[petsc-dev] DMPlex, Finite Volume, overlap and ghost cells...

[Matthew Knepley]

On Wed, Mar 11, 2015 at 3:34 AM, John O'Sullivan <
jp.osullivan at auckland.ac.nz> wrote:

>  Hi all,
>
> I've managed to get myself very confused about how to use DMPlex correctly
> for a distributed Finite Volume grid...
>
> My understanding was that along partition boundaries the ghost cells are
> used store cell information from neighbouring partitions so that the fluxes
> can be calculated.
>
> Though debugging through ex11 it seems like overlap is set equal to 1?
>
> I'm solving a simple pressure-diffusion equation on a 1D column (from an
> exo grid) which works fine on a single processor but not in parallel. I'm
> certainly not setting things up right or labeling correctly...
>
> Could someone please explain the most appropriate way to set up and label
> the DM, whether the overlap should be 0 or 1 and whether ghost cells should
> be placed on internal partition boundaries.
>

Yes, for FV the partition overlap should be 1, as it is in ex11. This means
that when the partition happens, we will
put a layer of cells on the other side of partition boundaries,

When DMPlexConstructGhostCells() is called, it will put ghost cells on the
other side of the true boundary. Both
kinds of cells will be separated from interior cells by the
DMPlexGetHybridBounds(&cMax), where cMax is the
first ghost cell.

Now you still need a way to get rid of faces between ghost cells (which
only occur in cells across a partition boundary).
To do this, you use the "ghost" label we make during partitioning:

    ierr = DMPlexGetLabel(dm, "ghost", &ghostLabel);CHKERRQ(ierr);
    ierr = DMLabelGetValue(ghostLabel, face, &ghost);CHKERRQ(ierr);
    if (ghost >= 0) continue;

What exactly is going wrong?

   Matt


> Thanks!
> John
>
>   --
> Dr John O'Sullivan
> Lecturer
> Department of Engineering Science
> University of Auckland, New Zealand
> email: jp.osullivan at auckland.ac.nz <https://lists.mcs.anl.gov/mailman/listinfo/petsc-dev>
> tel: +64 (0)9 923 85353
>
>


-- 
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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