Theory
is the absolute pressure to which the case is referenced:
- Every gauge pressure of the case is relative to : an outlet at 0 Pa gauge discharges to the atmosphere (Outlet gauge pressure), and the reported pressures are relative to it.
- It sets the density of the gas through the ideal-gas law.
- For the compressible solver it is the operating pressure, i.e. the absolute pressure on which the pressure field is built.
In altitude mode, is the pressure of the standard atmosphere's troposphere at Altitude, equation (1). In pressure mode it is Atmospheric pressure as typed, for example a measured barometric pressure or a test-cell pressure.
For an incompressible run only adds a constant to the reported absolute pressures and sets the density: the incompressible solver itself has no absolute pressure. The engine backpressure the report quotes, inlet static pressure absolute minus , is therefore the same number as the gauge inlet static pressure.
Equations
| where | meaning | unit |
|---|---|---|
| Ambient pressure | Pa | |
| Sea-level pressure, 101 325 | Pa | |
| Temperature lapse rate, 0.0065 | K/m | |
| Altitude | m | |
| Sea-level temperature of the standard atmosphere, 288.15 (not the case's temperature) | K | |
| Gravitational acceleration, 9.80665 | m/s² | |
| Molar mass of air, 0.0289644 | kg/mol | |
| Universal gas constant, 8.31447 | J/(mol K) |
Default and rule
- Default
- altitude (0 m, i.e. 101 325 Pa)
- Rule
Altitude, 0 m: Pa.
- Source
Equation (1) is the troposphere of the standard atmosphere (U.S. Standard Atmosphere, 1976), identical to the ISA in this range; its exponent evaluates to 5.25578.
Older case files that set a separate operating pressure are converted on loading: when that value differs from 101 325 Pa, the case opens in pressure mode with that value.
Valid range altitude | pressure
Choices
Altitude: -500 to 11 000 m. Pressure: 40 000 to 110 000 Pa absolute.
Standard atmosphere
Altitude mode gives the pressure of a standard day; the real barometric pressure varies by a few per cent with the weather. Use pressure mode when it is known.