Article,
Evolution of Kelvin-Helmholtz billows in nature and laboratory
Earth and Planetary Science Letters, 143 (1-4): 217--231 (1996)
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 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 both 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
lengthscale 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). The field observations were found to be in good agreement
with laboratory-based predictions.
