
FLUENT uses your boundary condition inputs at velocity inlets to compute the mass flow into the domain through the inlet and to compute the fluxes of momentum, energy, and species through the inlet. This section describes these calculations for the case of flow entering the domain through the velocity inlet boundary and for the less common case of flow exiting the domain through the velocity inlet boundary.
Treatment of Velocity Inlet Conditions at Flow Inlets
When your velocity inlet boundary condition defines flow entering the physical domain of the model, FLUENT uses both the velocity components and the scalar quantities that you defined as boundary conditions to compute the inlet mass flow rate, momentum fluxes, and fluxes of energy and chemical species.
The mass flow rate entering a fluid cell adjacent to a velocity inlet boundary is computed as
Note that only the velocity component normal to the control volume face contributes to the inlet mass flow rate.
Treatment of Velocity Inlet Conditions at Flow Exits
Sometimes a velocity inlet boundary is used where flow exits the physical domain. This approach might be used, for example, when the flow rate through one exit of the domain is known or is to be imposed on the model.

In such cases you must ensure that overall continuity is maintained in the domain.

In the pressurebased solver, when flow exits the domain through a velocity inlet boundary FLUENT uses the boundary condition value for the velocity component normal to the exit flow area. It does not use any other boundary conditions that you have input. Instead, all flow conditions except the normal velocity component are assumed to be those of the upstream cell.
In the densitybased solvers, if the flow exits the domain at any face on the boundary, that face will be treated as a pressure outlet with the pressure prescribed in the Outflow Gauge Pressure field.
Density Calculation
Density at the inlet plane is either constant or calculated as a function of temperature, pressure, and/or species mass fractions, where the mass fractions are the values you entered as an inlet condition.