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Returned solution is not converged

Posted Nov 30, 2016, 3:45 a.m. EST Heat Transfer & Phase Change, Geometry, Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 5.1 3 Replies

Jan Kolbe
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Hello COMSOLants,

I am new to COMSOL and want to do a stationary thermal simulation of high voltage power cables with COMSOL Multiphysics 5.1.
I have a soil geometry that is 20m*10m and three cables placed separated in the centre of the 20m width and in about 1.5m burial depth.
I started simple to get the simulation running first by adding only Joule Heating Physics. As I have some air domains I also added the surface-to-surface radiation to those parts.

Anyway, as soon as I start applying a voltage to the cables via the Electric current physics of joule heating the solver gets the following error even before and after adding the radiation physics no matter how coarse or fine I set the mesh settings:

"Failed to find a solution.
Maximum number of Newton iterations reached.
There was an error message from the linear solver.
The relative residual (0.029) is greater than the relative tolerance.
Returned solution is not converged.
- Feature: Stationary Solver 1 (sol1/s1)"

Any suggestions what could be wrong?

Thanks

Best regards
Jan

3 Replies Last Post Dec 2, 2016, 8:18 a.m. EST
Jeff Hiller COMSOL Employee

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Posted: 8 years ago Nov 30, 2016, 8:57 a.m. EST
Hi Jan,
There are many possible reasons for non-convergence.
One common rookie mistake that could lead to this is if you set up your model in such a way that it does not have any solution, or that it has multiple solutions. For instance, you'll want to check that there is somewhere for the heat generated in the system to flow to (There is no stationary solution if you have a net positive heat source and only insulation boundary conditions). Similarly, check that there is somewhere for currents injected into the system to leave. At the opposite end of the spectrum, a DC current problem without voltage specified anywhere has an infinite number of mathematical solutions, which differ from each other by a constant voltage and which the software can't choose from, so make sure you specify a gauge voltage somewhere to make the solution unique.
Like I said, there could be other reasons, so if the tips above don't help, you may want to post your file so other users can chime in with more educated answers.
Best,
Jeff
Hi Jan, There are many possible reasons for non-convergence. One common rookie mistake that could lead to this is if you set up your model in such a way that it does not have any solution, or that it has multiple solutions. For instance, you'll want to check that there is somewhere for the heat generated in the system to flow to (There is no stationary solution if you have a net positive heat source and only insulation boundary conditions). Similarly, check that there is somewhere for currents injected into the system to leave. At the opposite end of the spectrum, a DC current problem without voltage specified anywhere has an infinite number of mathematical solutions, which differ from each other by a constant voltage and which the software can't choose from, so make sure you specify a gauge voltage somewhere to make the solution unique. Like I said, there could be other reasons, so if the tips above don't help, you may want to post your file so other users can chime in with more educated answers. Best, Jeff
Jan Kolbe
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Posted: 8 years ago Dec 1, 2016, 5:46 a.m. EST
Hi Jeff,

thanks for your reply.
I checked all your tips and my simulation is working now.

Another question I figured out is if there is a possibility of defining a normal current in a 2D model?
So if I have the XY-plane I'm working at and want to define a current flowing in z-direction in one domain how can I do that?

Thanks for helping

Jan
Hi Jeff, thanks for your reply. I checked all your tips and my simulation is working now. Another question I figured out is if there is a possibility of defining a normal current in a 2D model? So if I have the XY-plane I'm working at and want to define a current flowing in z-direction in one domain how can I do that? Thanks for helping Jan
Jeff Hiller COMSOL Employee

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Posted: 8 years ago Dec 2, 2016, 8:18 a.m. EST
Hi Jan,
Since this question is not directly related to this thread, I would recommend you start a separate thread for it.
Best,
Jeff
Hi Jan, Since this question is not directly related to this thread, I would recommend you start a separate thread for it. Best, Jeff

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