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Error in pressure drop simulation

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Hi!
I get the following error when trying to simulate the pressure drop in a long pipe with a lot of inlets (se attached model).

Failed to find a solution.
Segregated group 1

Matrix has zero on diagonal for the SSOR update submatrix in the Vanka algorithm.
In segregated group 1:
Returned solution is not converged.

The model has been working before but when i made a few changes it won't run anymore.

Thank you for your help!


1 Reply Last Post Mar 12, 2015, 10:57 a.m. EDT
Frank van Gool COMSOL Employee

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Posted: 10 years ago Mar 12, 2015, 10:57 a.m. EDT
Dear Olof,

For CFD models I generally advise the following steps (in that order):
- enable pseudo timestepping, this can stabilize your model (already enabled in your model)
- try to reset your solver (remove all, and rebuilt it). This can set up the problem in a proper way
- Use ramping, that is try with lower velocities / pressure differences first, and slowly increase them
- Simplify the physics. I notice that you use mass flux boundaries. COMSOL finds a single pressure for these inlets, so that the total flux is equal to the prescribed value. You could try to rewrite these to velocity inlets, to simplify the model.
- switch to a direct solver

You can also take a few steps back, and check which modification caused your model to stop converging. That is typically the section that you need to modify.

Best regards,
Frank
Dear Olof, For CFD models I generally advise the following steps (in that order): - enable pseudo timestepping, this can stabilize your model (already enabled in your model) - try to reset your solver (remove all, and rebuilt it). This can set up the problem in a proper way - Use ramping, that is try with lower velocities / pressure differences first, and slowly increase them - Simplify the physics. I notice that you use mass flux boundaries. COMSOL finds a single pressure for these inlets, so that the total flux is equal to the prescribed value. You could try to rewrite these to velocity inlets, to simplify the model. - switch to a direct solver You can also take a few steps back, and check which modification caused your model to stop converging. That is typically the section that you need to modify. Best regards, Frank

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