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, Fully developed flow, 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.
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For incompressible flow, the Pressure list has two options, Static and Total. For weakly compressible and compressible flow, the static pressure should be specified in the text field.
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If Pressure is Static, and 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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If Pressure is Total, the Average check box is available and selected by default to prescribed the averaged total pressure in the weak forms. If it is cleared, the total pressure is imposed pointwise.
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 incompressible 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.
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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 reduce the amount of fluid entering the domain through the boundary.
Velocity
See the Inlet node Velocity section for the settings.
Fully Developed Flow
This boundary condition is applicable when the flow exits the domain into a long pipe or channel, at the end of which a flow profile is fully developed.
The channel can be thought of as a virtual extrusion of the outlet cross section. The outlet boundary must hence be flat in order for the fully developed flow condition to work properly. In 2D axisymmetric models, the outlet normal must be parallel to the symmetry axis.
Select an option to control the flow rate at the outlet:
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Average velocity, Uav.
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Flow rate, V0. Two-dimensional models also require an Entrance thickness, Dz, which is the out-of-plane thickness of the extruded entrance channel.
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Average pressure, Pav. Observe that Pav is the average pressure on the outflow.
The Apply condition on each disjoint selection separately check box is selected per default. When this setting is selected, the fully developed flow condition is applied separately on each disjoint selection. If this option is not selected, the condition is applied over the whole feature selection. The Apply condition on each disjoint selection separately should be disabled only if the flow conditions are known to be identical on each disjoint boundary.
The fully developed flow condition requires any volume force to be approximately aligned with the normal of the outlet boundary. The exception is gravity when the Include gravity setting is selected in the physics interface settings. Unless Use reduced pressure is also selected, an option to Compensate for hydrostatic pressure or Compensate for hydrostatic pressure approximation becomes available. It is selected per default and should only be deselected if the outlet normal is aligned with the gravity force and you want to specify an average pressure that includes the hydrostatic pressure.
For the Viscoelastic Flow interface, the fully developed outflow can be used in conjunction with the viscoelastic constitutive model. No additional inputs are required for the elastic stress tensor components. They are instead solved for on the outlet boundary to be consistent with the fully developed flow profile.
Constraint Settings
This section is displayed by clicking the Show button () and selecting Advanced Physics Options.
Prescribing Inlet and Outlet Conditions in the COMSOL Multiphysics Reference Manual
Fully Developed Flow (Outlet)