Overview of the self-sustaining mechanisms of wall turbulence

This paper reviews the ideas about why wall turbulence is self-sustaining. The paper is to distribute this information to fluid dynamists who are nonspecialists in turbulence. The production of turbulence and Reynolds stress centers on vortices and “streaks” of low speed fluid near the wall. There are two main categories of self-sustaining mechanisms. In one category, parent vortices interact with the wall and produce offspring vortices. In the second category, workers view the mechanisms as instabilities. Streak velocity profiles, profiles where low velocity fluid has accumulated in long streamwise regions of small spanwise extent, are unstable. These regions are caused by streamwise vortices in the near-wall region. The most dangerous perturbation is a sinuous streamwise mode. The mode may be a normal mode or a transient-growth mode. Ultimately the nonlinear development produces a streamwise vortex. In turn, the vortex can reproduce the streak profile. A third category is based on a common mathematical approach. The goal is to construct a low-order dynamical system of differential equations that display the elemental processes of turbulence.