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

[Eric Lindsey]

On Thu, Dec 5, 2013 at 8:55 AM, Matthew Knepley <knepley at mcs.anl.gov> wrote:

> 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.
>

The fault does extend all the way through the domain; there are traction
boundary conditions on those sides of the domain that match the stresses on
the fault. The fault itself has static friction -- so I think some
nonlinear iterations are required to solve for the slip that satisfies the
mohr-coulomb condition on the fault. The linear solves do finish in 1
iteration, but I think the Jacobi preconditioner for the fault
(fieldsplit_1) is pretty weak, which explains the number of nonlinear
iterations. Unless I've misunderstood 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
>

Thanks - I think this helped... It did prevent the zero pivot in the
plastic case, but now one of the inner linear solves fails to converge
(after 10,000 iterations), eventually leading to Pylith giving up with the
message:

WARNING! Fault opening with nonzero traction., v_fault: 175, opening:
1.41881e-09, normal traction: -6103.07

The normal traction is still negative and this doesn't happen in the
elastic case... I think this must be a numerical issue. So there is still
an issue with my solver once plastic yielding starts, I think.


> 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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>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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