## 15.7.2 Specifying the Operating Pressure for the System

The system Operating Pressure is used to calculate density using the ideal gas law. For non-adiabatic simulations, the Compressibility Effects under PDF Options can be enabled to account for cases where substantial pressure changes occur in time and/or space. In such cases it is assumed that the species mass fractions do not change with pressure, and the density is calculated as

 (15.7-1)

where is the density at the specified Operating Pressure ( ), and is the local mean pressure in a FLUENT cell.

When the Compressibility Effects option is enabled, the flow operating pressure (set in the Operating Conditions panel) can differ from the Non-Premixed model operating pressure. To distinguish this difference, the Operating Pressure name tag in the Species Model panel changes to Equilibrium Operating Pressure when the compressibility effects option is enabled.

SeeSection  15.16 for details about enabling compressibility effects.