Q: Boundary condition for a single slit experiment

[S W]

Does anyone know how to set up the boundary condition for a single slit?

In detail, we have two fields u_x, u_y for velocity in x,y direction. 

For example, using tau-method, the following code setup a Dirichlet hard wall:
problem.add_equation(u_y(y=0)=0)

But how to open a slit(hole) with Neumann bdy condition on the hole?

Many thanks!