COMSOL 6.2 - Inlet

The node adds a selection of boundary conditions that describe inlets in fluid-flow simulations.

	Gas Boundary Condition Equations

Select a — (the default), , or .

For click the (the default) or buttons.

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For specify that the velocity at the boundary is equal to a given u0 (SI unit: m/s) and enter the components in the matrix:

This condition specifies the normal stress which in most cases is approximately equal to the pressure. Enter the p0 (SI unit: Pa) at the boundary. The default is 0 Pa. Enter a relative pressure value in p0 (SI unit: Pa). Or if the reference pressure pref defined at the physics interface level is equal to 0 Pa, enter an absolute pressure in p0.

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The check box is selected by default. This option adjusts the inlet pressure,

, locally in order to reduce the amount of fluid exiting the domain through the boundary. If suppress backflow is deselected, the inlet boundary can become an outlet depending on the pressure field in the rest of the domain.

For define an , du (dimensionless). The magnitude of du does not matter, only the direction. du must point into the domain.

The 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 velocity is defined by the boundary condition of the adjacent boundary in the model.

The boundary conditions for the virtual inlet channel are inherited from the boundaries adjacent to the inlet channel. Virtual boundaries adjacent to walls (except slip walls) are treated as no-slip walls. Virtual boundaries adjacent to any other type of boundary are treated as slip-walls (or equivalently as symmetry boundaries).

Select a option — (the default), , or .

The option adds contributions to the inflow boundary, which force the flow toward the solution for a fully developed channel flow.

Select an option to control the flow rate at the inlet:

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, V0. Two-dimensional models also require an , Dz, which is the out-of-plane thickness of the extruded entrance channel.

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, Pav. Observe that Pav is the average pressure on the inflow boundary.

Fully developed inflow can be used in conjunction with the turbulence models. No additional inputs are required for the turbulence variables. They are instead solved for on the inlet boundary to be consistent with the fully developed flow profile.

A fully developed flow boundary cannot be adjacent to any Interior Wall feature

These settings are the same as for Wall. See Gas Boundary Condition. The only difference is that is the default.

This section is available for Bubbly Flow, Turbulent Flow interfaces. No additional inputs are required for .

No additional inlet conditions are required for the algebraic turbulence models: Algebraic yPlus and L-VEL.

For the interface, a value or expression for the υ0 should be specified. The default value is 3*bf.nul.

For transport-equation turbulence models, apart from Spalart–Allmaras, the following settings are available under the option (see Table 3-3 for the default values):

For the v2-f turbulence model, the additional choice between and appears. For , a value for the turbulent relative fluctuations at the inlet, ζ0, may be specified. When is selected, values or expressions for the dependent turbulence variables should be defined. Availability is based on the physics interface and the boundary condition chosen.

The turbulent length scale LT and turbulence intensity IT are related to the turbulence variables, see Theory for the Turbulent Flow Interfaces.