How to add boundary vertex to mesh

[amir90]

Hi,

In example 2 (linear static analysis) the essential boundary conditions are faces of the mesh.

1. I need to run a simple tetrahedral linear static analysis where I need to fix nodes.
   However, there is no addbdrVertex to the mesh. I found AdjustVDofs, but I am unsure how to use it, and have not found any examples of it.

2. I also need the forces to act on a few known nodes, How do I select individual nodal DOFs for the forces to act on?

Thanks.

[jakubcerveny]

Hi @amir90, in FEM it's incorrect to prescribe BCs or loads on zero measure sets (single nodes). It may look like it's working but when you refine the mesh, you get a different BC or load. In fact, as you keep refining, your stresses will go to infinity. You should fix (or load) boundary elements, i.e., edges in 2D and faces in 3D.

[tzanio]

To add to @jakubcerveny's answer -- if you really need to, you can use the EliminateVDofs method of BilinearForm to eliminate a list of Dirichlet degrees of freedom.

See

mfem/fem/bilinearform.hpp

Lines 478 to 487 in 18f0cf0

	/// Eliminate the given @a vdofs. NOTE: here, @a vdofs is a list of DOFs.
	void EliminateVDofs(const Array<int> &vdofs, const Vector &sol, Vector &rhs,
	DiagonalPolicy dpolicy = DIAG_ONE);
	/// Eliminate the given @a vdofs, storing the eliminated part internally.
	/** This method works in conjunction with EliminateVDofsInRHS() and allows
	elimination of boundary conditions in multiple right-hand sides. In this
	method, @a vdofs is a list of DOFs. */
	void EliminateVDofs(const Array<int> &vdofs,
	DiagonalPolicy dpolicy = DIAG_ONE);

as well as #35, #80 (comment) and #737.