Large-scale motions and self-similar structures in compressible turbulent channel flows

TL;DR

This study uses direct numerical simulations to analyze the scale and structure of motions in compressible turbulent channel flows, revealing that outer-region energy distribution is mainly influenced by Reynolds number effects, and confirming the presence of self-similar structures.

Contribution

It provides new insights into the scale characteristics and self-similar structures in compressible turbulence, with detailed analysis of their geometrical properties.

Findings

Outer-region energy distribution dominated by Reynolds number effects.

Outer motion length scales are unaffected by Mach number crossing the sound barrier.

Self-similar structures have specific aspect ratios: 15.5 for velocity/temperature, 1.8 for density/pressure.

Abstract

In this work, we study the scale characteristics of the log- and outer-region motions and structures in subsonic and supersonic turbulence. To this end, a series of direct numerical simulations of the compressible turbulent channel flow at medium Reynolds numbers are performed. Based on this database, the streamwise and spanwise length scales of the outer-region motions are investigated by the two-point correlations and the one-dimensional spectra. The energy distribution among the multi-scale structures in the outer region is found to be dominated by the semilocal friction-Reynolds-number effects rather than the Mach-number effects. This conclusion not only holds for the velocity fluctuations but also the fluctuations of the thermodynamic variables. Besides, the streamwise and spanwise length scales of the outer motions do not alter significantly when the flow passes the sound barrier as reported by a previous experimental study (Bross et al., J. Fluid Mech., vol. 911, 2021, A2). On the other hand, the self-similar structures populating the logarithmic region are investigated by adopting a linear coherence spectrum. The streamwise/wall-normal aspect ratio of the self-similar wall-attached structures of the streamwise velocity and temperature fluctuations is approximately 15.5, and the counterpart of density and pressure fluctuations is 1.8. The present study confirms the existence of self-similar structures in compressible wall turbulence and assesses their geometrical characteristics.