Operating pressure is significant for incompressible ideal gas flows because it directly determines the density: the incompressible ideal gas law computes density as . You must therefore be sure to set the operating pressure appropriately.

In low-Mach-number compressible flow, the overall pressure drop is small compared to the absolute static pressure, and can be significantly affected by numerical roundoff. To understand why this is true, consider a compressible flow with . The pressure changes, , are related to the dynamic head, , where is the static pressure and is the ratio of specific heats. This gives the simple relationship , so that as . Therefore, unless adequate precaution is taken, low-Mach-number flow calculations are very susceptible to roundoff error.

Operating pressure is significant for low-Mach-number compressible flows because of its role in avoiding roundoff error problems. Ensure that you set the operating pressure appropriately. You may want to specify a floating operating pressure instead of a constant operating pressure for low-Mach-number, time-dependent compressible flows with average pressure in the domain varying in time. See Floating Operating Pressure for details.

Operating pressure is less significant for higher-Mach-number compressible flows. The pressure changes in such flows are much larger than those in low-Mach-number compressible flows, so there is no real problem with roundoff error and, therefore, no real need to use gauge pressure. In fact, it is common convention to use absolute pressures in such calculations. Since Ansys Fluent always uses gauge pressure, you can simply set the operating pressure to zero, making gauge and absolute pressures equivalent.

If the density is assumed constant or if it is derived from a profile function of temperature, the operating pressure is not used in the density calculation.

Note that the default operating pressure is 101325 Pa.

Ansys Fluent avoids the problem of roundoff error (discussed in The Significance of Operating Pressure) by subtracting the operating pressure (generally a large pressure roughly equal to the average absolute pressure in the flow) from the absolute pressure, and using the result (termed the gauge pressure). The relationship between the operating pressure, gauge pressure, and absolute pressure is shown below. The absolute pressure is simply the sum of the operating pressure and the gauge pressure:

(8–74)

All pressures that you specify and all pressures computed or reported by Ansys Fluent are gauge pressures.

The criteria for choosing a suitable operating pressure are based on the Mach-number regime of the flow and the relationship that is used to determine density. For example, if you use the ideal gas law in an incompressible flow calculation (for example, for a natural convection problem), you should use a value representative of the mean flow pressure.

To place this discussion in perspective, Table 8.1: Recommended Settings for Operating Pressure shows the recommended approach for setting operating pressures. The default operating pressure is 101325 Pa.

Table 8.1: Recommended Settings for Operating Pressure

Density Relationship	Mach Number Regime	Operating Pressure
ideal gas law		0 or mean flow pressure
ideal gas law		mean flow pressure
profile function of temperature	incompressible	not used
constant	incompressible	not used
incompressible ideal gas law	incompressible	mean flow pressure

You will set the Operating Pressure in the Operating Conditions Dialog Box.

 Setup →   Cell Zone Conditions → Operating Conditions...