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.
The Normal inflow velocity is specified as
u =
−nU
0, 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.
This option specifies the normal stress, which in most cases is approximately equal to the pressure. 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.
The Mass flow rate option sets the integrated mass flow across the entire boundary, 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.
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.
The Standard flow rate Qsv sets a standard volumetric flow rate, according to the SEMI standard E12-0303. The mass flow rate is specified as the volumetric flow rate of a gas at standard density — the
Mean molar mass Mn divided by a
Standard molar volume Vm (that is, the volume of one mole of a perfect gas at standard pressure and standard temperature). The flow occurs across the whole boundary in the direction of the boundary normal and is computed by a surface (3D) or line (2D) integral. 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.
The Standard flow rate (SCCM) boundary condition is equivalent to the
Standard flow rate boundary condition, except that the flow rate is entered directly in SCCMs (standard cubic centimeters per minute) without the requirement to specify units. Here, the dimensionless
Number of SCCM units Qsccm should be specified.
The Fully developed flow option adds contributions to the inflow boundary, which force the flow towards 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.
) and selecting Advanced Physics Options.