Outlet
This condition should be used on boundaries for which there is a net outflow from the domain. To obtain a numerically well-posed problem, it is advisable to also consider the Inlet conditions when specifying an Outlet condition. For example, if the velocity is specified at the inlet, the pressure may be specified at the outlet, and vice versa. Specifying the velocity vector at both the inlet and the outlet may cause convergence difficulties. Selecting appropriate outlet conditions for the Navier-Stokes equations is a nontrivial task. Generally, if there is something interesting happening at an outflow boundary, the computational domain should be extended to include this phenomenon.
Boundary Condition
The available Boundary condition options for an outlet are Pressure and Velocity.
Pressure Conditions
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.
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The Normal flow option changes the no tangential stress condition to a no tangential velocity condition. This forces the flow to exit (or enter) the domain perpendicularly to the outlet boundary.
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The Suppress backflow check box is selected by default. This option adjusts the outlet pressure in order to prevent fluid from entering the domain through the boundary.
Velocity
See the Inlet node Velocity section for the settings.
Constraint Settings
This section is displayed by clicking the Show button () and selecting Advanced Physics Options.
Prescribing Inlet and Outlet Conditions