[petsc-users] Issues creating DMPlex from higher order mesh generated by gmsh

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

> On Fri, May 5, 2023 at 10:55 AM Vilmer Dahlberg via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
>> Hi.
>>
>>
>> I'm trying to read a mesh of higher element order, in this example a mesh
>> consisting of 10-node tetrahedral elements, from gmsh, into PETSC. But It
>> looks like the mesh is not properly being loaded and converted into a
>> DMPlex. gmsh tells me it has generated a mesh with 7087 nodes, but when I
>> view my dm object it tells me it has 1081 0-cells. This is the printout I
>> get
>>
>
> Hi Vilmer,
>
> Plex makes a distinction between topological entities, like vertices, edges
> and cells, and the function spaces used to represent fields, like velocity
> or coordinates. When formats use "nodes", they mix the two concepts
> together.
>
> You see that if you add the number of vertices and edges, you get 7087,
> since for P2 there is a "node" on every edge. Is anything else wrong?

Note that quadratic (and higher order) tets are broken with the Gmsh reader. It's been on my todo list for a while.

As an example, this works when using linear elements (the projection makes them quadratic and visualization is correct), but is tangled when holes.msh is quadratic.

$ $PETSC_ARCH/tests/dm/impls/plex/tutorials/ex1 -dm_plex_filename ~/meshes/holes.msh -dm_view cgns:s.cgns -dm_coord_petscspace_degree 2

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