Inlet
This condition should be used on boundaries for which there is a net flow into the domain. To obtain a numerically well-posed problem, it is advisable to also consider the Outlet conditions when specifying an Inlet condition. For example, if the pressure is specified at the outlet, the velocity may be specified at the inlet, and vice versa. Specifying the velocity vector at both the inlet and the outlet may cause convergence difficulties.
Boundary Condition
The available Boundary condition options for an inlet are Velocity, Fully developed flow, Mass flow, and Pressure. After selecting a Boundary Condition from the list, a section with the same or a similar name displays underneath. For example, if Velocity is selected, a Velocity section, where further settings are defined, is displayed.
Velocity
The Normal inflow velocity is specified as u = −nU0, where n is the boundary normal pointing out of the domain and U0 is the normal inflow speed.
The Velocity field option sets the velocity vector to u = u0. The components of the inlet velocity vector u0 should be defined for this choice.
Pressure Conditions
This option specifies the normal stress, which in most cases is approximately equal to the pressure.
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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 cleared by default to prescribe the total pressure pointwise. If it is selected, the averaged total pressure is imposed in the weak forms instead.
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 Suppress backflow option adjusts the inlet pressure locally in order to reduce the amount of fluid exiting the domain through the boundary. If you clear the suppress backflow option, the inlet boundary can become an outlet depending on the pressure field in the rest of the domain.
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Flow direction controls in which direction the fluid enters the domain.
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For Normal flow, it prescribes zero tangential velocity component.
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For User defined, an Inflow velocity direction du (dimensionless) should be specified. The magnitude of du does not matter, only the direction. du must point into the domain.
Mass Flow
The mass flow at an inlet can be specified by the Mass flow rate or a Pointwise mass flux.
The Apply condition on each disjoint selection separately check box is selected per default. When this setting is selected, the mass 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.
Mass Flow Rate
The Mass flow rate option sets the integrated mass flow over the boundary selection, the Normal mass flow rate to a specific value, m. The mass flow is assumed to be parallel to the boundary normal, and the tangential flow velocity is set to zero.
For 2D components, the Channel thickness dbc is used to define the area across which the mass flow occurs. This setting is not applied to the whole model. Line or surface integrals of the mass flow over the boundary evaluated during postprocessing or used in integration coupling operators do not include this scaling automatically. Such results should be appropriately scaled when comparing them with the specified mass flow.
Pointwise Mass Flux
The Pointwise mass flux sets the mass flow parallel to the boundary normal. The tangential flow velocity is set to zero. The mass flux is a model input, which means that COMSOL Multiphysics can take its value from another physics interface when available. When User defined is selected a value or function Mf should be specified for the Mass flux.
Fully Developed Flow
The Fully developed flow option adds contributions to the inflow boundary, which force the flow toward the solution for a fully developed channel flow. The channel can be thought of as a virtual extrusion of the inlet cross section. The inlet boundary must hence be flat in order for the fully developed flow condition to work properly. In 2D axisymmetric models, the inlet normal must be parallel to the symmetry axis.
Select an option to control the flow rate at the inlet:
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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. Note that Pav is the average pressure on the inflow boundary.
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 inlet 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 inlet 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 inflow 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 inlet boundary to be consistent with the fully developed flow profile.
Viscoelastic Stress
For the Viscoelastic Flow interface, the inlet conditions for the elastic stress components are required. Note, that if several branches are specified in the Fluid Property node, the initial values entered above are applied to all branches.
Constraint Settings
This section is displayed by clicking the Show button () and selecting Advanced Physics Options.
In the COMSOL Multiphysics Reference Manual:
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Prescribing Inlet and Outlet Conditions
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Normal Stress Boundary Condition
Fully Developed Flow (Inlet)