Regeneration mechanisms of near-wall turbulence structures

TLDR

Direct numerical simulations of a highly constrained plane Couette flow with a minimal domain size—matching the typical spanwise spacing of near‑wall streaks—were used to investigate the dynamics of near‑wall turbulent structures. The study revealed a well‑defined, quasi‑cyclic regeneration cycle of near‑wall structures comprising streak formation by streamwise vortices, streak breakdown, and vortex regeneration, with vortex regeneration directly driven by streak breakdown and the cycle breaking when the domain width is reduced below the natural streak spacing.

Abstract

Direct numerical simulations of a highly constrained plane Couette flow are employed to study the dynamics of the structures found in the near-wall region of turbulent flows. Starting from a fully developed turbulent flow, the dimensions of the computational domain are reduced to near the minimum values which will sustain turbulence. A remarkably well-defined, quasi-cyclic and spatially organized process of regeneration of near-wall structures is observed. This process is composed of three distinct phases: formation of streaks by streamwise vortices, breakdown of the streaks, and regeneration of the streamwise vortices. Each phase sets the stage for the next, and these processes are analysed in detail. The most novel results concern vortex regeneration, which is found to be a direct result of the breakdown of streaks that were originally formed by the vortices, and particular emphasis is placed on this process. The spanwise width of the computational domain corresponds closely to the typically observed spanwise spacing of near-wall streaks. When the width of the domain is further reduced, turbulence is no longer sustained. It is suggested that the observed spacing arises because the time scales of streak formation, breakdown and vortex regeneration become mismatched when the streak spacing is too small, and the regeneration cycle at that scale is broken.