On Tue, May 15, 2018 at 8:31 AM, Griffith, Boyce Eugene < boy...@email.unc.edu> wrote:
> > > On May 15, 2018, at 10:23 AM, John Peterson <jwpeter...@gmail.com> wrote: > > > > On Tue, May 15, 2018 at 8:17 AM, Griffith, Boyce Eugene < > boy...@email.unc.edu> wrote: > >> >> >> On May 15, 2018, at 10:13 AM, John Peterson <jwpeter...@gmail.com> wrote: >> >> >> >> On Tue, May 15, 2018 at 5:37 AM, Griffith, Boyce Eugene < >> boy...@email.unc.edu> 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 ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Libmesh-users mailing list Libmesh-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmesh-users
