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8.14.3 Setting the Operating Pressure



The Significance of Operating Pressure


Operating pressure is significant for incompressible ideal gas flows because it directly determines the density: the incompressible ideal gas law computes density as $\rho = \frac{p_{\rm op}}{\frac{R}{M_w} T}$. You must therefore be sure to set the operating pressure appropriately.

Operating pressure is significant for low-Mach-number compressible flows because of its role in avoiding roundoff error problems, as described in Section  8.14.2. Again, you must be sure to set the operating pressure appropriately. For time-dependent compressible flows, you may want to specify a floating operating pressure instead of a constant operating pressure. See Section  9.6.4 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 there is therefore no real need to use gauge pressure. In fact, it is common convention to use absolute pressures in such calculations. Since 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.



How to Set the Operating Pressure


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 (e.g., for a natural convection problem), you should use a value representative of the mean flow pressure.

To place this discussion in perspective, Table  8.14.1 shows the recommended approach for setting operating pressures. Remember that the default operating pressure is 101325 Pa.


Table 8.14.1: Recommended Settings for Operating Pressure
Density
Relationship
Mach Number
Regime
Operating
Pressure
ideal gas law ${\rm M} > 0.1$ 0 or
$\approx$ mean flow pressure
ideal gas law ${\rm M} < 0.1$ $\approx$ mean flow pressure
profile function
of temperature
incompressible not used
constant incompressible not used
incompressible
ideal gas law
incompressible $\approx$ mean flow pressure

You will set the Operating Pressure in the Operating Conditions panel.

Define $\rightarrow$ Operating Conditions...


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