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9.6.1 When to Use the Compressible Flow Model

Compressible flows can be characterized by the value of the Mach number:

 {\rm M} \equiv u/c (9.6-1)

Here, $c$ is the speed of sound in the gas:

 c = \sqrt{\gamma RT} (9.6-2)

and $\gamma$ is the ratio of specific heats $(c_p/c_{\rm v})$.

When the Mach number is less than 1.0, the flow is termed subsonic. At Mach numbers much less than 1.0 ( ${\rm M}< 0.1$ or so), compressibility effects are negligible and the variation of the gas density with pressure can safely be ignored in your flow modeling. As the Mach number approaches 1.0 (which is referred to as the transonic flow regime), compressibility effects become important. When the Mach number exceeds 1.0, the flow is termed supersonic, and may contain shocks and expansion fans which can impact the flow pattern significantly. FLUENT provides a wide range of compressible flow modeling capabilities for subsonic, transonic, and supersonic flows.

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