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 saisanthoshm88 July 9, 2013 20:23

Transient Simulations

Could some one please suggest on how to judge the convergence in transient simulations, I mean in a steady state simulation it is possible to judge the convergence using monitor points. But in a transient simulation, the monitor points also show a time dependent trend so what criteria could be used to judge convergence in transient simulations.

And is it always fine to initialize a transient simulation with a steady state simulation for better convergence.

 hmasenger July 9, 2013 23:49

yes.it is always a good way to run a steady state simulation before transient one to have an accurate initial condition.
for the convergence question, i know every time step should be converged just like a steady state run .you should let results in all sub steps converge to a constant value

 saisanthoshm88 July 10, 2013 02:17

Hamed, thanks for your response but monitoring a quantity for each time step could be difficult because as time progresses each time step may even converge within 2 iterations

 hmasenger July 10, 2013 02:46

why don't you set the RMS convergence criteria to a small value like 10e-5 or 10e-6 to let the solver go further iterations to converge?

 saisanthoshm88 July 10, 2013 03:02

No I think it's not a recommended practice, because the simulation then takes longer.

 hmasenger July 10, 2013 04:13

you can set maximum iteration loops to a specific number to prevent from consuming time in another word for example,by setting RMS to 10e-6 and a maximum iteration to 50 you can run the solver up to 50 iteration and maybe the RMS equal to 10e-4.I hope you get the point.

 monkey1 July 10, 2013 05:43

I would monitor a related value that is supposed to reach convergence.
E.g. when I simulate the flow and dispersion through a domain I always monitor the pressure difference between in and outlet. That one beeing related to the required result variables / flow solution and supposed to reach a "steady state".

 ghorrocks July 13, 2013 06:57

The concept for assessing convergence in transient simulations is the same as for steady state - you pick some important points and see how it converges with different variations on the thing you are controlling (mesh size, convergence tolerance, time step size etc). It is just in a transient simulation the thing you are assessing does not necessarily converge to a single value, but is a function of time. This complicates things a little but is still pretty easy to handle.

The technique I usually use is to run the transient simulation for long enough for the important physics to start doing something, but as short as possible. Let's say that is after 1s of simulated time. Then you do a mesh convergence study using the result of important monitor points at the 1s time step. Then you have a single value just like a steady state simulation to do a normal convergence assessment on (including Richardson extrapolation if you wish).

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