For high-Mach-number flows, compressibility affects turbulence through so-called
"dilatation dissipation", which is normally neglected in the modeling of
incompressible flows [708]. Neglecting the dilatation dissipation fails
to predict the observed decrease in spreading rate with increasing Mach number for compressible
mixing and other free shear layers. To account for these effects in the 
-
models in Ansys Fluent, the dilatation dissipation term, 
, can be included in the 
equation. This term is modeled according to a proposal by Sarkar [566]:
 | (4–68) |
where 
is the turbulent Mach number, defined as
 | (4–69) |
where 
(
) is the speed of sound.
Note: The Sarkar model has been tested for a very limited number of free shear test cases, and should be used with caution (and only when truly necessary), as it can negatively affect the wall boundary layer even at transonic and supersonic Mach numbers. It is disabled by default. For details, see Model Enhancements in the Fluent User's Guide.