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

[Maximilian Hartig]

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 <mailto: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.

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