[petsc-users] DMPlex tetrahedra facets orientation

[Nicolas Barral]

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 ?

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/>