The numerical decomposition of turbulent fluctuations in a compressible boundary layer

In many flows the turbulence is weakly compressible even at large Mach numb er. For example, in a compressible boundary layer with Ma < 5, the differen ces relative to an incompressible boundary layer are understood as being ca used by density variations that accompany variations in temperature across the layer. Turbulent fluctuations in a compressible boundary layer are ther efore expected to be dominated by the effects of nonconstant temperature, a nd low Mach number theories in which acoustic fluctuations are not dominant should be applicable to the fluctuating field. However, the analysis of co mpressible boundary layer DNS data reveals the presence of significant acou stic fluctuations. To distinguish between acoustic and thermal effects, a n umerical decomposition procedure for compressible boundary layer fluctuatio ns is applied to determine the acoustic and nonacoustic fluctuations, Excep t for very near the wall, where the decomposition procedure is not valid, i t is found that the nonacoustic fluctuations are only weakly coupled to the acoustic fluctuations at Mach numbers as high as 6.