[petsc-users] DMPlex tetrahedra facets orientation

[Matthew Knepley]

On Sun, Mar 7, 2021 at 8:52 AM Nicolas Barral <
nicolas.barral at math.u-bordeaux.fr> wrote:

> Matt,
>
> Thanks for your answer.
>
> However, DMPlexComputeCellGeometryFVM does not compute what I need
> (normals of height 1 entities). I can't find any function doing that, is
> there one ?
>

The normal[] in DMPlexComputeCellGeometryFVM() is exactly what you want.
What does not look right to you?

  Thanks,

    Matt


> So far I've been doing it by hand, and after a lot of experimenting the
> past weeks, it seems that if I call P0P1P2P3 a tetrahedron and note x
> the cross product,
> P3P2xP3P1 is the outward normal to face P1P2P3
> P0P2xP0P3              "                P0P2P3
> P3P1xP3P0              "                P0P1P3
> P0P1xP0P2              "                P0P1P2
> Have I been lucky but can't expect it to be true ?
>
> (Alternatively, there is a link between the normals and the element
> Jacobian, but I don't know the formula and can  find them)
>
>
> Thanks,
>
> --
> Nicolas
>
> On 08/02/2021 15:19, Matthew Knepley wrote:
> > On Mon, Feb 8, 2021 at 6:01 AM Nicolas Barral
> > <nicolas.barral at math.u-bordeaux.fr
> > <mailto:nicolas.barral at math.u-bordeaux.fr>> wrote:
> >
> >     Hi all,
> >
> >     Can I make any assumption on the orientation of triangular facets in
> a
> >     tetrahedral plex ? I need the inward facet normals. Do I need to use
> >     DMPlexGetOrientedFace or can I rely on either the tet vertices
> >     ordering,
> >     or the faces ordering ? Could DMPlexGetRawFaces_Internal be enough ?
> >
> >
> > You can do it by hand, but you have to account for the face orientation
> > relative to the cell. That is what
> > DMPlexGetOrientedFace() does. I think it would be easier to use the
> > function below.
> >
> >     Alternatively, is there a function that computes the normals -
> without
> >     bringing out the big guns ?
> >
> >
> > This will compute the normals
> >
> >
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DMPLEX/DMPlexComputeCellGeometryFVM.html
> > Should not be too heavy weight.
> >
> >    THanks,
> >
> >      Matt
> >
> >     Thanks
> >
> >     --
> >     Nicolas
> >
> >
> >
> > --
> > 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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