Theory
The incompressible solver works with a constant density and a kinematic pressure; it has no density field. A mass-flow inlet is written as OpenFOAM's flowRateInletVelocity with massFlowRate and rhoInlet (OpenFOAM user guide, 2026), which imposes the volume flow of equation (1). The same density converts the mass flow wherever the program needs a velocity (Mach estimate, turbulence estimate, mesh plan).
The box defaults to the density a run uses, computed on the Material step from the temperature and the ambient pressure (ideal gas). A value typed here overrides the Material density for this inlet only.
For a compressible case the field is hidden and not stored: the compressible solver divides the mass flow by the solved density on the inlet face, so the mass flow is imposed exactly, independent of any inlet density (in a validation run the solved inlet mass flux equals the requested value).
Equations
| where | meaning | unit |
|---|---|---|
| Volume flow rate imposed at the inlet | m³/s | |
| Mass flow rate entered | kg/s | |
| Inlet density | kg/m³ |
Default and rule
- Default
- the density of the Material step
- Rule
The Material step's density (computed, or the manual override there).
- Source
Behaviour of OpenFOAM's flow-rate inlet as documented (OpenFOAM user guide, 2026); the compressible behaviour is confirmed by a validation run of the program.
Valid range > 0; incompressible mass-flow inlets only
Values
Positive. Keep it equal to the Material density unless the inlet gas really differs from the rest of the domain; the incompressible solver itself uses one density everywhere.
Different from the Material density
The solver's pressure is converted to pascals with the Material density, so a different inlet density changes the imposed volume flow but not the density of the solved flow.