Intrinsic compressibility effects in near-wall turbulence

TL;DR

This paper investigates how intrinsic compressibility effects influence near-wall turbulence, revealing their role in velocity profile shifts, turbulence intensity increases, and vortex weakening through direct numerical simulations.

Contribution

It provides a clear quantification of intrinsic compressibility effects on turbulence, distinguishing them from mean property variations, and offers a theoretical explanation for observed phenomena.

Findings

Intrinsic compressibility causes an upward shift in the velocity profile.

Compressibility reduces turbulent shear stress and weakens near-wall vortices.

Compressibility effects increase streamwise turbulence intensity.

Abstract

The impact of intrinsic compressibility effects -- changes in fluid volume due to pressure variations -- on high-speed wall-bounded turbulence has often been overlooked or incorrectly attributed to mean property variations. To unambiguously quantify these intrinsic compressibility effects, we perform direct numerical simulations of compressible turbulent channel flows with nearly uniform mean properties. Our simulations reveal that intrinsic compressibility effects yield a significant upward shift in the logarithmic mean velocity profile that can be attributed to the reduction in the turbulent shear stress. This reduction stems from the weakening of the near-wall quasi-streamwise vortices. We in turn attribute this weakening to the spontaneous opposition of sweeps and ejections from the near-wall expansions and contractions of the fluid, and provide a theoretical explanation for this mechanism. Our results also demonstrate that intrinsic compressibility effects are responsible for the increase in the inner-scaled streamwise turbulence intensity in compressible flows compared to incompressible flows, previously regarded to be an effect of mean property variations.