[petsc-users] Define sub-dm from original dm and vector mapping

[Mike Michell]

Dear PETSc developer team,

Hi, this is a follow-up question regarding generating sub-dm for surface
from the original volumetric dm (
https://lists.mcs.anl.gov/pipermail/petsc-users/2023-March/048263.html).

I have created a short code as written below as a piece of it to create a
sub-dm on the surface, extracted from the original volume dmplex:

-------------------------------------
    call DMClone(dm_origin, dm_wall, ierr);CHKERRA(ierr)

    ! label for face sets
    call DMGetLabel(dm_wall, "Face Sets", label_facesets,
ierr);CHKERRA(ierr)
    call DMPlexLabelComplete(dm_wall, label_facesets, ierr);CHKERRA(ierr)

    ! label for vertex on surface
    call DMCreateLabel(dm_wall, "Wall", ierr);CHKERRA(ierr)
    call DMGetLabel(dm_wall, "Wall", label_surf, ierr);CHKERRA(ierr)
    call DMPlexGetChart(dm_wall, ist, iend, ierr);CHKERRA(ierr)
    do i=ist,iend
      call DMLabelGetValue(label_facesets, i, val, ierr);CHKERRA(ierr)
      if(val .eq. ID_wall) then
        call DMLabelSetValue(label_surf, i, ID_wall, ierr);CHKERRA(ierr)
      endif
    enddo
    call DMPlexLabelComplete(dm_wall, label_surf, ierr);CHKERRA(ierr)

    ! create submesh
    call DMPlexCreateSubmesh(dm_wall, label_surf, ID_wall, PETSC_TRUE,
dm_sub, ierr);CHKERRA(ierr)
    call DMPlexGetSubpointMap(dm_sub, label_sub, ierr);CHKERRA(ierr)
-------------------------------------

Now I can define a vector on dm_sub and view it via PETSc_Viewer. However,
it is unclear how to map a vector defined on the volumetric dm (dm_wall)
into a vector defined on the surface dm (dm_sub). I guess label_sub created
via DMPlexGetSubpointMap() can help with it, but it is difficult to get a
solid picture without an example. Could I get a comment for it?

Thanks,
Mike
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