# Transient Simulations

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 July 9, 2013, 20:23 Transient Simulations #1 Senior Member   --------- Join Date: Oct 2010 Posts: 302 Rep Power: 8 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. __________________ Best regards, Santhosh.

 July 9, 2013, 23:49 #2 Senior Member     hamed Join Date: Apr 2009 Posts: 148 Rep Power: 8 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

 July 10, 2013, 02:17 #3 Senior Member   --------- Join Date: Oct 2010 Posts: 302 Rep Power: 8 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 __________________ Best regards, Santhosh.

 July 10, 2013, 02:46 #4 Senior Member     hamed Join Date: Apr 2009 Posts: 148 Rep Power: 8 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?

 July 10, 2013, 03:02 #5 Senior Member   --------- Join Date: Oct 2010 Posts: 302 Rep Power: 8 No I think it's not a recommended practice, because the simulation then takes longer. __________________ Best regards, Santhosh.

 July 10, 2013, 04:13 #6 Senior Member     hamed Join Date: Apr 2009 Posts: 148 Rep Power: 8 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.

 July 10, 2013, 05:43 #7 Senior Member   Join Date: Jul 2011 Location: Berlin, Germany Posts: 120 Rep Power: 6 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".

 July 13, 2013, 06:57 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,954 Rep Power: 85 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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