[petsc-users] Advice on creating vectors defined on lower dimensional manifolds of a DMPlex

[Matthew Knepley]

On Mon, Oct 6, 2025 at 1:11 PM Aldo Bonfiglioli <aldo.bonfiglioli at unibas.it>
wrote:

> Dear all,
>
> what is the best approach for defining vectors that "sit" on the (vertices
> and/or faces) of a given stratum of the "Face Sets" of a DMPlex?
>
> DM Object: 3D plex 1 MPI process
>  type: plex
> 3D plex in 3 dimensions:
>  Number of 0-cells per rank: 9261
>  Number of 1-cells per rank: 59660
>  Number of 2-cells per rank: 98400
>  Number of 3-cells per rank: 48000
> Labels:
>  marker: 1 strata with value/size (1 (14402))
>  celltype: 4 strata with value/size (0 (9261), 1 (59660), 3 (98400), 6
> (48000))
>  depth: 4 strata with value/size (0 (9261), 1 (59660), 2 (98400), 3
> (48000))
>  Face Sets: 6 strata with value/size (1 (800), 2 (800), 3 (800), 4 (800),
> 5 (800), 6 (800))
>
> These vectors are going to be used (for example) to store stresses and
> heat flux on solid surfaces.
>
> To be more specific: suppose stratum 3 of the "Face Sets" is a solid wall.
>
> I want to create a vector that that stores quantities computed on the
> (800) faces of that wall OR the vertices of that wall.
>

It should be simple to just create such vectors. You request a submesh
using that label

  DM subdm;
  DMLabel label, sublabel;

  DMGetLabel(dm, "Face Sets", &label);
  DMLabelDuplicate(label, &sublabel);
  DMPlexLabelComplete(dm, sublabel);
  DMPlexCreateSubmesh(dm, sublabel, 3, PETSC_TRUE, &subdm)
  DMLabelDestroy(&sublabel);

Now you can define a PetscFE over this submesh in the same way as any other
mesh. Moreover, the subdm contains a mapping back to the original DM, from
which you can create a mapping of dofs, so that you can inject the
subvector into a larger field if you wish.

If you want to use fields on submeshes inside a PetscDS, so that the Plex
manages the solve, the procedure is slightly different, but I can detail it
if you want.

  Thanks,

     Matt

> Thanks,
>
> Aldo
>
> --
> Dr. Aldo Bonfiglioli
> Associate professor of Fluid Mechanics
> Dipartimento di Ingegneria
> Universita' della Basilicata
> V.le dell'Ateneo Lucano, 10 85100 Potenza ITALY
> tel:+39.0971.205203 fax:+39.0971.205215
> web: https://urldefense.us/v3/__http://docenti.unibas.it/site/home/docente.html?m=002423__;!!G_uCfscf7eWS!agxuKTesyLfH3uplL5jGjn6KTo6lQTqlAtD7aN9x4Nao7Q1B1yi933LeiaBDdRvOl-Eusg-tJFIjzfmXLiuH$  <https://urldefense.us/v3/__http://docenti.unibas.it/site/home/docente.html?m=002423__;!!G_uCfscf7eWS!dx5g28NqJW34oxLLKP1Fjtp65c0KkvUjelPzjza0lBJtf6uu5ROFqpa2GTX5Cle8L7S_YjHssSDqe6szXd2PEYvYVHHq5mtW8EU$>
>
>

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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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