Problem solving fourth-order PDE

[mekong]

@Stein

You must substitute the known boundary condition for - n[i] * v.dx( i ) in place of it,

No, because my boundary conditions (BCs) are Dirichlet BCs. They do not constraint n^i \partial_i v, see

bc_u = DirichletBC( Q.sub( 0 ), u_profile, boundary )
bc_v = DirichletBC( Q.sub( 1 ), v_profile, boundary )
[...]
bcs = [bc_u, bc_v]
[...]
problem = NonlinearVariationalProblem( F, psi, bcs, J )

I agree with you that I could drop the term - n[i] * (v.dx( i )) * nu_u * ds because nu_u vanishes, but this previous post of Dokken’s implies that leaving that term sholud be harmless, because it is zero. Can I get some problems if I leave it ?