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Detailed Log information - No convergence because of spatial or time discretization ?

Posted 3 juil. 2018, 05:09 UTC−4 Electromagnetics, Low-Frequency Electromagnetics Version 5.3 4 Replies

Jan
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Hello,

see attached the detailed log of my time dependent simulation with linear material properties. The model fails to converge and I don't know why. By looking at the log, can one see where the problem might be?

From my point of view, "SolEst" and "LinRes" look okay. I think the problem might be with"LinErr". But what does this mean practical at all?

Kind regards, Jan

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4 Replies Last Post 5 juil. 2018, 07:25 UTC−4
Jan
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Posted: 6 years ago 3 juil. 2018, 05:16 UTC−4

I have to add, that the model basically works. I only changed one little thing from the working model: a rotation was added.

I have to add, that the model basically works. I only changed one little thing from the working model: a rotation was added.
Henrik Sönnerlind COMSOL Employee

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Posted: 6 years ago 4 juil. 2018, 02:12 UTC−4
Updated: 6 years ago 4 juil. 2018, 05:28 UTC−4

Hi Jan,

This indicates that the system matrix is ill-conditioned. There can be many underlying reasons, ranging from the actual physical properties of the problem you are solving to erroneous boundary conditions or material properties.

If the solver for the 'innermost' problem (the linear solver) in your iterations reports this kind of problem, the 'outer' nonlinear solver will not accept the solution as converged, even though its own criteria are fulfilled.

You can force the nonlinear solver to accept the solutions nevertheless. Change the settings in the 'Error' section in the 'Direct' node in the solver sequence. This is usually safe in a nonlinear problem, since a 'bad' solution will give extra iterations. Your graph indicates that there are no large residuals, so suppressing the check on the solution from the linear solver seems OK.

Regards,
Henrik

-------------------
Henrik Sönnerlind
COMSOL
Hi Jan, This indicates that the system matrix is ill-conditioned. There can be many underlying reasons, ranging from the actual physical properties of the problem you are solving to erroneous boundary conditions or material properties. If the solver for the 'innermost' problem (the linear solver) in your iterations reports this kind of problem, the 'outer' nonlinear solver will not accept the solution as converged, even though its own criteria are fulfilled. You can force the nonlinear solver to accept the solutions nevertheless. Change the settings in the 'Error' section in the 'Direct' node in the solver sequence. This is usually safe in a nonlinear problem, since a 'bad' solution will give extra iterations. Your graph indicates that there are no large residuals, so suppressing the check on the solution from the linear solver seems OK. Regards, Henrik
Jan
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Posted: 6 years ago 4 juil. 2018, 05:24 UTC−4
Updated: 6 years ago 4 juil. 2018, 05:30 UTC−4

Hi Henrik,

thank you very much for your reply. I am testing the model without the Error check right now.

Regards,
Jan

Hi Henrik, thank you very much for your reply. I am testing the model without the Error check right now. Regards, Jan
Jan
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Posted: 6 years ago 5 juil. 2018, 07:25 UTC−4

Hi again,

I tried Henriks tip with the Error check and the model does solve for a few time steps now (see the attached plot of the Convergence). Unfortunately, after some succesfull time steps, the nonlinear solver fails to converge.

I checked if the dependent variables show questionable behaviour. They look very smooth and not suspicious. From my point of view, the results look physically correct, but the solver says otherwise...

Kind regards, Jan

Hi again, I tried Henriks tip with the Error check and the model does solve for a few time steps now (see the attached plot of the Convergence). Unfortunately, after some succesfull time steps, the nonlinear solver fails to converge. I checked if the dependent variables show questionable behaviour. They look very smooth and not suspicious. From my point of view, the results look physically correct, but the solver says otherwise... Kind regards, Jan

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