A Dense Current Flowing down a Sloping Bottom in a Rotating Fluid

A density-driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. The behavior of the dense fluid descending the slope has been investigated by systematically varying four parameters: the rotation rate, the bottom slope, the flow rate of the dense fluid, and the density of the dense fluid. Over a wide range of parameter values, the following three flow types were found: a laminar regime in which the dense current had a constant thickness behind the head, a wave regime in which wavelike disturbances appeared on the interface between the dense and fresh fluids, and an eddy regime in which periodic formation of cyclonic eddies in the fresh overlying ambient fluid was observed. All of the experiments revealed that increasing the slope angle and the density of the bottom fluid allowed the flow to evolve from the laminar to the wave regime. Furthermore, increasing rotation rate induced the formation of eddies. A theoretical solution for the downslope velocity field has been found using a steady-state model. Comparison between the theoretical and experimental downslope velocities gave good agreement. The wave regime was observed to occur for values of the Froude number greater than 1. The laminar regime was found for values of the Froude number less than 1. The amount of mixing between the dense and the ambient fluids was measured. Mixing increased significantly when passing from the laminar to the wave regime, that is, with increasing Froude number. Good agreement between the amount of mixing observed in the ocean and in the laboratory experiments is encouraging and makes the waves observed in the present experiments a possible candidate for the mixing observed during oceanic dense current overflows.