[petsc-users] DMPlex H27 elements

[Matthew Knepley]

On Fri, Jan 4, 2019 at 10:04 AM Yann Jobic via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear Petsc Users,
>
> I'm using DMPlexCreateFromCellList to create my DM. I would like to have
> an order 2 geometry.
>
> It's working fine in 2D for elements of type Q9.
>

I do not see how that is possible. The topology does not work that way, and
there
is no way a 9 vertex quad was interpolated, so something else is happening.

Moreover, anything you input still has affine geometry. Toby has been
working on
non-affine geometry, and all the code is there, but right now you have to
build and
populate the coordinate space by hand. What this means is that you create a
quadratic space for the coordinate DM (rather than the default linear), get
the
global coordinate vector out, and stick in the Q9 coordinates. The topology
is still
the same no matter what coordinates you read in.

We will get around to making an interface for this soon.

  Thanks,

     Matt


> I checked that it's working correctly by using
> DMPlexComputeCellGeometryFEM, and compute the value of the determinant
> (CheckMeshGeometry of dm/impls/plex/examples/tutorials/ex2.c)
>
> I can import H8 elements, it's working fine.
>
> But for H27 element, it's not working. I really don't know how to order
> my vertex in the cells array. So far, the determinant is zero...
>
> I don't know where to look in order to find this information. I tried
> the function DMPlexGetRawFaces_Internal of the file
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/src/dm/impls/plex/plexinterpolate.c.html#DMPlexGetRawFaces_Internal
>
> but it didn't help me.
>
> Could you please point me where to look ?
>
> Thanks, and happy new year !
>
> Yann
>
>

-- 
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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