[petsc-users] accessing fields from previous, converged solution.

[Matthew Knepley]

On Thu, Aug 17, 2017 at 3:13 AM, Maximilian Hartig <imilian.hartig at gmail.com
> wrote:

> Thanks for your help Matt.
>
> On 16. Aug 2017, at 16:17, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Wed, Aug 16, 2017 at 8:56 AM, Maximilian Hartig <imilian.hartig at gmail.
> com> wrote:
>
>> Hello,
>>
>> I have a problem with several fields that I solve with PetscFE and TS. I
>> now need to access the solution from the previous timestep to compute the
>> residual for the current timestep.
>> I tried a TSMonitor with the following code in it:
>>
>> TSGetDM(ts,&dm);
>> DMClone(dm,&dm_aux);
>> DMGetDS(dm,&prob_aux);
>> DMSetDS(dm_aux,prob_aux);
>> DMCreateGlobalVector(dm_aux,&old_solution);
>> VecCopy(u,oldsolution);
>> PetscObjectCompose((PetscObject) dm, “A”, (PetscObject) old_solution);
>>
>> VecDestroy(&old_solution);
>> DMDestroy(&dm_aux);
>>
>> hoping that it would create an auxiliary field that I could access in the
>> evaluation of the residual. It did that but messed with the discretisation
>> of the initial problem in some way. So I figure that adding auxiliary
>> fields to a dm after having fed it to a TS context is not something you
>> should be doing.
>>
>> Is there a way to access the fields of the solution for the previous
>> timestep during the evaluation of the current residual?
>>
>
> First, I can show you how to do what you are asking for. I think you can
> simply
>
> PetscObjectQuery((PetscObject) dm, "old_solution", (PetscObject *)
> &old_solution);
> if (!old_solution) {
>   DMCreateGlobalVector(dm, &old_solution);
>   PetscObjectCompose((PetscObject) dm, "old_solution", old_solution);
> }
> VecCopy(u, oldsolution);
>
>
> Unfortunately, this produces an error and tells me “An object cannot be
> composed with an object that was composed with it."
>
>
> Second, I think a better way to do this than composition is to use
>
>   DMGetNamedGlobalVector(dm, "old_solution", &old_solution);
>
>
> Yes, this seems to work and I do not get an error while running the
> monitor. Also the discretisation seems to be fine. But the old solution now
> is not available as an auxiliary field.
>
>
> Third, I can say that I am profoundly troubled by this. This interferes
> with the operation of
> the time integrator, and I cannot see a reason for this. If you are
> keeping track of the integral
> of some quantity, I would update that in TSPostStep() and request that
> integral instead of the
> previous solution. We do this for some non-Newtonian rheologies.
>
>
> I have an “if” condition in my residual which I need to evaluate to
> determine the correct formulation for my residuals. I use actual fields to
> keep track of the integrals. But when I use the actual field to evaluate
> the “if” condition, due to the implicit nature of the solver I jump “back
> and forth” over that condition. I need the “if” condition to be independent
> from the field for which it determines the residual.
> The actual application is as follows:
>  I have, amongst others, a displacement and a stress field.
> I evaluate for my stress given a displacement increment delta u.
> If the resulting stress is > yield stress, I need a plasticity
> formulation, if it is smaller, I can use elasticity.
>

Hmm, I think this is a formulation problem. You should be using the
"correct" formulation at the implicit step, since otherwise
the residual will be inconsistent with what you tested when evaluated at
the new step. I have the same kind of elasto-plastic
system in PyLith (http://www.geodynamics.org/cig/software/pylith/) which we
solve implicitly without much problem.

  Thanks,

    Matt


> Thanks,
> Max
>
>
>   Thanks,
>
>      Matt
>
>
>> Thanks,
>> Max
>
>
>
>
> --
> 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
>
> http://www.caam.rice.edu/~mk51/
>
>
>


-- 
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

http://www.caam.rice.edu/~mk51/
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