I have a question about boundary condition assembling procedure. I would like to impose a boundary condition like that : du/dn + a.u + b.d²u/dt² = 0 to implement BGT radiation condition (or similar) where
u is the unknow solution du/dn define the normal derivative d²u/dt² the tangential second derivative and a and b are constants.
For the weak formulation, in 2d, the tangential derivative of u is needed.
To impose the normal derivative and the linear part, I use that kind of assembling : getfem::generic_assembly assem; assem.push_mi(mim); assem.push_mf(mf_u); std::vector<T> tmp(1, m_a); //m_a is a constant
assem.push_data(tmp); assem.push_mat(F); assem.set("F=data(1);" "M(#1,#1)+=-sym(comp(Base(#1).Base(#1))(:,:).F(p))"); assem.assembly(Robin_region);
What is the best way to introduce the tangente part?