Buoyant mixing of unstably stratified fluids in a vertical square tube

Quantitative time dependent measurements of irreversible mixing caused by the development of Rayleigh-Taylor instability on an initially unstable two-layer stratification of miscible fluids are taken from a series of laboratory experiments. The experiments are conducted by overturning a stable stratification in a high-aspect ratio tank with square cross-section, and are observed until the flow is quiescent.

Using a light attenuation technique we obtain detailed time-dependent measures of the irreversible mixing, and its efficiency during flow evolution. We find that the flow passes through a number of distinct phases, beginning with classical t2 grow of the instability before a rapid (but gradually slowing) growth of the instability along the entire length of the tube. The still statically unstable mean stratification continues to drive turbulent mixing along the tube, ultimately leading to a weakly stably stratified final state with a gradual exponential decay of the remaining disturbances.