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Old   April 21, 2011, 09:31
Default Quick question about convergence.
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Ammar Tareen
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Dear All,

In some cfd software like fluent or solidWorks, when we carry out a simulation, the software decides to stop the simulation when a convergent solution is reached.

In openfoam, on the other hand, we always have to specify an ending time of our simulation. My question: is it possible to have openfoam determine when a convergent solution is reached and have it stop the simulation automatically?

Or do we have to somehow figure out when the system should be in equilibrium and specify that time?

Any help or pointers appreciated,
Regards,
~Ammar.
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Old   April 21, 2011, 10:10
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Ammar Tareen
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Simply stated, in steady state solvers, (e.g. rhoSimpleFoam, the one I am using) why is there time involved in the control Dictionary? Aren't all the time derivatives supposed to be zero?
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Old   April 21, 2011, 10:42
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Vesselin Krastev
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1) Concerning the timeStep settings in the steady state solvers, in such cases they are simply iteration counters: for instance, setting a timeStep equal to 1 with an endTime of 1000 in simpleFoam (or rhoSimpleFoam or other steady solvers) means that the solver in question will perform 1000 SIMPLE loops on velocity, pressure and other (eventual) quantities before stopping. But you can obtain the same identical result by setting a timeStep of 0.1 and an endTime of 100 and so on.

2) About the issue of stopping the simulation at a given level of convergence, you can set your own convergence level in the fvSolution dictionary (take a look here Continuity. Stop critertion of PISO and SIMPLE in OpenFOAM solvers. ) : usually 10^-06 for all quantities is enough for assuming a substantially converged solution, but of course it depends of the kind of application we're talking about...

hope this helps

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Old   April 21, 2011, 10:48
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Ammar Tareen
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1) Thank you Vesselin, makes a lot of sense.

2) So is that only way to go about it? Here's what I am worried about. let's say I run my case for a 100 iterations, from 0 to 99. But let's also assume that my required tolerances were reached at step 40, meaning I would get a nice solution at that step. So what happens in all those iterations after step 40? Are they wasted? That's the kind of thing I want to avoid.

Thanks for your help

~Ammar.
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Old   April 21, 2011, 10:56
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Vesselin Krastev
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Quote:
Originally Posted by atareen64 View Post
So what happens in all those iterations after step 40? Are they wasted?

No, if you set the convergence criterion like is shown in the post I mentioned, once the criterion is satisfied the solver stops (which means that in this case the solution procedure is stopped at time step 41). Instead, if the criterion is not satisfied during the solution iterative procedure, the solver will stop only at the endTime set in the controlDict dictionary.

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Old   April 21, 2011, 10:58
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Oh wow thank you that seems like exactly what I was looking for!

The link wasn't opening before but now it's working. I'll try it right now! THANKS A LOT :-)
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