Experiments on the evolution of gravitational instability of an overturned, initially stably stratified fluid

Unstable density stratifications were created in the laboratory by rapid overturning of a narrow tank containing an initially stable density structure. The experiments were carried out for two different initial density distributions: (i) a two-layer (steplike) and (ii) a linear stratification. For the former case the depth of the mixing layer was found to increase linearly with time. The number of convective elements (thermal-like flow structures) present at the front of the mixing layer was observed to decrease with time, through the mechanism of subsequent pairing. In the case of an initially linear stratification the flow evolution is characterized by a number of distinct stages: different modes of instability emerge subsequently through the entire fluid column, leading to the formation of horizontal layers, which finally break up into thermal-like convective flow structures.