Inlet

The available options for an inlet are , , and . After selecting a from the list, a section with the same or a similar name displays underneath. For example, if is selected, a section, where further settings are defined, is displayed.

The is specified as u = −nU0, where n is the boundary normal pointing out of the domain and U0 is the normal inflow speed.

The option sets the velocity vector to u = u0. The components of the inlet velocity vector u0 should be defined for this choice.

This option specifies the normal stress, which in most cases is approximately equal to the pressure.

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For incompressible flow, the list has two options, and . For weakly compressible and compressible flow, the static pressure should be specified in the text field.

If is 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.

If is , the check box is available and unselected by default to prescribe the total pressure pointwise. If it is selected, the averaged total pressure is imposed in the weak forms instead.

When is selected and not selected in the interface section, the (named 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 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.

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

The mass flow at an inlet can be specified by the or a .

The 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 should be disabled only if the flow conditions are known to be identical on each disjoint boundary.

The option sets the integrated mass flow over the boundary selection, the 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 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.

The 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 is selected a value or function Mf should be specified for the .

For the Viscoelastic Flow interface, the inlet conditions for the elastic stress components are required. Note, that if several branches are specified in the node, the initial values entered above are applied to all branches.

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

The 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 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 setting is selected in the physics interface settings. Unless is also selected, an option to or 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.