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-   -   Case comparison of steady state and time-averaged transient solutions (http://www.cfd-online.com/Forums/cfx/107097-case-comparison-steady-state-time-averaged-transient-solutions.html)

k.vafiadis September 17, 2012 12:11

Case comparison of steady state and time-averaged transient solutions
 
Hello, I want to find the differences (compare cases) between a steady state solution and a time-averaged transient solution, for example a contour of Velocity in Stn Frame.trnavg versus a contour of Velocity in Stn Frame.
Case comparison would be a choice if I had to compare same things.
Is any reference to the documentation available?
Thanks in advance :)

sukram-hx October 19, 2012 14:47

Hello,
this exactly is the same thing I am trying to do in CFX-Post for several days now. It seems there is no way to compare different variables (e.g. velocity.trnavg and velocity) in the case comparison mode at the same time.
To overcome this problem, I had two ideas (unfortunately, both are (currently) not working):

1): For the steady-state simulation, definition of additional variables in CFX-Pre, whose names have the suffix ".Trnavg", and make them a copy of the standard variables. The problem is: CFX does not allow the "." in the name of additonal variables --> :-/

2): The second idea required a little bit more effort for the implementation. In both simulations, an additional variable is created with the same name (for example "mode"). In the steady simulation, the variable is assigned a constant value of "0", whereas the same variable in the transient simulation gets the value "1". Now you have to re-run your cases with these additional variables in order to include them into your result files. After opening both result files in CFX-Post you can create an expression of the following form and assign it to a variable:

MyTemperatureSteadyTrnavg=(1-mode)*Temperature+mode*Temperature.Trnavg

Because mode is 1 for transient runs and 0 for steady ones, the new variable automatically links to the desired variable of each simulation type.

Using these "combined" variables however works fine for contour plots but fail also when you try to switch to the case comparison mode. An error message with something like "The variable MyTemperatureSteadyTrnavg is not available in the whole domain" appears --> :-/

So far I am also looking for some way to compare transient and steady cases with respect to differences.
Maybe this is at least a little bit helpful for your studies.
If you have some further ideas, please let me know.

Regards,
Markus

ghorrocks October 20, 2012 05:37

This sounds like an annoying problem. This sounds worthy of reporting to ANSYS as a feature request.

I have not been able to try this, so am not sure if it will work:
In the transient simulation generate a variable and make it equal to the Velocity.trnavg. In the steady state simulation generate a variable with the same name and make it equal to Velocity. Hopefully then you can use the compare case stuff in CFD-Post to compare the variables together.


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