[CIG-SHORT] convergence troubles: drucker-prager material with fault

[Matthew Knepley]

On Wed, Dec 4, 2013 at 11:23 PM, Eric Lindsey <elindsey at ucsd.edu> wrote:

> Hi guys,
>
> I'm having some trouble getting the model with a fault in an elastoplastic
> rheology (plane strain) to converge. I'm pretty sure I've fixed all the
> boundary conditions to behave correctly now. The LU method fails entirely
> once yielding occurs, and the approximate method (multiplicative fieldsplit
> using ml and jacobi for the two domains) does not converge. More details:
>
> The first thing I tried was using (solver04.cfg) from the tutorial, which
> is a schur fieldsplit with LU and the custom Pylith preconditioner. In the
> case of a fault in a purely elastic material, this procedure is working
> fine. The nonlinear solve does take over 100 iterations, I'm not sure if
> this is just a function of my mesh; anyway I'm not too worried about
> optimization yet.
>

Is this a through going fault? The linear solve is exact, so we would not
expect this.


> In the plastic case, whenever stresses near the bends in the fault begin
> to exceed the yield criterion (I increase them slowly to this value over
> several time steps), I get a Zero Pivot error. But maybe this is not
> unexpected for a plastic rheology?
>

Using this option

fs_fieldsplit_0_pc_factor_shift_type = nonzero

will prevent a 0-pviot, but the preconditioner becomes weaker.

   Matt

I also tried the solver options for an approximate solution with
> multiplicative fieldsplit suggested by Brad (solver08.cfg): The elastic
> case still works fine; it uses way more KSP iterates but still finishes in
> about the same total time. But the plastic case still throws a Zero Pivot.
> When I ran it earlier without increasing snes_max_it, I got a string of
> these messages instead:
>
> "Nonlinear solve did not converge due to DIVERGED_FNORM_NAN iterations 0"
>
> In either case, I think I must need a better solver. I've gone back
> through the tutorial videos for the solvers, but I'm not very clear on how
> to apply the nonlinear options from the driven cavity problem to my
> situation, or really where to start. My input files are attached, but I'm
> happy to send any additional files or output as needed. Any insight,
> suggestions, or wild guesses?
>
> Thanks
> Eric
>
>
>
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