are some plots with the options used to create them.  Any values not specified were at their default value:

 

  • Tree:
    • Default Boundary – Off
    • Contour 1 – On
    • Default Legend View 1 – On
    • Wireframe – On
  • Detail – Geometry:
    • Domains –  All Domains
    • Locations – Default Boundary
    • Variable – Total Mesh Displacement
    • Range – Local
    • # of Contours – 11
  • Detail – Render
    • Show Contour Bands – On
    • Show Contour Lines – On
    • Line Width – 1
    • Constant Coloring – Off

 

  • Tree:
    • Default Boundary – Off
    • Contour 1 – On
    • Default Legend View 1 – On
    • Wireframe – On
  • Detail – Geometry:
    • Domains –  All Domains
    • Locations – Default Boundary
    • Variable – Total Mesh Displacement
    • Range – Local
    • # of Contours – 256
  • Detail – Render
    • Show Contour Bands – On
    • Show Contour Lines – OFF

 

  • Tree:
    • Default Boundary – On
    • Contour 1 – On
    • Default Legend View 1 – On
    • Wireframe – On
  • Detail – Geometry:
    • Domains –  All Domains
    • Locations – Default Boundary
    • Variable – Total Mesh Displacement
    • Range – Local
    • # of Contours – 10
  • Detail – Render
    • Show Contour Bands – OFF
    • Show Contour Lines – On
    • Line Width – 5

Every once in a while you need a black and white contour plot with isopleths.  You can do that in CFD Post very easily with contour options and by going into the Cases part of the tree and editing the Default Boundary options.  On the render tab, set the color to 0.   Make sure that the Default Boundary is checked so it shows up.  Now go to the Contour and:

  • Adjust the number of contours to a reasonable number (10 is good)
  • Under the Labels tab check Show Numbers
  • Under the Render tab:
    • Uncheck Show Contour Bands
    • Check Show Contour Lines
    • Set line width to a larger number, 2 to 5 should work depending on your geometry
    • Check Constant Coloring
    • Make sure Color Mode is at Default

 

I could fill up another 20 screens with all the options available.  Just get in there and play with it.

 

Multiple Result Steps

If your result file has multiple result sets in it, maybe it is a transient run or has multiple load steps, you can access those results by clicking on the little Clock icon at the top of the CFD Post window:

 

You get a dialog that shows each of the steps in your *.RST along with times.  To look at a specific result, click on it and hit apply.

 

I have not found a way for this to work with modal results yet…  For now you need to use a macro to split your modes into separate RST files.

Animation

One of the key reasons to use CFD Post for an FEA model is to get keyframe animations.  Clinton wrote a great article on how to use this great feature a while back.   It doesn’t really do the type of 0 to result animation that we do in the FEA world (like say a mode shape animation)  What it is good at is transients.

It has lots of nice options to make a high quality file.  It also has the ability to animate the motion of the camera, which may be nice to have.  Also, if you are animating a contour plot you want to enable global ranges on your contour. To do this read the section on global variables in the “Things to Know” section below.

The defaults make an OK animation. To get a good animation you need to click on the Options button and then make sure that “White Background” is checked (or you will get JPEG’y distortion) and on the Advanced tab set Quality to High.  Last, check Don’t Encode Last MPEG frame so you can loop.

I’ve included a couple of animations I did with it here:

Basic plot:

 

Fancy with camera moving.  Note that when converting to GIF you loose colors (GIF is only 256 colors).  The AVI looks much nicer.

 

 

A Valve fly through. I put this one on YouTube:

http://www.youtube.com/user/padtinc#p/u/6/0Tg0caQThaA

Here is the animated gif:

 

Things to Know

There are some limitations.  Take a look at the help for CFD Post in section 8.14.5.1

Setting units is hidden a bit.  Go to Edit->Options->Units and set them to what you want.

If you want a deformed shape on your plot, you access that by RMB’ing on the graphics window, but not on your part.  Then choose Deformation and then the level you want. Auto usually works best.