Theory
The ambient pressure comes from the Material step (from the altitude through the standard atmosphere, or typed). The compressible solver imposes the absolute value of equation (1) on the outlet (OpenFOAM user guide, 2026), and the gas density everywhere follows from the absolute pressure through the ideal-gas law. The incompressible solver imposes only the gauge pressure (as a kinematic pressure); for it the absolute value sets the gas density: the density of the Material step is computed at the outlets' mean absolute pressure (the operating pressure). The compressible solver's pressure limits are 0.1 and 2 times the outlet pressure, and its starting field is the operating pressure.
Equations
| where | meaning | unit |
|---|---|---|
| Absolute outlet pressure | Pa | |
| Ambient pressure (Material step) | Pa | |
| Outlet gauge pressure (Outlet gauge pressure) | Pa |
Default and rule
- Default
- ambient pressure + gauge pressure
- Rule
Computed from the gauge pressure; typing it sets the gauge pressure.
- Source
The compressible outlet value is the program's sum of the Material step's ambient pressure and the gauge pressure; boundary condition as documented for OpenFOAM (OpenFOAM user guide, 2026).
Valid range read-only: follows the ambient pressure and the gauge pressure
Typed value
Typing the absolute value stores absolute minus ambient as the gauge pressure; a later change of the ambient pressure keeps the gauge pressure.