Get side normal vector

[LP1012]

Hi All,

This should be (I hope) a super quick question, but I am having terrible luck finding what I need in the documentation. I am using libMesh features in MOOSE development, and I need the normal vector an element side (so a face in 3D, edge in 2D) to compute the dot product between it and an incoming particle angle (for work in Magpie, for anyone interested...). Anyways, I am not finding a method for calling the side's normal vector... Would anyone have any suggestions?

Here is a clip of the code of interest:

const auto range = _mesh.getBoundaryElementRange();
  for (const BndElement * bnd_elem : *range)
  {
    if (bnd_elem->_bnd_id != boundary_id)
      continue; // skip other boundaries
    const auto elem = bnd_elem->_elem;
    dof_id_type elem_id = elem->id();
    const auto side = bnd_elem->_side;
    Real side_volume = elem->side_ptr(side)->volume();

    RealVectorValue side_normal_vec = elem->build_side_ptr(side)->normal(); // THIS LINE IS BROKE
    Real volume_scaler = side_normal_vec * _direction;

    volume_sum += side_volume * volume_scaler;

    elem_volumes.emplace_back(elem_id, side_volume);
  }

[roystgnr]

A side can be non-planar, so most users call get_normals() to get normal vectors at all the side quadrature points where they're doing other FE computations.

There's a new method, that works like

RealVectorValue side_normal_vec = elem->side_vertex_average_normal(side);

that might work for you, but IIRC it was added for finite volume users and it's only currently implemented for first-order geometric elements.