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8.3.6 Ideal Gas Law for Compressible Flows

For compressible flows, the gas law is as following:

 \rho = \frac{p_{\rm op} + p}{\frac{R}{M_w} T} (8.3-6)


$p$ = the local relative (or gauge) pressure predicted by FLUENT
$p_{\rm op}$ = the Operating Pressure

Density Inputs for the Ideal Gas Law for Compressible Flows

The inputs for the ideal gas law are as follows:

1.   Enable the ideal gas law for a compressible fluid by choosing ideal-gas from the drop-down list to the right of Density in the Materials panel.

Specify the ideal gas law individually for each material that you want to use it for. See Section  8.3.7 for information on specifying the ideal gas law for mixtures.

2.   Set the operating pressure by defining the Operating Pressure in the Operating Conditions panel.

Define $\rightarrow$ Operating Conditions...


The input of the operating pressure is of great importance when you are computing density with the ideal gas law. Equation  8.3-6 notes that the operating pressure is added to the relative pressure field computed by the solver, yielding the absolute static pressure. See Section  8.14 for recommendations on setting appropriate values for the operating pressure. By default, Operating pressure is set to 101325 Pa.

3.   Set the molecular weight of the homogeneous or single-component fluid (if no chemical species transport equations are to be solved), or the molecular weights of each fluid material (species) in a multicomponent mixture. For each fluid material, enter the value of the Molecular Weight in the Materials panel.

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