EVOLUTION OF KELVIN-HELMHOLTZ BILLOWS IN NATURE AND LABORATORY

A mixing mechanism prevalent in natural flows is the formation and bre akdown of vortical billows known as Kelvin-Helmholtz (K-H) instabiliti es. Here we present field examples of K-H billow occurrences in the at mosphere and oceans, Laboratory experiments aimed at studying certain key features of K-H billows are also discussed, wherein the billows we re generated in a two-layer stratified tilt-tank, It is shown that sma ll-scale turbulent mixing is present within billows from the early sta ges 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 Delta Ur/lambda approximate to 5, where Delta U is the velocity shear and lambda is the wavelength, When Ut/lambda < 5, the Thorpe scale L(T) and Ihe maximum Thorpe displacement (L(T))( max), normalized by the local billow height L(b), are independent of b oth the horizontal location within the billow and time with L(T)/L(b) approximate to (0.49 +/- 0.03) and (L(T))(max)/L(b) approximate to (0. 89 +/- 0.02). After the collapse starts, however, the pertinent length scale ratios in the 'core' of the billow show values similar to those of fully developed turbulent patches, i.e., L(T)/L(b) approximate to ( 0.29 +/- 0.04) and (L(T))(max)/L(b) approximate to (0.68 +/- 0.04). Th e field observations were found to be in good agreement with laborator y-based predictions.