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"Unable to find solution" with 2 VERY simple models

Posted Jul 22, 2011, 11:13 a.m. EDT Low-Frequency Electromagnetics, Studies & Solvers Version 4.2, Version 4.2a 3 Replies

Adam Steele
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In the files attached I was trying to see the B-field around a very simple permanent magnet.

In the one case, its a ring using the "2D axisymetric" geometry and in the other it's a square magnet with symmetry along the z-axis. In both cases I get 'Unable to find solution errors" but I cannot for the life of me imagine why.

A 3D model, also attached has no problem with a solution. So what am I doing wrong in the 2D cases?


Thanks,
Adam
3 Replies Last Post Apr 26, 2012, 11:33 a.m. EDT
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Hello Adam Steele

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Jacob Block
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Posted: 1 decade ago Oct 10, 2011, 2:46 p.m. EDT
I'm a newbie at Comsol and am having the exact same problem. Why don't either of these converge to a solution? Help would be GREATLY appreciated! Thanks!!

And just FYI, your simple_axial.mph doesn't have the aluminum defined inside the r2 domain. I feel like that should still converge though, so I'm not sure what the deal is. I've also tried making fillet-edged rectangles thinking maybe it was an issue with discontinuity. I've tried changing the sigma of air to be non zero. I've tried changing the mesh density, etc. If someone could make a 2D model like the ones attached converged I might be able to find what the difference is.

Thanks again.
I'm a newbie at Comsol and am having the exact same problem. Why don't either of these converge to a solution? Help would be GREATLY appreciated! Thanks!! And just FYI, your simple_axial.mph doesn't have the aluminum defined inside the r2 domain. I feel like that should still converge though, so I'm not sure what the deal is. I've also tried making fillet-edged rectangles thinking maybe it was an issue with discontinuity. I've tried changing the sigma of air to be non zero. I've tried changing the mesh density, etc. If someone could make a 2D model like the ones attached converged I might be able to find what the difference is. Thanks again.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Oct 11, 2011, 10:09 a.m. EDT
Hi

I'm not sure exactly what is wrong, the error is more a warning (in this case) that the solution is not very precise, as it solves :

Failed to find a solution.
The relative error (13) is greater than the relative tolerance.
Returned solution is not converged.


If you use mf rather than mfnc it solves without issues.

One thing, round off the angles of the magnetic field, with a small fillet radius, you will get less singulatity numerical effects

--
Good luck
Ivar
Hi I'm not sure exactly what is wrong, the error is more a warning (in this case) that the solution is not very precise, as it solves : Failed to find a solution. The relative error (13) is greater than the relative tolerance. Returned solution is not converged. If you use mf rather than mfnc it solves without issues. One thing, round off the angles of the magnetic field, with a small fillet radius, you will get less singulatity numerical effects -- Good luck Ivar
BUAS BMELab
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Posted: 1 decade ago Apr 26, 2012, 11:33 a.m. EDT
Hi Adam,

You maybe already find the solution for your problem. But I give my answer to solve (hopefully) the question.

I got exactly the same experience as you. The 3D model gave me quite a good result but I had no chance to get an 2D solution (Error: Failed to find a solution. The relative error (3.4) is greater than the relative tolerance. Returned solution is not converged).

The Problem was that I set any reference for my problem (so there was a missing equation).
I had to add a Zero Magnetic Scalar Potential: Right-click on Magnetic Fields, No Currents (mfnc) -> Points* -> Zero Magnetic Scalar Potential. Then select a point (it's nummerically better to choose a point on the symmetric axis and away from the magnets).

* It's important to set the reference at one point and not on a line.

Best regards

BMELab, Aymeric
Hi Adam, You maybe already find the solution for your problem. But I give my answer to solve (hopefully) the question. I got exactly the same experience as you. The 3D model gave me quite a good result but I had no chance to get an 2D solution (Error: Failed to find a solution. The relative error (3.4) is greater than the relative tolerance. Returned solution is not converged). The Problem was that I set any reference for my problem (so there was a missing equation). I had to add a Zero Magnetic Scalar Potential: Right-click on Magnetic Fields, No Currents (mfnc) -> Points* -> Zero Magnetic Scalar Potential. Then select a point (it's nummerically better to choose a point on the symmetric axis and away from the magnets). * It's important to set the reference at one point and not on a line. Best regards BMELab, Aymeric

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