At the outlet, where a uniform pressure is commonly applied, there must not be any flow features which will conflict with this uniform pressure boundary. Additionally, the flow should be approximately normal to the plane of the outlet. Sometimes the boundary must be extended to achieve this result. The following figure illustrates these concepts.
Pressure boundary conditions should not be specified on surfaces that meet at a corner. This often causes flow recirculation zones that can lead to analysis instability. It is not physically possible to maintain the specified pressure uniformly over all surfaces of a corner. The work-around is to extend the opening and to apply the boundary condition to only one surface of the extension.