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Not convergent when the mesh is dense, why?
Posted Oct 22, 2012, 9:09 a.m. EDT RF & Microwave Engineering, Studies & Solvers Version 4.2a 5 Replies
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Convergence problem is kinda mysterious...
This time convergence is nor reached when the mesh is dense. I would say perhaps too dense in some region. Some coarser mesh leads to convergence. What's the possible reason for this?
From my point of view, the sub-domain with more elements has more weight in the quantity used to evaluate convergence. Then importance of sub-domain with too dense mesh may be overemphasized. The convergence becomes sensitive to the behavior there. So too dense mesh only in some part may not be a good idea. What do you think?
Gracias!
Best
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you should have a dense mesh in regions with high gradients on the dependent variables, such to resolve the dependent variables correctly, no reason to have a dense mesh where the gradients is low, but this concerns ALL dependent variables, often we check only 1-2, and forget about the others, not to say the local Langrange variables.
So I agree that the mesh density can influence the error, but I'm not sure that is the main reason
--
Good luck
Ivar
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Some of the most common reasons for non-convergence have nothing to do with the mesh and refining the mesh cannot be expected to help fix the problem.
A couple of such reasons:
- The problem is not well-posed, i.e. there isn't a solution to the problem as posed, or there are multiple solutions
- The initial values provided to the solver are not sufficiently close to the solution of the problem.
If your model is still refusing to converge despite your refining the mesh, I would suggest looking into those two possibilities.
Best,
JF
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Thanks for the reply!
As for the two possible reasons you suggested, since the problem does have a unique physical solution, I think only the second is probable. But generally how can one improve the model in this direction? Although putting some approximation as the background field may be one method, it's not really practical to get such an good approximation.
Best!
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- As far as getting better intial conditions, you could try ramping up the nonlinearities or ramping up the loads. There was a minicourse on solvers at the COMSOL Conference earlier this month that covered this topic.