Problem with the Coulomb sliding law

Extension of Elmer in computational glaciology

Problem with the Coulomb sliding law

Postby MTrevers » 29 Aug 2017, 22:54

I've come across a problem when applying the Coulomb sliding law to my simulation.

The output is completely different when setting the Cauchy flag on or off (which I know shouldn't be the case). With the flag set to False, the flow solution doesn't converge. When True, it does but the results are completely insensitive to the value of $As in the .sif (attached).

I've looked at the code and I can see that in the Friction_Coulomb function, StressValues is receiving zeros each time, so this is the reason for the incorrect behaviour. By contrast, FlowValues is receiving sensible numbers for velocity and pressure.

Without being able to debug the code, I can't get any closer to the route of the problem. Is anyone able to help further? Have I made some mistake in the .sif?

Thanks
Matt
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case_coulomb.sif
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MTrevers
 
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Re: Problem with the Coulomb sliding law

Postby gagliar » 01 Sep 2017, 09:52

Dear Matt,

The issue here is that stresses are unknown (and therefore set to zero) for the first time you enter the Stokes solver. This will be always the case for the first time step of a transient simulation or for the first iteration of a steady state simulation, as in your case. To solve this, you will need at least 2 steady state iterations, which can be forced by setting
Code: Select all
Steady State Max Iterations = 5
Steady State Min Iterations = 2

in the Simulation section.

Hope it helps,
Olivier
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Re: Problem with the Coulomb sliding law

Postby gagliar » 05 Sep 2017, 10:30

Dear Matt,

I have modified the user function such that if the Cauchy normal stress is zero (which should be the case at a first iteration of a steady state problem or at the first time step of a transient one), then the normal stress is approximated by the isotropic pressure. If you do only one iteration, you should then get the same result with Cauchy or deviatoric stress. But, I would recommand to go for more than one iteration anyway to got the true evaluation of the normal stress (even if I agree that it should not change that much the evaluation of the effective pressure). On top of that, I would recommand to define an effective pressure variable so that you can have it evaluated correctly even at the first iteration.

Can you test this (you will need to get the last Elmer/Ice version and re-compile).

Hope it helps,
Olivier
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