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

(1–26)

Here, is the speed of sound in the gas:

(1–27)

and is the ratio of specific heats .

Equation 1–27 applies for ideal gas. In the general case of real fluids, the speed of sound is defined in terms of the isentropic compressibility as:

(1–28)

where is the fluid density, is the pressure, and the subscript denotes that the partial derivative of density with respect to pressure is taken at constant entropy.

When the Mach number is less than 1.0, the flow is termed subsonic. At Mach numbers much less than 1.0 ( 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 that can impact the flow pattern significantly. Ansys Fluent provides a wide range of compressible flow modeling capabilities for subsonic, transonic, and supersonic flows.