[petsc-users] DMPlexCreateFromDAG for P2 elements

[Asitav Mishra]

Stefano,

Thanks a lot for your prompt response. I have a higher-order mesh, so will
need to create my own PetscSection to add the local coordinates looks  like.
At this point, FEM residuals and Jacobian computations are not as
important. But, could you direct me to any examples in creating
PetscSection for such elements?

Thanks,
Asitav

On Tue, Oct 8, 2019 at 3:18 PM Stefano Zampini <stefano.zampini at gmail.com>
wrote:

> DMPlex abstracts the mesh topology. PetscSection defines the dofs
> associated to the mesh entities.
> Why do you need P2 elements? For the discrete solution or because you have
> a high order mesh?
>
> If it is only for the solution, you should create a linear mesh
> (cell-vertex only), interpolate the mesh, and then use
> PetscFECreateDefault (
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/FE/PetscFECreateDefault.htm
> )
> +
> DMAddField (
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMAddField.html
> )
> +
> DMCreateDS (
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMCreateDS.html
> )
>
> Instead, if you have a high-order mesh, you should create a linear mesh,
> and then manipulate yourself the PetscSection defining the local
> coordinates.
> However, I’m not sure how complete is the support for high-order meshes
> when it comes to compute FEM residuals and assemble Jacobians
>
>
> On Oct 8, 2019, at 9:54 PM, Asitav Mishra via petsc-users <
> petsc-users at mcs.anl.gov> wrote:
>
> Hi,
>
> Does Petsc support DMPlexCreateFromDAG for 2nd order (P2) elements, or any
> orders higher than one (P1) for that matter? I know it is possible to
> create DMPlex mesh from GMSH files with P2 elements, but not sure how the
> 'cone/support' structures are built. One approach is to ignore the higher
> order nodes in an element to create the 'cones', but is there a way to
> preserve the P2 element information in the 'cones'?
>
> For example: how would the depth=1 cone structure look like for a mesh
> with two P2 triangles sharing a face?
>             2
>           / | \
>         /   |   \
>      6     |     8
>     /       |       \
>   /         |         \
> 0   0    5    1    3
>   \         |         /
>     \       |       /
>      4     |     7
>         \   |   /
>           \ | /
>            1
>
> Higher order nodes: [4, 5, 6, 7, 8]. Can we construct cones with
> cells-to-nodes (depth = 1)?:
>
> numPoints = [9, 2];                                             // 9 nodes
> instead of just 4?
> coneSizes = [6, 6,    0, 0, 0, 0, 0, 0, 0, 0, 0];
> cones        = [0, 1, 2, 4, 5, 6,    2, 1, 3, 5, 7, 8];
> etc.
>
> Any help would be greatly appreciated.
>
> Thanks,
> Asitav
>
> --
> Asitav Mishra, PhD
> Research Engineer II, NIA
>
>
>

-- 
Asitav Mishra, PhD
Research Engineer II, NIA
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