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MK August 27, 2008 10:00

About the iteration panel.
Hello everybody I am analysing an unsteady problem via Fluent. In the iteration panel, we must fill Time Step Size, Number of Time Steps, and Maximum Iterations per Time Step. I can not understand these items very well, especially their relation with the convergence. You see, when the problem converges, Fluent doesn't stop iteration. Why? Another problem is that I do not know how to specify the Time Step Size and Number of Time Steps Values? Regards.

NRD August 27, 2008 17:54

Re: About the iteration panel.
Since you use an implicit time stepping, FLUENT iterates for each time step. For unsteady case, when FLUENT says that the solution is converged, it means, the solution has converged for that time step and then it will move forward in time.

You can specify time step size depending on the CFL condition

MK August 29, 2008 08:16

Re: About the iteration panel.
Dear NRD If at the end of iteration the problem did not converge, should I change the problem parameters and do the iteration again? Could you please introduce me a reference on this subject? Thanks a lot.

NRD September 4, 2008 12:18

Re: About the iteration panel.
That's correct.

If the solution does not converge for the number of iterations that you specified then you can try reducing the time step and rerun the simulation. If you are not particular about the level of accuracy then you can just specify number of iterations and run it even if the case does not converge at each time step. Since it is implicit time marching, the code is unconditionally stable (i.e. code will be stable even if the time step is quite large but will degrade the accuracy).

FLUENT documentation is the best reference on this.

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