EVOLUTION OF KELVIN-HELMHOLTZ BILLOWS IN NATURE AND LABORATORY

Abstract

A mixing mechanism prevalent in natural flows is the formation and breakdown of vortical billows known as Kelvin-Helmholtz (K-H) instabilities. Here we present field examples of K-H billow occurrences in the atmosphere and oceans. Laboratory experiments aimed at studying certain key features of K-H billows are also discussed, wherein the billows were generated in a two-layer stratified tilt-tank. It is shown that small-scale turbulent mixing is present within billows from the early stages of their evolution, but mixing becomes intense and the billows are destroyed as they achieve a maximum height and initiate collapse at a non-dimensional time of ΔUt/λ #, where ΔU is the velocity shear and λ is the wavelength. When Υ Ut/λ < 5, the Thorpe scale LT and the maximum Thorpe displacement (LT)max, normalized by the local billow height Lb, are independent of both the horizontal location within the billow and time withLT /Lb # (0.49 +/- 0.03) and (LT)max/Lb # (0.89 +/- 0.02). After the collapse starts, however, the pertinent lengthscale ratios in the 'core' of the billow show values similar to those of fully developed turbulent patches, i.e., LT/Lb # (0.29 +/- 0.04) and (LT)max/Lb # (0.68 +/- 0.04). The field observations were found to be in good agreement with laboratory-based predictions.