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Convergence with coupled implicit solver

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Old   October 28, 2005, 08:47
Default Convergence with coupled implicit solver
Henrik Ström
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I have an unsteady simulation that I have run earlier versions of using the segregated solver, and that worked just fine. However, now I want to add a very fast chemical reaction to my system, which will make the problem stiff. This causes the segregated solver to have problems as the reaction starts after a few hundred timesteps. I therefore want to change to the coupled (implicit) solver and also possibly use the special stiff-chemistry ODE solver that is built-in in the coupled solver.

My problem is, though, that when I change to the coupled solver, the solution diverges already in the first timestep (that is, before the reaction even takes place and for a situation where the segregated solver still works OK)... So, what can I do to get it to work? I have tried very low Courant numbers, but that does not seem to help (only slow down the rate at which it diverges). Are there any other parameters that I should experiment with? (FAS in the explicit formulation is not applicable since it is an unsteady simulation).

Any help or hints is greatly appreciated!

Thanks, Henrik
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Old   October 29, 2005, 03:57
Default Re: Convergence with coupled implicit solver
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Hi 1-use very small time step for first itration 2-use very small cournt number in first 3-enable stiff model 4- refine your mesh goodbye
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