Genuine compressibility effects in wall-bounded turbulence

Compressible wall-bounded turbulence is generally assumed to be devoid of genuine compressibility effects, meaning that the effect of finite fluid dilatation is regarded as "small," at least in the absence of disturbing pressure gradients. In the present paper we attempt to answer the basic question of how small these effects are, by interrogating a DNS database of compressible channel flow and by using Helmholtz decomposition to infer the relative magnitude and correlations between the solenoidal and the dilatational parts of turbulence velocity fields. Not surprisingly, we find dilatational velocity fluctuations to be much smaller than solenoidal ones, but perhaps unexpectedly, we find that finite correlation between the two components accounts for a nonnegligible fraction (about 10%) of the turbulent shear stress near walls, and for up to 4% of the wall skin friction. Quadrant analysis of the dilatational velocity fluctuations shows that the largest contribution to the turbulent shear stress results from significant correlation between positive streamwise solenoidal velocity fluctuations (i.e., high-speed streaks), and positive vertical dilatational velocity fluctuations, which tend to mitigate the intensity of wall-ward sweep events.