[petsc-users] PetscFE questions

[Matthew Knepley]

On Thu, Oct 20, 2016 at 9:42 AM, Julian Andrej <juan at tf.uni-kiel.de> wrote:

> Thanks for the suggestion. I guess DMCreateSubDM can work, but is
> cumbersome to handle for the normal solution process since the mass
> matrix for example is not a seperate field.
>

I did not understand what you meant by "parts of the physics". If you just
want to make a different operator, then swap out the PetscDS from the DM.
That holds the pointwise functions and discretizations.


> src/snes/examples/tutorials/ex77 handles a seperate field for the
> nullspace, if anyone is interested in that.
>
> An intuitive way was just copying the DM and describing a new problem on
> it.
>
>   DM dm_mass;
>   PetscDS ds_mass;
>   Vec dummy;
>   PetscInt id = 1;
>   petsc_call(DMCreateGlobalVector(dm, &dummy));
>   petsc_call(DMClone(ctx->dm, &dm_mass));
>   petsc_call(DMGetDS(dm_mass, &ds_mass));
>   petsc_call(PetscDSSetDiscretization(ds_mass, 0, (PetscObject)fe));
>   petsc_call(PetscDSSetJacobian(ds_mass, 0, 0, mass_kernel, NULL, NULL,
> NULL));
>   petsc_call(PetscDSAddBoundary(ds_mass, PETSC_TRUE, "wall", "marker",
> 0, 0, NULL, (void (*)())ctx->exact_funcs[0], 1, &id, ctx));
>   petsc_call(DMCreateMatrix(dm_mass, &ctx->M));
>   petsc_call(DMPlexSNESComputeJacobianFEM(dm_mass, dummy, ctx->M,
> ctx->M, NULL));
>
> is this an intended way to assemble a jacobian based on a weak form?
> The memory overhead for a DM copy isn't huge on the first sight.
>

Its O(1).


> And a much more important question. Is there any mathematical
> description how exactly you handle dirichlet boundary conditions here?
>

Right now, you can do two things:

  1) Handle it yourself

or

  2) eliminate particular dofs

If you use 2), these dofs are eliminated from the global vector. They
remain in the
local vector, and boundary values are inserted before local vectors are
passed to
assembly routines.

   Matt


> On first sight it looks like condensing the nodes only to
> non-essential nodes and then projecting them back in the solution
> vector. If thats teh case I don't understand how you "augment" the
> solution with the boundary nodes.
>
> Regards
> Julian
>
>
> On Wed, Oct 19, 2016 at 11:51 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > On Tue, Oct 18, 2016 at 7:38 AM, Julian Andrej <juan at tf.uni-kiel.de>
> wrote:
> >>
> >> Hi,
> >>
> >> i have general question about PetscFE. When i want to assemble certain
> >> parts of physics separately, how can i do that? I basically want to
> >> assemble matrices/vectors from the weak forms on the same DM (and
> >> avoid copying the DM) and use them afterwards. Is there a convenient
> >> way for doing that?
> >>
> >> The "workflow" i'm approaching is something like:
> >>
> >> - Setup the DM
> >> - Setup discretization (spaces and quadrature) for each weak form i
> >> want to compute
> >> - Compute just the weak form i want right now for a specific
> >> discretization and field.
> >>
> >> The reason is i need certain parts of the "complete" Jacobian for
> >> computations of eigenproblems and like to avoid computing those more
> >> often than needed.
> >
> >
> > The way I envision this working is to use DMCreateSubDM(). It should
> extract
> > everything correctly for the subset of fields you select. However, I have
> > not
> > extensively tested, so if something is wrong let me know.
> >
> >   Thanks,
> >
> >      Matt
> >
> >>
> >> Regards
> >> Julian
> >
> >
> >
> >
> > --
> > 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
>



-- 
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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