[petsc-users] Problem with interpolating projected field of quadratic order

[Sander Arens]

>
> 1) Does the test run for you?


I ran the tests and they all pass.

2) If so, then could it be that you assume something about the order of
> functions?


I'm not really sure what you mean with this.

So the values in 'u' are the FEM coefficients for the coordinate function
> {x, y}. Why are you calling 'x' projected and
> 'y' interpolated?
>

No, u is a vector with three fields which are obtained in the following way:

1) Project the coordinates onto field nr 0 so that I have the coordinates
for all the dofs
2) Project the coordinates onto field nr 1, so that I get the coordinates
at the centroid of the cell
3) Project field nr 0 onto field nr 2, so that I get the coordinates at the
centroid of the cell

I then output the values of the different fields, so that I can compare the
values of field 1 and 2.

What I would expect is that the values of field 1 and 2 would be the same.
This is the case for field 0 being P1, but not for P2.
I also projected the coordinates onto a P2 field in
snes/examples/tutorials/ex77.c, but this was done using DMPlexCreateBoxMesh
and gave no problems.
Maybe it's some assumption on the vertex numbering in
DMPlexCreateFromCellList or DMPlexCreateFromFile that I'm not aware of?

I hope I made the problem clearer now?

Thanks,
Sander

On 18 June 2016 at 16:33, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Jun 18, 2016 at 9:26 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Sat, Jun 18, 2016 at 8:56 AM, Sander Arens <Sander.Arens at ugent.be>
>> wrote:
>>
>>> Hello all,
>>>
>>> Recently I've been trying to run a fem problem using DMPlex, PetscFE,
>>> etc. using a mesh created with gmsh. I used DMPlexReverseCell to get rid of
>>> the negative jacobian determinants. However, I noticed another problem:
>>> when I projected the coordinates on a quadratic fem-field and interpolated
>>> them they didn't match the interpolated values of the original coordinate
>>> field anymore.
>>>
>>> I attached a simple test to reproduce this. With linear interpolation I
>>> see no problem, it's only when I start using quadratic interpolation.
>>>
>>> I have no idea why this is giving problems. Am I missing something
>>> simple here or is this a bug?
>>>
>>
>> Tests for this already exist
>>
>>
>> https://bitbucket.org/petsc/petsc/src/996a00f1a1693e2d85044ed633fe1848cf357f2a/src/dm/impls/plex/examples/tests/ex3.c?at=master&fileviewer=file-view-default
>>
>> with run parameters here:
>>
>>
>> https://bitbucket.org/petsc/petsc/src/996a00f1a1693e2d85044ed633fe1848cf357f2a/config/builder.py?at=master&fileviewer=file-view-default#builder.py-58
>>
>> and specifically for P2 triangles
>>
>>
>> https://bitbucket.org/petsc/petsc/src/996a00f1a1693e2d85044ed633fe1848cf357f2a/config/builder.py?at=master&fileviewer=file-view-default#builder.py-72
>>
>> 1) Does the test run for you?
>>
>>   ./config/builder2.py check src/dm/impls/plex/examples/tests/ex3.c
>>
>> 2) If so, then could it be that you assume something about the order of
>> functions?
>>
>
> I am having a hard time understanding what you are doing here:
>
>   /* View the projected coordinates field */
>   ierr = VecGetSubVector(u, fields[0], &subvec);CHKERRQ(ierr);
>   ierr = PetscObjectSetName((PetscObject) subvec, fieldNames[0]);CHKERRQ(ierr);
>   ierr = VecViewFromOptions(subvec, NULL, "-projectedcoordinates_vec_view");CHKERRQ(ierr);
>   ierr = VecRestoreSubVector(u, fields[0], &subvec);CHKERRQ(ierr);
>
>   /* View the interpolated coordinates field */
>   ierr = VecGetSubVector(u, fields[1], &subvec);CHKERRQ(ierr);
>   ierr = PetscObjectSetName((PetscObject) subvec, fieldNames[1]);CHKERRQ(ierr);
>   ierr = VecViewFromOptions(subvec, NULL, "-interpolatedcoordinates_vec_view");CHKERRQ(ierr);
>   ierr = VecRestoreSubVector(u, fields[1], &subvec);CHKERRQ(ierr);
>
>
> So the values in 'u' are the FEM coefficients for the coordinate function
> {x, y}. Why are you calling 'x' projected and
> 'y' interpolated?
>
>   Thanks,
>
>      Matt
>
>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> Thanks,
>>> Sander
>>>
>>
>>
>>
>> --
>> 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
>>
>
>
>
> --
> 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
>
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