Author(s): Adrien Lefauve; P. F. Linden
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Keywords: No Keywords
Abstract: Exchange flows - Buoyancy-driven exchange flows naturally arise where relatively large bodies of fluid have different densities on either side of a relatively narrow constriction. In a gravitational field, this difference in buoyancy results in a horizontal hydrostatic pressure gradient along the constriction, of opposite sign above and below a ‘neutral level’. This pressure gradient drives a counter-flow through the constriction, in which fluid from the negatively buoyant reservoir flows below the neutral level towards the positively buoyant reservoir, and vice versa. Such buoyancy-driven exchange flows result in little to no net volume transport, but crucially, in a net buoyancy transport between the reservoirs which tends to homogenise buoyancy differences in the system. In addition, irreversible mixing often occurs across the interface between the two counterflowing layers of fluid, creating an intermediate layer of partially mixed fluid. Applications - The net transport and mixing of the active scalar field (e. g. heat, salt or other solutes) and of other potential passive scalar fields having different concentrations in either reservoirs (e. g. pollutants or nutrients) have a wide range of consequences, recognised since the Antiquity. Aristotle offered the first recorded explanation of the movement of salty water within the Mediterranean Sea. Since then, exchange flows through the straits of Gibraltar and the Bosphorus have driven much speculation and research, due to their crucial roles in the water and salt balances of the Mediterranean Sea. More recently, it has been recognised that nutrient transport from the Atlantic partially supported primary production in Mediterranean ecosystems. The quantification, modelling and discussion of the past and current impact of exchange flows in straits, estuaries or between lakes continues to generate a vast literature. Broader importance - More fundamentally, exchange flows are stably stratified shear flows, a canonical class of flows widely used in the mathematical study of stratified turbulence, dating back at least to O. Reynolds and G. I. Taylor. Multi-layered stratified shear flows have complex and still puzzling hydrodynamic stability and turbulent mixing properties. The straightforward and steady forcing of exchange flows make them ideal laboratory stratified shear flows because of the ability to sustain, over long time periods, high levels of turbulent intensity and mixing representative of large-scale natural flows.
Year: 2022