Specify force $\vec{F}$ or pressure $P$ at the boundary. For the case of surface gravity waves (water waves is an example of), the position of the interface between the two fluids (water an air in the case of water waves) is unknown, so besides the kinematic boundary condition at the interface we also prescribe a dynamic boundary condition, which is the prescription of the pressure at the interface (pressure on both sides of the interface have to be the same).

Timothy S Choe
2003-03-15