use compute_no_normal_flux_constraints within step-35

[Giovanni Formica]

Dear All,

I'm trying to slightly modify the step-35 code in order to better understand how boundary conditions work in this case study.

The boundary id 3 is the right-hand line of the domain ("nsbench2.inp" 2D example), and I'd like to impose a zero tangential velocity in an alternative way the tutorial does.

In the diffusion_step of the tutorial, we have a loop over each component, and in the case 3 of the boundary ids we simply impose that the components != 0 are zero.

Maybe I'm wrong, but this situation is valid only with orthogonal faces and not with curved boundaries.

Therefore, I'd like to substitute the present call

                case 3:
                      if (d != 0)
                          VectorTools::interpolate_boundary_values(
                                  dof_handler_velocity,
                                  boundary_id,
                                  Functions::ZeroFunction<dim>(),
                                  boundary_values);
                      break;

with something like this:

                case 3:
                    VectorTools::compute_no_normal_flux_constraints(
                                dof_handler_velocity,
                                0,
                                boundary_id,
                                Functions::ZeroFunction<dim>(), // incorrect, I know!
                                        boundary_values);
                      break;

I understood that "ZeroFunction" has to be actually a "AffineConstraints", but I'd like to avoid complicating the code too much, at the moment (I'm a deal.ii beginner).

Any help or suggestions  are greatly appreciated.

Best!