[petsc-users] DMPlex tetrahedra facets orientation

[Matthew Knepley]

On Sun, Mar 7, 2021 at 4:13 PM Nicolas Barral <
nicolas.barral at math.u-bordeaux.fr> wrote:

> On 07/03/2021 16:54, Matthew Knepley wrote:
> > On Sun, Mar 7, 2021 at 8:52 AM Nicolas Barral
> > <nicolas.barral at math.u-bordeaux.fr
> > <mailto: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?
>
>
> So it turns out it's not what I want because I need non-normalized
> normals. It doesn't seem like I can easily retrieve the norm, can I?
>

You just want area-weighted normals I think, which means that you just
multiply by the area,
which comes back in the same function.

  Thanks,

    Matt


> If not, I'll fallback to computing them by hand for now. Is the
> following assumption safe or do I have to use DMPlexGetOrientedFace?
>  >  if I call P0P1P2P3 a tet and note x the cross product,
>  >  P3P2xP3P1 is the outward normal to face P1P2P3
>  >  P0P2xP0P3              "                P0P2P3
>  >  P3P1xP3P0              "                P0P1P3
>  >  P0P1xP0P2              "                P0P1P2
>
> Thanks
>
> --
> Nicolas
> >
> >    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>
> >      > <mailto: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/>
>


-- 
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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