# Outlets

Identify one or more outlet surfaces and define the conditions.

1. On the Fluids ribbon, select the Outlet tool.

Tip: To find and open a tool, press Ctrl+F. For more information, see Find and Search for Tools.
2. Select one or more surfaces as the outlets.
3. In the microdialog, select and define the desired outlet condition.
• Gauge Pressure: Define the gauge value of the static pressure of the exiting fluid at the outlet surface. The gauge value of pressure is defined as the pressure relative to the atmospheric pressure (p_gauge = p_absolute - p_atmospheric). In the case of an incompressible subsonic flow, the typical fluid pressure at the outlet is equal to the atmospheric pressure. For example, if the atmospheric pressure is 14.7 psi (pounds-per-square-inch) and the static pressure at the outlet is equal to the atmospheric pressure, the gauge value of static pressure should be entered as 0. The velocity and temperature are automatically calculated at the outlet surface. Gauge pressure specification is the most commonly used outlet condition where the exit velocity or flow rate are unknown.
• Average Velocity: Define the average velocity of the exiting fluid at the outlet surface. A spatially varying velocity profile is applied on the outlet surface along a locally perpendicular direction at each point on the surface. This velocity profile is calculated based on a fully developed pipe flow assumption (laminar or turbulent) that models the existence of a boundary layer, whose average value is equal to the user-specified average velocity magnitude. A fully developed velocity profile has zero velocity on wall boundaries and maximum velocity farthest from the walls. The exact velocity variation is based on the flow Reynolds number. Use this condition when a known exit velocity must be enforced at an outlet surface.
• Volumetric Flow Rate: Define the volume of the fluid per unit time exiting the outlet surface. The application of this condition follows a similar approach to the Average Velocity condition, where a fully developed velocity profile is computed such that the specified flow rate is enforced on the outlet surface. This velocity profile is calculated based on a fully developed pipe flow assumption (laminar or turbulent) that models the existence of a boundary layer. A fully developed velocity profile has zero velocity on wall boundaries and maximum velocity farthest from the walls. The exact velocity variation is based on the flow Reynolds number. Use this condition when a known flow rate must be enforced at an outlet surface.
Tip: Gauge pressure is defined relative to the atmospheric pressure. To set the outlet pressure equal to the atmospheric pressure, enter 0.