Using a steady state for initial conditions in transient...

MarkHerskind · May 18, 2021, 04:22

I'm trying to run a simulation, which is pretty unstable...

I would like to initialize my transient simulation from a steady state simulation...

My setup is in Workbench 21R1 using RSM, and I do 5000 iterations in my SS, which converges fine..

When I start my transient simulation I get this warning:

Quote:

Warning: The time step size (1e-06) in the session did not match the time step size in the data file (1), and has been overwritten by the value from the data file

So it sets the timestep to 1s, which off course diverges...

I can't seem to find the option to use my defined timestep (1e-6)..

I usually work i OF/CFX, so Fluent is a bit unknown to me, so sorry if this is very simple issue...

Thanks
Mark

Chris13 · December 12, 2022, 09:30

Hi Mark,

since I encounter the same warning with a comparable setup, I wonder if you found a solution? I contacted Ansys support, but they couldn't help me quickly and will call me back. Maybe you have a faster solution?

Best regards
Chris

AlexanderZ · December 13, 2022, 00:59

run fluent standalone (without workbench) there should be no problems

Chris13 · December 16, 2022, 06:14

I can't because fluent standalone doesn't support it's own in-house cluster management system / scheduler (RSM+ARC). You have to use workbench for that. Ansys support told me this will be looked into in future releases. Both, support of RSM in fluent standalone and the time-step size problematic.
For now there are two workarounds:

1. set the physical time step within the previous stationary simulation even though you don't need it (rpsetvar 'physical-time-step 0.001) this will then be saved in the settings file and the warning is gone.

2. use the "automatic initialization of the solution and case modification" option within fluent (the transient simulation setup) to set the time-step automatically between initialization and the first time-step. You will still get the warning and the time-step will be set back to the default value but it's then being overwritten.

I hope this explanation is clear and helpfull.

December 13, 2022, 00:59		#3
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	run fluent standalone (without workbench) there should be no problems __________________ best regards ****************************** press LIKE if this message was helpful

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December 12, 2022, 09:30		#2
Chris13 New Member Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 9	Hi Mark, since I encounter the same warning with a comparable setup, I wonder if you found a solution? I contacted Ansys support, but they couldn't help me quickly and will call me back. Maybe you have a faster solution? Best regards Chris

December 16, 2022, 06:14		#4
Chris13 New Member Chris Join Date: Nov 2016 Location: Germany Posts: 27 Rep Power: 9	I can't because fluent standalone doesn't support it's own in-house cluster management system / scheduler (RSM+ARC). You have to use workbench for that. Ansys support told me this will be looked into in future releases. Both, support of RSM in fluent standalone and the time-step size problematic. For now there are two workarounds: 1. set the physical time step within the previous stationary simulation even though you don't need it (rpsetvar 'physical-time-step 0.001) this will then be saved in the settings file and the warning is gone. 2. use the "automatic initialization of the solution and case modification" option within fluent (the transient simulation setup) to set the time-step automatically between initialization and the first time-step. You will still get the warning and the time-step will be set back to the default value but it's then being overwritten. I hope this explanation is clear and helpfull.