[petsc-users] indices into Vec/Mat associated to a DMPlex
Matteo Semplice
matteo.semplice at unito.it
Wed Nov 15 02:11:22 CST 2017
Hi.
I am struggling with indices into matrices associated to a DMPLex mesh.
I can explain my problem to the following minimal example.
Let's say I want to assemble the matrix to solve an equation (say
Laplace) with data attached to cells and the finite volume method. In
principle I
- loop over the faces of the DMPlex (height=1)
- for each face, find neighbouring cells (say points i and j in the
DMPlex) and compute the contributions coming from that face (in the
example it would be something like (u_j-u_i)*<x_j-x_i,n> with
x=centroids, n=scaled normal to face and <,> the inner product)
- insert the contributions +/-<x_j-x_i,n> in rows/columns n(i) and n(j)
of the matrix where n(k) is the index into Vec/Mat of the unknown
associated to the k-th cell
My problem is how to find out n(k). I assume that the Section should be
able to tell me, but I cannot find the correct function to call. I see
that FEM methods can use DMPlexMatSetClosure but here we'd need a
DMPlexMatSetCone...
On a side note, I noticed that the grad field in PetscFVFaceGeom is not
computed by DMPlexComputeGeometryFVM. Is it meant or could it be used to
store the precomputed <x_j-x_i,n> values? But even if so, this wouldn't
sovle the problem of where to put the elements in the matrix, right?
Matteo
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