The available Boundary condition options for an outlet are Pressure, Laminar outflow, and Velocity.

This option specifies the normal stress, which in most cases is approximately equal to the pressure. The tangential stress component is set to zero. If the reference pressure pref, defined at the physics interface level, is equal to 0, the value of the Pressure p0, at the boundary, is the absolute pressure. Otherwise, p0 is the relative pressure at the boundary.

When Include gravity is selected and Use reduced pressure not selected in the interface Physical model section, the Compensate for hydrostatic pressure approximation (named Compensate for hydrostatic pressure for compressible flows) check box is available and selected by default. When it is selected, the hydrostatic pressure is automatically added to the pressure entered in p0 user input.

See the Inlet node Velocity section for the settings.

This boundary condition is applicable when the flow exits the domain into a long pipe or channel, at the end of which a laminar flow profile is fully developed. The normal stress at the outlet is determined from the flow conditions at the end of a fictitious channel appended to the outlet boundary. The outflow can be specified by the Average velocity Uav, the Flow rate V0, or the Exit pressure pexit.

For the Entrance pressure option, when Include gravity is selected and Use reduced pressure not selected in the interface Physical model section, the Compensate for hydrostatic pressure approximation (named Compensate for hydrostatic pressure for compressible flows) check box is available and selected by default. When it is selected, the hydrostatic pressure is automatically added to the pressure entered in pexit user input.

The Exit length Lexit should be significantly greater than 0.06ReD, where Re is the Reynolds number, and D is the outlet length scale (hydraulic diameter), in order that the flow can adjust to a fully developed laminar profile.

The Constrain outer edges to zero option forces the laminar profile to go to zero at the bounding points or edges of the outlet channel. Otherwise, the velocity is defined by the boundary condition of the adjacent boundary in the computational domain. For example, if one end of a boundary with a Laminar outflow condition connects to a slip boundary, the laminar profile will have a maximum at that end.

This section is displayed by clicking the Show button () and selecting Advanced Physics Options.