Konstantine Kourbatski
Ansys Employee

Fluent calculates gauge pressure, which is defined as p_gauge = p_abs - p_op. Thus, for a system without pressure variations,  p_gauge = 0 and p_op = p_abs. For more information, please refer to Sec. 8.14. Operating Pressure of Fluent User Guide.

The operating pressure is a global value used for all fluids zone. If you need to specify different pressures in different fluid zones, then this can be accomplished by prescribing different gauge pressures to those zones.