[petsc-users] DMPlex assemble submatrices with overlapping dofs

[Fabian Jakub]

Dear Matt, dear PETSc Devs,


I have a dmplex mesh with wedge type cells (2d triangle mesh extruded in
the vertical, i.e. prism cells).

Let me call the faces with 3 edges "top/bot" faces and the faces with 4
edges "side" faces

The mesh can be subdivided into two regions.

The upper part where a petsc section has only dofs on the "top/bot"
faces and a lower part where i have dofs on the "side" faces as well.

At the moment I take care of this layout when I assemble the matrices.

I was wondering if I could assemble a submatrix for each part, then
create a combined matrix from the two regions.

What I tried so far was to

clone the dm twice (topdm and botdm)

generate a section with the corresponding layouts

e.g. now

topdm has 2 cells with dofs on 3 faces

botdm has 1 cell with dofs on 5 faces

DMCompositeCreate()

DMCompositeAddDM(topdm and botdm)


Now, looking at the matrix generated on the composite DM

or calling DMCompositeGetGlobalISs

I see that the dofs on the face between the two sub meshes are not shared.

My question is:

Is this generally the way to go about it or do you suggest a completely
different approach

i.e. instead of "sharing" the dofs on the interfacing faces I could
"couple" the meshes with matrix entries in the composite Mat

Does that sound more practical?

Or how should I go about telling the composite DM about the shared dofs?

Many Thanks!

Fabian