[petsc-users] DMPlex tetrahedra facets orientation
Nicolas Barral
nicolas.barral at math.u-bordeaux.fr
Mon Mar 8 03:02:31 CST 2021
On 07/03/2021 22:56, Matthew Knepley wrote:
> On Sun, Mar 7, 2021 at 4:51 PM Nicolas Barral
> <nicolas.barral at math.u-bordeaux.fr
> <mailto:nicolas.barral at math.u-bordeaux.fr>> wrote:
>
>
> 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>
> > <mailto: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>>
> > > <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:
> > >
> > > 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).
>
>
> Yes. Now that I have pretty much settled on it, I will put it in the
> manual. It is currently here:
>
> https://gitlab.com/petsc/petsc/-/blob/main/src/dm/impls/plex/plexinterpolate.c#L56
>
> All normals are outward facing, but hopefully the ordering in the sourse
> file makes sense.
Thanks Matt, but I'm not sure I understand well. What I do so far is:
ierr = DMPlexGetCone(dm, c, &cone);CHKERRQ(ierr);
for (i=0; i<dim+1; ++i) {
f = cone[i];
ierr = DMPlexComputeCellGeometryFVM(dm, f, &area, NULL,
&vn[i*dim]);CHKERRQ(ierr);
if (dim == 3) { area *= 2; }
for (j=0; j<dim; ++j) { vn[i*dim+j] *= area; }
So in 3D, it seems that:
(vn[9],vn[10],vn[11]) is the inward normal to the facet opposing vertex0
(vn[6],vn[7],vn[8]) " " 1
(vn[3],vn[4],vn[5]) " " 2
(vn[0],vn[1],vn[2]) " " 3
in 2D:
(vn[2],vn[3]) is a normal to the edge opposing vertex 0
(vn[4],vn[5]) " " 1
(vn[0],vn[1]) " " 2
Yet in 2D, whether the normals are inward or outward does not seem
consistent across elements.
What am I wrongly assuming ?
Thanks,
--
Nicolas
>
> Thanks,
>
> Matt
>
> 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>>>
> > > > <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
> <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/>
>
>
>
> --
> 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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