[petsc-users] basis/basisDer/Jac in FE

[Matthew Knepley]

On Thu, Oct 27, 2022 at 1:20 PM Yann Jobic <yann.jobic at univ-amu.fr> wrote:

> There is this factor also in front of the rectangle, which should 1/4
> for a unit rectangle of dim 2, but it's also 0.5.
>

Hi Yann,

The reference element in Petsc is [-1, -1] to [1, 1]. This matches
FEniCS/Firedrake and also makes
it easier to use orthogonal polynomials.

  Thanks,

     Matt


> Thanks,
> Yann
>
>
> Le 10/27/2022 à 6:23 PM, Mark Adams a écrit :
> > Area of a unit triangle is (1x1)/2 I would guess.
> >
> > On Thu, Oct 27, 2022 at 11:32 AM Yann Jobic <yann.jobic at univ-amu.fr
> > <mailto:yann.jobic at univ-amu.fr>> wrote:
> >
> >     Hello,
> >
> >     I'm trying to understand how to use the lowlevel Petsc Finit Element
> >     Framework. I've got few questions about it.
> >
> >     1) I'm checking the determinant of the Jacobian transformation from
> >     real
> >     space to the parametric one. The source code is in :
> >
> https://petsc.org/main/src/dm/impls/plex/plexgeometry.c.html#DMPlexComputeTriangleGeometry_Internal
> <
> https://petsc.org/main/src/dm/impls/plex/plexgeometry.c.html#DMPlexComputeTriangleGeometry_Internal
> >
> >     I don't quite understand the 0.5 factor in front of it.
> >     Geometrically, it should be the ratio of the area/volume of the reel
> >     element over the referenced one. It's not the case here no ?
> >     It seems that, for other elements, this factor still applies.
> >     What this factor represent ?
> >
> >     2) I only checked triangle elements here. The ordering of the test
> >     functions and its derivatives is different it seems, with also the
> 0.5
> >     factor. Am i mistaken ?
> >
> >     I could not find any informations about it in the user-doc. Maybe
> it's
> >     written somewhere else ?
> >
> >     Many Thanks,
> >
> >     Best regards,
> >
> >     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/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20221027/e503df06/attachment.html>