Outlet Considerations for Incompressible Flows

Outlet Configuration

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. If the outlet boundary is too close to where the flow has to turn, you should extend the outlet so that the flow can align itself, and exit normal to the outlet plane.

In this image, the outlet is poorly configured, allowing the flow to leave the domain as it is turning. This is not accurate because the pressure distribution is not physically uniform where the flow is turning:

In this image, the outlet is extended, allowing the flow to complete the turn, and align itself with the outlet plane:

Outlets at Corners

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.