[petsc-users] DMPlex Interpolation

[Matthew Knepley]

On Sun, Aug 28, 2022 at 8:51 AM Mike Michell <mi.mike1021 at gmail.com> wrote:

> Hi, thank you for the reply.
>
> I was able to manage mapping from cell-center to vertex. Basically, in
> Fortran, it seems DMCreateInterpolation() requires the optional scaling
> vector as a mandatory argument, which is strange.
>

It needs a custom Fortran wrapper to check for the NULL input from Fortran.


> My dmplex has zero overlap layer over the procs, and there is no ghost
> cell inside physical boundary. In this case, it seems mapping between cell
> center to vertex returns zero (in case the global cell vector initialized
> to zero) value at the nodes, which are located at physical boundary. To
> manage this problem, is it mandatory to have ghost cells. Is this correct
> understanding?
>

I cannot quite understand. Are you saying that you have inhomogeneous
Dirichlet conditions on the boundary, and a 0 guess in the interior, and
you get all zeros from interpolation? Yes, we have no way of getting
boundary values into the operator. You could conceivably make an operator
that mapped between local vectors, but it is hard to see why you would want
this. In the solver, we usually  just care about the unknowns we are
solving for.

Did I understand your question, or is it about something else?


> Also, my mapping accuracy itself seems to be improved. For simple test,
> cell-center's x-coordinate value of each cell mapped to node and printed to
> the vertex field as .vtu format, and the mapped x-vertex-coordinate is
> quite different with the actual nodal coordinate values what PETSc
> intrinsically provides through DMGetCoordinatesLocal(). I believe I am
> doing something wrong, probably arguments in PetscFECreateLagrange() can
> improve the mapping accuracy in finite-element space?
>

Yes, a cellwise field is much less accurate than a linear field. Pushing
the values up to a linear field does not make them more accurate.

  Thanks,

     Matt


> Thanks,
>
>
>> On Thu, Aug 25, 2022 at 7:12 PM Mike Michell <mi.mike1021 at gmail.com>
>> wrote:
>>
>>> Hi, this is a duplication of
>>> https://lists.mcs.anl.gov/pipermail/petsc-users/2022-August/046746.html
>>> for in-depth question.
>>>
>>> I wrote a short code as attached to test interpolation between two
>>> DMPlex objects. Goal is to map solution field defined on
>>> cell-centroid(DMPlex-1) into vertex(DMPlex-2) field or vice versa.
>>>
>>> Basically, DMCreateInterpolation() fails in the example code and it was
>>> not allowed to get useful error messages. The DMPlex is created by loading
>>> a 2D square box in Gmsh. Can I get some comments on that?
>>>
>>
>> Sure, I am looking at it.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> Thanks,
>>>
>>
>>
>> --
>> 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/>
>>
>

-- 
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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