[petsc-users] Inquiry about resetting a petscsection for a dmplex

[Matthew Knepley]

On Thu, Feb 27, 2025 at 6:12 PM neil liu <liufield at gmail.com> wrote:

> Dear Pestc community,
>
> I am currently working on a 3D adaptive vector FEM solver. In my case, I
> need to solve two systems: one for the primal equation using a low-order
> discretization and another for the adjoint equation using a high-order
> discretization.
>
> Afterward, I need to reset the section associated with the DMPlex.
> Whichever is set first—20 DOFs (second-order) or 6 DOFs (first-order)—the
> final mapping always follows that of the first-defined configuration.
>
> Did I miss something?
>
> When solving two systems like this on the same mesh, I recommend using
DMClone(). What this does is create you a new
DM with the same backend topology (Plex), but a different function space
(Section). This is how I do everything internally in Plex. Does that make
sense?

  Thanks,

     Matt

> Thanks,
>
>
> Xiaodong
>
> PetscErrorCode DMManage::SetupSection(CaseInfo &objCaseInfo){
> PetscSection s;
> PetscInt edgeStart, edgeEnd, pStart, pEnd;
> PetscInt cellStart, cellEnd;
> PetscInt faceStart, faceEnd;
>
> PetscFunctionBeginUser;
> DMPlexGetChart(dm, &pStart, &pEnd);
> DMPlexGetHeightStratum(dm, 0, &cellStart, &cellEnd);
> DMPlexGetHeightStratum(dm, 1, &faceStart, &faceEnd);
> DMPlexGetHeightStratum(dm, 2, &edgeStart, &edgeEnd); /* edges */;
> PetscSectionCreate(PetscObjectComm((PetscObject)dm), &s);
> PetscSectionSetNumFields(s, 1);
> PetscSectionSetFieldComponents(s, 0, 1);
> if (objCaseInfo.getnumberDof_local() == 6){
> PetscSectionSetChart(s, edgeStart, edgeEnd);
> for (PetscInt edgeIndex = edgeStart; edgeIndex < edgeEnd; ++edgeIndex) {
> PetscSectionSetDof(s, edgeIndex, objCaseInfo.numdofPerEdge);
> PetscSectionSetFieldDof(s, edgeIndex, 0, 1);
> }
> }
> else if(objCaseInfo.getnumberDof_local() == 20){
> PetscSectionSetChart(s, faceStart, edgeEnd);
> for (PetscInt faceIndex = faceStart; faceIndex < faceEnd; ++faceIndex) {
> PetscSectionSetDof(s, faceIndex, objCaseInfo.numdofPerFace);
> PetscSectionSetFieldDof(s, faceIndex, 0, 1);
> }
> //Test
> for (PetscInt edgeIndex = edgeStart; edgeIndex < edgeEnd; ++edgeIndex) {
> PetscSectionSetDof(s, edgeIndex, objCaseInfo.numdofPerEdge);
> PetscSectionSetFieldDof(s, edgeIndex, 0, 1);
> }
> }
> //
> PetscSectionSetUp(s);
> DMSetLocalSection(dm, s);
> PetscSectionDestroy(&s);
>
> //Output map for check
> ISLocalToGlobalMapping ltogm;
> const PetscInt *g_idx;
> DMGetLocalToGlobalMapping(dm, &ltogm);
> ISLocalToGlobalMappingView(ltogm, PETSC_VIEWER_STDOUT_WORLD);
> ISLocalToGlobalMappingGetIndices(ltogm, &g_idx);
>
> PetscFunctionReturn(PETSC_SUCCESS);
> }
>


-- 
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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