
Summary
You will enter the following information for a pressure outlet boundary:
All values are entered in the Pressure Outlet panel (Figure 7.8.1), which is opened from the Boundary Conditions panel (as described in Section 7.1.4). Note that open channel boundary condition inputs are described in Section 23.10.2.
Defining Static Pressure
To set the static pressure at the pressure outlet boundary, enter the appropriate value for Gauge Pressure in the Pressure Outlet panel. This value will be used for subsonic flow only. Should the flow become locally supersonic, the pressure will be extrapolated from the upstream conditions.
Remember that the static pressure value you enter is relative to the operating pressure set in the Operating Conditions panel. Refer to Section 7.3.1 regarding hydrostatic pressure.
FLUENT also provides an option to use a radial equilibrium outlet boundary condition. To enable this option, turn on Radial Equilibrium Pressure Distribution. When this feature is active, the specified gauge pressure applies only to the position of minimum radius (relative to the axis of rotation) at the boundary. The static pressure on the rest of the zone is calculated from the assumption that radial velocity is negligible, so that the pressure gradient is given by
(7.81) 
where is the distance from the axis of rotation and is the tangential velocity. Note that this boundary condition can be used even if the rotational velocity is zero. For example, it could be applied to the calculation of the flow through an annulus containing guide vanes.

Note that the radial equilibrium outlet condition is available only for 3D and axisymmetric swirl calculations.

Defining Backflow Conditions
Backflow properties consistent with the models you are using will appear in the Pressure Outlet panel. The specified values will be used only if flow is pulled in through the outlet.
If the cell zone adjacent to the pressure outlet is moving (i.e., if you are using a rotating reference frame, multiple reference frames, mixing planes, or sliding meshes) and you are using the pressurebased solver, the velocity in the dynamic contribution to total pressure (see Equation 7.33) will be absolute or relative to the motion of the cell zone, depending on whether or not the Absolute velocity formulation is enabled in the Solver panel. For the densitybased solvers, the velocity in Equation 7.33 (or the Mach number in Equation 7.34) is always in the absolute frame.

Even if no backflow is expected in the converged solution, you should always set realistic values to minimize convergence difficulties in the event that backflow does occur during the calculation.

Defining Radiation Parameters
If you are using the P1 radiation model, the DTRM, the DO model, or the surfacetosurface model, you will set the Internal Emissivity and (optional) Black Body Temperature. See Section 13.3.15 for details. (The Rosseland radiation model does not require any boundary condition inputs.)
Defining Discrete Phase Boundary Conditions
If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the pressure outlet. See Section 22.13 for details.
Defining Open Channel Boundary Conditions
If you are using the VOF model for multiphase flow and modeling open channel flows, you will need to specify the Free Surface Level, Bottom Level, and additional parameters. See Section 23.10.2 for details.