[petsc-users] node DG with DMPlex

[Matthew Knepley]

On Mon, Mar 16, 2020 at 5:20 PM Yann Jobic <yann.jobic at univ-amu.fr> wrote:

> Hi all,
>
> I would like to implement a nodal DG with the DMPlex interface.
> Therefore, i must add the internal nodes to the DM (GLL nodes), with the
> constrains :
> 1) Add them as solution points, with correct coordinates (and keep the
> good rotational ordering)
> 2) Find the shared nodes at faces in order to compute the fluxes
> 3) For parallel use, so synchronize the ghost node at each time steps
>

Let me get the fundamentals straight before advising, since I have never
implemented nodal DG.

  1) What is shared?

      We have an implementation of spectral element ordering (
https://gitlab.com/petsc/petsc/-/blob/master/src/dm/impls/plex/examples/tutorials/ex6.c).
Those share
      the whole element boundary.

  2) What ghosts do you need?

  3) You want to store real space coordinates for a quadrature?

      We usually define a quadrature on the reference element once.

  Thanks,

    Matt


> I found elements of answers in those threads :
> https://lists.mcs.anl.gov/pipermail/petsc-users/2016-August/029985.html
>
> https://lists.mcs.anl.gov/mailman/htdig/petsc-users/2019-October/039581.html
>
> However, it's not clear for me where to begin.
>
> Quoting Matt, i should :
> "  DMGetCoordinateDM(dm, &cdm);
>    <Set field information into cdm instead of dm>
>   DMCreateLocalVector(cdm, &coordinatesLocal);
>   <Fill in higher order coordinate values>
>   DMSetCoordinatesLocal(dm, coordinatesLocal);"
>
> However, i will not create ghost nodes this way. And i'm not sure to
> keep the good ordering.
> This part should be implemented in the PetscFE interface, for high order
> discrete solutions.
> I did not succeed in finding the correct part of the source doing it.
>
> Could you please give me some hint to begin correctly thoses tasks ?
>
> Thanks,
>
> Yann
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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