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-   -   how to identify transient simulation converged (http://www.cfd-online.com/Forums/cfx/26927-how-identify-transient-simulation-converged.html)

littlelz January 22, 2009 08:59

how to identify transient simulation converged
 
I am just wondering how to know that my transition simulation has converged. actually, I am running a steady state simulation on airfoil case, and I found local convergence problem which, I guess, is caused by seperation bubble moving, i.e. it is actually a transient problem. So I start running transient simulation. what 's your comments on my case, and how do I know it is already converged (I have no experience on transient simulation).

tnanks, anyway

littlelz

sa January 22, 2009 14:27

Re: how to identify transient simulation converged
 
when your lift and drag curve reaches to some periodic fluctuations.

Glenn Horrocks January 22, 2009 18:26

Re: how to identify transient simulation converged
 
Hi,

Assuming lift and drag are your key outputs, do a sensitivity check of mesh size, convergence tolerance and time step size (if transient) to assess the level of accuracy of your answer.

Glenn Horrocks

littlelz January 27, 2009 06:26

Re: how to identify transient simulation converged
 
Thanks for your reply.

That's what I am concerning; the timestep size in transient simulation. In my case, as I set the time step size very small, say 1e-8, it is converged quickly, 3-5 coeff loops. as I increase it, say 5e-7, it is very slow to reach convergence. What's that mean? how do I decide a proper tiemsetp size? Please advice

Glenn Horrocks January 27, 2009 17:25

Re: how to identify transient simulation converged
 
Hi,

You find the correct timestep size by a sensitivity study. If you are getting 3-5 coeff loops at a timestep of 1e-8 then that is a good place to start as that is about the point where the transient solver usually works best - but check with a sensitivity analysis.

Glenn Horrocks

anne January 27, 2009 19:13

Re: how to identify transient simulation converged
 
oh, I see.

many thanks for your help. all the best


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