[petsc-users] How to construct DMPlex of cells with different topological dimension?

[Matthew Knepley]

On Wed, Nov 3, 2021 at 8:06 AM TARDIEU Nicolas <nicolas.tardieu at edf.fr>
wrote:

> Dear Matt,
>
> Thank you for the additional information.
> I do agree that I will have to write code to describe the interpolation I
> need for a multi-model problem.
> Let me try to illustrate my point on the simple mesh I have attached to
> this email. It is done with Gmsh but other generators I use can do the same.
> The mesh is composed of :
>
> - a 3D part, volume_3d, where continuum mechanics equations will be
> assigned (displacement dof)
> - a 2D part, neumann_2d, which is a face of volume_3d and that will be
> used to integrate a Neumann condition assigned to volume_3d (it shares the
> displacement dof of volume_3d)
> - a 2D part, shell_2d, where shell equations will be assigned
> (displacement+rotation dof)
> - a 1D part, beam_1d, where beam equations will be assigned
>  (displacement+rotation dof)
>
> I can imagine that it is impossible, only from the knowledge of the mesh,
> to guess the interpolation. But I would be happy to try to write a small
> DMPlex test to try to describe such a model with a little help from you or
> another PETSc practitioner.
>

That is a good idea. I think we can use the pieces we have to build what
you want automatically. Cn you send me the *.msh for this.
I do not even have GMsh running on my machine, so I cannot process the .geo
:) When we read in GMah files, we mark the cell type.
However, I think we automatically try to find lower dimensional pieces in
the existing mesh (since that is what people usually want, like
your neumann section), so we would need a way to indicate that we are
specifying lower dimensional pieces that are not part of a
volumetric piece.

  Thanks,

     Matt


> Regards,
> Nicolas
> ------------------------------
> *De :* knepley at gmail.com <knepley at gmail.com>
> *Envoyé :* mercredi 3 novembre 2021 12:14
> *À :* TARDIEU Nicolas <nicolas.tardieu at edf.fr>
> *Cc :* petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
> *Objet :* Re: [petsc-users] How to construct DMPlex of cells with
> different topological dimension?
>
> On Wed, Nov 3, 2021 at 4:20 AM TARDIEU Nicolas via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
> Dear all,
>
> As a practitioner of structural mechanics, I modestly wish to support
> Yuan's argument. Model mixing, and consequently finite element dimension
> mixing, is (perhaps unfortunately) an inescapable practice of this
> engineering field. This has consequences not only on the nature of the
> meshes we use but also on the quasi-systematic recourse to linear relations
> (also named multipoints constraints, MPC) between the degrees of freedom of
> the different models (for example, linking a shell to a 3D part requires to
> write kinematic connections between displacements and rotations).
> DMPlex handling such meshes would be a really great contribution for the
> structural mechanics community.
>
>
> Nicolas,
>
> From my point of view, this is supported, so it would be good to
> understand what you do not think is supported:
>
> 1) Multiple models
>
>      This is supported in the sense that you can use any data layout, and
> assemble any system you want into it. You can use
>      beams along edges and shells on faces if you want. I have examples of
> different equations in different pieces of the domain.
>
> 2) Mesh with pieces hanging off
>
>      This is supported in the sense that you can input such a mesh. You
> have to describe it completely, but it can be input and iterated over.
>
> 3) Mesh Interpolation
>
>      If you only want to input vertices _and_ you want to input different
> dimensional objects, you will have to write code. There is no way to guess
>      what you mean from a collection of vertices. You could
>
>      a) Directly  call GetRawFaces() to do the interpolation per cell
> before input (seems easier)
>
>      b) Rewrite the interpolation loop to check whether cell types have
> been set and just call the appropriate interpolation if they have
>
> I guess you are arguing that I should add the code for 3). Most people I
> have interacted with use a mesh generator to make the meshes, so
> I concentrated on importing those formats. Can meshes like these be made
> with those generators?
>
>   Thanks,
>
>      Matt
>
>
> Best regards,
> Nicolas
>
>
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-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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