[petsc-users] DMPlex H27 elements

[Yann Jobic]

On 05/01/2019 02:36, Matthew Knepley wrote:
> On Fri, Jan 4, 2019 at 10:04 AM Yann Jobic via petsc-users 
> <petsc-users at mcs.anl.gov <mailto: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.

I get the idea. So at the beginning i start with the Q4/H8 dmplex, then 
i fill from the global coordinate vector the missing coordinates, and 
attach this new coordinate vector to the DM. Then the interpolation will 
be correct ? Can i use DMproject and all the petsc functions ?

In order to output the results, i've got to write my own vtk interface 
no ? (by splitting the Q9/H27 in 4xQ4 or 8xH8).

Or is it working with the hdf5 one ?

Does the p4est interface work with this setup ?

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

Great !

Thanks,

Yann

>
>   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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