[petsc-users] DMPlex tetrahedra facets orientation

[Nicolas Barral]

On 07/03/2021 22:30, Matthew Knepley wrote:
> On Sun, Mar 7, 2021 at 4:13 PM Nicolas Barral 
> <nicolas.barral at math.u-bordeaux.fr 
> <mailto: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>
>      > <mailto: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.
>

Ah by the area times 2, of course, my bad.
Do you order height-1 elements in a certain way ? I need to access the 
facet (resp. edge) opposite to a vertex in a tet (resp. triangle).

Thanks

-- 
Nicolas


>    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>>
>      >      > <mailto: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/>