Compressible Turbulence and Shock Waves

Abstract

If the velocity scale associated with the turbulent fluctuations is a substantial fraction of the mean speed of sound, compressibility effects are expected to arise. This direct effect of compressibility, which is also the most well documented, appears to be responsible for the reduced spreading rate of turbulent mixing layers under compressible conditions, and the slower decay of hypersonic wakes. A different circumstance where the compressibility effects can become significant occurs when an otherwise quasi-incompressible turbulent flow is subjected to a rapidly changing environment. For example a turbulent flow passing through a shock wave or a steep expansion wave may suddenly enter in a state of disequilibrium. The sudden change may also generate significant fluctuations of acoustic and entropy modes. Thus the recovery of the turbulence from this disequilibrium may also exhibit compressibility effects. Other phenomena where the global behavior of the flow is strongly affected by compressibility include shock induced separations of a turbulent flow, phenomena involving the Ranque-Hilsh effect and other effects requiring chemical or thermodynamic nonequilibrium. The problem of shock wave boundary layer interaction in a hypersonic free-stream involves all of the above processes.