Pressure outlet boundary conditions require the specification of a static (gauge) pressure at the outlet boundary. The value of the specified static pressure is used only while the flow is subsonic. Should the flow become locally supersonic, the specified pressure will no longer be used; pressure will be extrapolated from the flow in the interior. All other flow quantities are extrapolated from the interior.
A set of "backflow'' conditions is also specified should the flow reverse direction at the pressure outlet boundary during the solution process. Convergence difficulties will be minimized if you specify realistic values for the backflow quantities.
Several options in FLUENT exist, where a radial equilibrium outlet boundary condition can be used (see Section 7.8.1 for details), and a target mass flow rate for pressure outlets (see Section 7.8.4 for details) can be specified.
For an overview of flow boundaries, see Section 7.2.