Re: [Libmesh-users] normal vectors on surface meshes

[Paul T. B. <ptb...@gm...>]

Not to butt-in, but just to confirm, I also compute this "manually" as John
suggests for things like normal pressure on membranes, etc. I don't believe
there's a canned function for it.

On Tue, May 15, 2018 at 10:38 AM, John Peterson <jwp...@gm...>
wrote:

> On Tue, May 15, 2018 at 8:31 AM, Griffith, Boyce Eugene <
> bo...@em...> wrote:
>
> >
> >
> > On May 15, 2018, at 10:23 AM, John Peterson <jwp...@gm...>
> wrote:
> >
> >
> >
> > On Tue, May 15, 2018 at 8:17 AM, Griffith, Boyce Eugene <
> > bo...@em...> wrote:
> >
> >>
> >>
> >> On May 15, 2018, at 10:13 AM, John Peterson <jwp...@gm...>
> wrote:
> >>
> >>
> >>
> >> On Tue, May 15, 2018 at 5:37 AM, Griffith, Boyce Eugene <
> >> bo...@em...> wrote:
> >>
> >>> Folks --
> >>>
> >>> Is there an easy way in the library to get normal vectors on surface
> >>> meshes (e.g. 2D elements in 3D space, 1D elements in 2D space, etc)?
> It
> >>> seems like most of the support for this assumes that these are only
> needed
> >>> for doing surface integrals on volumetric meshes, but I am sure I am
> >>> overlooking something.
> >>>
> >>
> >> The "simplest" way is to build an FE object and only pre-request the
> >> normals by calling:
> >>
> >> const std::vector<Point> & face_normals = fe_face->get_normals();
> >>
> >>
> >> I may be making a blunder here, but this doesn't seem to work if the
> >> dimension of the quadrature rule is equal to the dimension of the mesh
> ---
> >> the normals do not appear to be computed.
> >>
> >
> >
> > Hmm... yes generally you would use a dim-1 dimensional quadrature rule in
> > this context, e.g. the following (from adaptivity_ex4) should work:
> >
> > std::unique_ptr<FEBase> fe_face (FEBase::build(dim, fe_type));
> > std::unique_ptr<QBase> qface(fe_type.default_quadrature_rule(dim-1));
> > fe_face->attach_quadrature_rule (qface.get());
> > const std::vector<Point> & face_normals = fe_face->get_normals();
> > fe_face->reinit(elem, s);
> >
> >
> > I need to integrate on a surface mesh (for instance, a boundary mesh
> > extracted from a volumetric mesh --- although it could simply be a
> surface
> > that is not derived from a volumetric mesh) and to get the normals ---
> > something like:
> >
> > std::unique_ptr<FEBase> fe_surface(FEBase::build(dim, fe_type));
> > std::unique_ptr<QBase> quad_surface(fe_type.default_
> quadrature_rule(dim));
> > fe_surface->attach_quadrature_rule (quad_surface.get());
> > const std::vector<Point> & face_normals = fe_surface->get_normals();
> > fe_surface->reinit(elem);
> >
>
>
> I see now what you are saying. Yeah, the normals are only computed during
> *side* reinits, since they are outward normals relative to
> higher-dimensional element.
>
> So, for a 2D element I would compute them manually by computing dx/d(xi)
> \cross dx/d(eta), (those two quantities are available from the FE object to
> help you).
>
> --
> John
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