Experiments on mixing in a density driven current

Laboratory experiments, investigating mixing in a density driven current flowing down a sloping bottom in a rotating homogenous fluid, were performed. All the experiments were run inside a tank positioned on a rotating table. Four parameters, i.e. the rotation rate, the bottom slope, the flowrate, and the density of the dense fluid, were varied, spanning a wide range of Froude and Reynolds numbers. Different flow types were observed, while changing the above parameters, both in different experiments and simultaneously in the same experiment, as the current descended the slope. The mixing, due to the density driven current, was evaluated and the dependence of mixing on slope, Froude and Reynolds number was investigated. In particular, mixing increased with increasing Froude numbers. For low Froude numbers the magnitude of the mixing was comparable to mixing in the ocean. For large Froude and Reynolds numbers, mixing was comparable, or slightly lower, than in previous laboratory experiments that presented the classic turbulent entrainment behaviour. We suggest an empirical parameterization for entrainment in a dense current that takes into account subcritical mixing, which could be of fundamental importance when determining the final water mass characteristics of a dense overflow current descending the continental slope.