On the Kelvin–Helmholtz route to turbulence

Abstract

AbstractIn their transition from a laminar state to turbulence, some unstable flows pass through a set of well-defined stages involving different and distinct processes. This is so, in particular, for Kelvin–Helmholtz instability, although details of its transition still retain many mysterious aspects. Billows develop in the primary stage of this stratified shear flow instability, separated by thin braids in which the shear is relatively high. Fluid is statically unstable within the billows and consequently potentially prone to convective instability. Numerical studies by Mashayek & Peltier (J. Fluid Mech., this issue, vol 708, 2012a,b, pp. 5–44 and 45–70) have discovered several new types of secondary instability in the braids and billow cores that may hasten the eventual transition to turbulence. The instabilities are illustrated by the authors in colour figures, remarkable for their beauty and (recalling William Blake’s ‘The Tyger’) ‘fearful symmetry’. But are they helpful in establishing the subsequent turbulence in the natural environment?