The connecting boundary condition
between the domains ensures that the velocity potential and
the velocity in the x-direction are equal on the connecting boundary
between the domains, except on the wall surface. Since the
velocity potentials on the connecting boundary between the domains
are equal, the velocities in the z-direction are equal too. The
relation between the velocity potential and the pressure are derived
by the Bernoulli equation, Eq. 1. If the velocity potentials
are the same, the pressure is also the same on the connecting