Operating condition in Fluent -- CFD Online Discussion Forums

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"For incompressible flows that do not involve any pressure boundaries,

ANSYS FLUENT adjusts the gauge pressure field after each iteration to keep it from floating. This is done using the pressure in the cell located at (or nearest to) the reference pressure location. The pressure value in this cell is subtracted from the entire gauge pressure field; as a result, the gauge pressure at the reference pressure location is always zero. If pressure boundaries are involved, the adjustment is not needed and the reference pressure location
is ignored.
The reference pressure location is, by default, the cell center at or closest to (0,0,0). There may be cases in which you might want to move the reference pressure location, perhaps locating it at a point where the absolute static pressure is known (e.g., if you are planning to compare your results with experimental data). To change the location, enter new (X,Y,Z) coordinates for Reference Pressure Location in the Operating Conditions dialog box."