[Dune] DGF format and local ordering of vertices

[Sacconi, Andrea]

Hi all,

Andreas is right, I want the surface normal for each element, since it explicitly appears in one of my equations.
As far as I know, it's not possible to use any DUNE command to calculate it, so I implemented it by hand, using the cross product between edges, as Andreas has already perfectly figured out.
But you'll agree that, if you took one of the six faces of a cube, and looking at that face from outside, you saw some triangles with a clockwise orientation for their three corners and some others with a counter-clockwise one, it would mean big troubles, won't you?

Tomorrow I'll try to see what happens. I'll let you know.
By the way, the problem arose in using GMSH for generating a mesh for the "crust" of the unit cube. It's a borderless object, made of intrinsically 2D elements embedded in R^3; for the unit cube testing the orientation is trivial (the normal has all components equal to 0 but one, that can be either 1 or -1). For more weird geometries, as far as I know, there is no way of knowing. But for my case, a cube could be a good starting point.

Best,

Andrea
__________________________________________________________

Andrea Sacconi
PhD student, Applied Mathematics
AMMP Section, Department of Mathematics, Imperial College London,
London SW7 2AZ, UK
a.sacconi11 at imperial.ac.uk

________________________________________
From: dune-bounces+a.sacconi11=imperial.ac.uk at dune-project.org [dune-bounces+a.sacconi11=imperial.ac.uk at dune-project.org] on behalf of Dedner, Andreas [A.S.Dedner at warwick.ac.uk]
Sent: 27 September 2012 20:27
To: Martin Nolte; dune at dune-project.org
Subject: Re: [Dune] DGF format and local ordering of vertices

I was assuming Andrea wanted the surface normal - assuming a closed orientable surface the
outer normal does make sense and can be obtained using cross product of edges - if orientation
is preserved that is.
________________________________________
From: dune-bounces+a.s.dedner=warwick.ac.uk at dune-project.org [dune-bounces+a.s.dedner=warwick.ac.uk at dune-project.org] on behalf of Martin Nolte [nolte at mathematik.uni-freiburg.de]
Sent: 27 September 2012 20:14
To: dune at dune-project.org
Subject: Re: [Dune] DGF format and local ordering of vertices

As Andreas already pointed out, there is no such guarantee and requiring such a
thing will break some grid managers (e.g., AlbertaGrid or ALUGrid). Moreover, in
the case of a hypersurface, the term orientation does not even make sense.

What do you want this information for? If you just want the (covariant) normal,
you can obtain them from the intersections. If you can come up with a situation
where a face normal is needed independent of the intersection, let us know.

Best,

Martin

On 09/27/2012 06:10 PM, Sacconi, Andrea wrote:
> Hi everyone,
>
> I'd like to ask you a question about the Dune Grid Format (DGF) and the ordering
> of the corners in an element of the mesh.
>
> Suppose I put my vertices and my simplices in a dgf file, and then I use it to
> feed the grid constructor (Alberta, ALUGRID, whatever).
> In the online documentation about the DGF, under the field "Simplex", I read
> that "no ordering of local vertices is required".
> In my particular case, however, I do need a special ordering, since I'm dealing
> with hypersurfaces. To set an example: if you have the unit cube as initial
> geometry, I need that, looking from outside, all the triangles have corners
> counter-clockwise oriented s.t. the unit normal points outward. I do need that
> this ordering (used in .dgf file) is preserved!
>
> Is there any way of knowing whether this important information is preserved?
> Otherwise, the computation of the normal to each triangle could lead to weird
> results (strong nonsymmetric time-dependent behaviour for initial symmetric
> configurations).
>
> Thanks for your help!
>
> Andrea
> __________________________________________________________
>
> Andrea Sacconi
> PhD student, Applied Mathematics
> AMMP Section, Department of Mathematics, Imperial College London,
> London SW7 2AZ, UK
> a.sacconi11 at imperial.ac.uk
>
>
> _______________________________________________
> Dune mailing list
> Dune at dune-project.org
> http://lists.dune-project.org/mailman/listinfo/dune

--
Dr. Martin Nolte <nolte at mathematik.uni-freiburg.de>

Universität Freiburg                                   phone: +49-761-203-5630
Abteilung für angewandte Mathematik                    fax:   +49-761-203-5632
Hermann-Herder-Straße 10
79104 Freiburg, Germany

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