An outlet boundary condition is used where the flow is predominantly directed out of the domain. The hydrodynamic boundary condition specification (that is, those for mass and momentum) for a subsonic outlet involves some constraint on the boundary static pressure, velocity or mass flow. For all other transport equations, the outlet value of the variable is part of the solution.
Important: If you accidentally set the velocity at an outlet to be flowing into the domain, the outlet will behave as an inlet: wall building will not be turned on.
Information on setting outlet values in CFX-Pre is available. For details, see Boundary Details: Outlet in the CFX-Pre User's Guide.
Information on the mathematical implementation of an outlet boundary condition is available.