Theory
The outlet is a fixed static pressure (fixedValue in 0/p), with zero-gradient velocity for outflow and zero velocity imposed on faces where the flow turns back in (inletOutlet) (OpenFOAM user guide, 2026). The flow, and therefore the pressure drop, is set by the inlets; the outlet pressure only fixes the level of the pressure field.
The incompressible solver works with the kinematic pressure and has no absolute pressure: the program writes , equation (1), and the whole field is relative to the ambient. Because only pressure gradients drive an incompressible flow, a different outlet value shifts the whole field by that constant and leaves the pressure drop unchanged. The compressible solver needs the absolute pressure, which sets the density: it gets , equation (2) (Outlet absolute pressure).
Reported pressures are relative to the ambient (the operating pressure): the outlet reads its own gauge value. The engine backpressure is the inlet static pressure relative to the ambient; it equals the static pressure drop exactly when every outlet's gauge pressure is 0.
Equations
| where | meaning | unit |
|---|---|---|
| Outlet gauge pressure (this setting) | Pa | |
| Density of the Material step | kg/m³ | |
| Ambient pressure of the Material step | Pa |
Default and rule
- Default
- 0 Pa (discharge to the ambient)
- Rule
0 Pa.
- Source
Boundary conditions as documented for OpenFOAM (OpenFOAM user guide, 2026); the conversions to kinematic and absolute pressure are the program's.
Valid range any; relative to the ambient pressure of the Material step
Values
Any value; a positive gauge pressure models a downstream resistance (a tailpipe or a further silencer not in the model).
Several outlets
Each outlet has its own value. Different values change how the flow splits between the outlets.