Case comparison of steady state and time-averaged transient solutions

k.vafiadis · September 17, 2012, 13:11

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, 15:47

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, 06:37

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.

September 17, 2012, 13:11	Case comparison of steady state and time-averaged transient solutions	#1
k.vafiadis New Member Kyriakos Vafiadis Join Date: Feb 2011 Location: Kozani, Greece Posts: 29 Rep Power: 15	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 __________________ -- Kyriakos Vafiadis Mechanical Engineer, PhD candidate

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Solver for transonic flow?	Martin Hegedus	OpenFOAM Running, Solving & CFD	22	December 16, 2015 05:59
error message	cuteapathy	CFX	14	March 20, 2012 07:45
time step and iterations in steady state problem using transient solver	jing113cn	FLUENT	2	January 15, 2010 04:18
time to reach steady state with transient simulati	prabhu	CFX	2	April 21, 2008 11:18
Transient vs Steady State	Adam	CFX	1	April 12, 2007 12:34

October 19, 2012, 15:47		#2
sukram-hx New Member Markus Join Date: Dec 2011 Posts: 1 Rep Power: 0	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+modeTemperature.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

October 20, 2012, 06:37		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,857 Rep Power: 144	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.