This condition should be used on boundaries for which there is a net outflow from the domain. To obtain a numerically well posed problem, it is advisable to also consider the Inlet condition(s) when specifying an Outlet condition. For example, if the velocity is specified at the inlet, the pressure may be specified at the outlet, and vice versa. Specifying the velocity vector at both the inlet and the outlet may cause convergence difficulties. Selecting appropriate outlet conditions for the Navier-Stokes equations is a non-trivial task. Generally, if there is something interesting happening at an outflow boundary, the computational domain should be extended to include this phenomenon.