Boundary condition in eigenvalue problem

[fb77 (François Bouchut)]

You may look at this post for the implementation of the slip condition on a curved boundary.

Boundary condition

If you want to avoid using a Lagrange multiplier (because it enlarges the matrix to invert), some other ways to penalize the normal boundary conditions are possible. In 3D you can use for example the boundary vertices. You have to define first a continuous approximate normal on the boundary (label 1 is for boundary) fespace Vh1(Th,[P2,P2,P2]); varf bdryn([w1,w2,w3],[v1,v2,v3])=int2d(Th,1)(v1*N.x+v2*N.y+v3*N.z); Vh1 [normalappx,normalappy,normalappz]; normalappx[]=bdryn(0,Vh1); Then you can ad…