Finding elements in the opposite side of a square domain

[adhithya96]

Is there any way to find the element on the opposite side in libMesh ?

I was able to find the nodes by using PointLocator but for elements I wasn't able to find anything on libMesh Doxygen. I am trying to use it to implement Bloch boundary condition in MOOSE.

[roystgnr]

libMesh optionally lets you construct a node-to-elem map; I believe MOOSE always constructs it.

[roystgnr]

Though, now that I think about it a little longer ... why weren't you able to find the elements by using PointLocator? Searching for elements rather than nodes is the default behavior.

[adhithya96]

So I am am trying to use FunctorDirichletBC and FunctorNeumannBC to apply bloch boundary condition for displacement and stress.

  //Get point  locator  
  const auto pl = _mesh.getPointLocator();
  //Locate node on the other side of the geometry
  //The translation vector will be a user parameter
  libMesh::Point _translation_vec= libMesh::Point(_lattice_vec, 0.0, 0.0);
  const auto new_node = pl->locate_node(*_current_node + _translation_vec);

I am using this to find the opposite node for the dirichlet case but for NeumannBC I need to find the element. This is the code in FunctorNeumannBC.C which I am trying to modify'

  const Moose::ElemSideQpArg space_arg = {_current_elem, _current_side, _qp, _qrule, _q_point[_qp]};
  return _sign * _coef(space_arg, Moose::currentState()) *
         _functor(space_arg, Moose::currentState()) * _test[_i][_qp];

Here instead of _current_elem, I need to find the _new_elem on the opposite side.

[roystgnr]

Check out the PointLocator docs; you can just use operator() in the simplest case.

[adhithya96]

While going through the PointLocator docs I found this virtual const Elem * operator() (const Point &p, const std::set< subdomain_id_type > *allowed_subdomains=nullptr) const =0. When I try to modify the code I am getting the following error

/home/adhithyar/Documents/Project/static1d/src/BlochNeumannBCReal.C:55:3: error: 'virtual' outside class declaration
   55 |   virtual const Elem* _new_elem(*_current_node + _translation_vec);
      |   ^~~~~~~
/home/adhithyar/Documents/Project/static1d/src/BlochNeumannBCReal.C:55:34: error: '_current_node' was not declared in this scope; did you mean '_current_side'?
   55 |   virtual const Elem* _new_elem(*_current_node + _translation_vec);
      |                                  ^~~~~~~~~~~~~
      |                                  _current_side
/home/adhithyar/Documents/Project/static1d/src/BlochNeumannBCReal.C:61:53: error: '_new_side' was not declared in this scope
   61 |  const Moose::ElemSideQpArg space_arg = {_new_elem, _new_side, _qp, _qrule, _q_point[_qp]};

I tried adding the required headers and inheriting the base class which was used in the dirchlet case but I keep getting _current_node was declared in this scope

[roystgnr]

We're digressing from "how do I use libMesh APIs" to "how do I use C++" at this point, I'm afraid. For that I recommend reading Stroustrop cover-to-cover, though I admit I might have more time to be helpful myself if I didn't regularly do things like reading Stroustrop cover-to-cover...

You've possibly found the right API; you just need to call it. Something like const Elem * elem = (*pl)(*_current_node + _translation_vec); would be the equivalent to the code you quoted originally.

On the other hand, I'm assuming that when you say "the element on the opposite side" you meant "an element on the opposite side". The astute viewer of your diagram will note two elements on the opposite side of each node; in 3D you could have any number of elements sharing a vertex on the opposite side of a node. If you're happy getting any of those then operator() really is what you want; if you want all of those we have other PointLocator APIs that will give you the whole batch.