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

[John P. <jwp...@gm...>]

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);

where "s" is the numerical index of a side.

-- 
John