Buoyancy transfer across a diffusive interface

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Abstract
An experimental investigation of various aspects of buoyancy transfer across a diffusive density interface that separates stably stratified, turbulently convecting layers of relatively fresh cold water overlying hot salty water is described. It is argued that the interfacial layer should possess a double boundary-layer structure, in which the thicknesses of the salt and heat interfacial layers are determined by a balance between the opposing effects of diffusion and entrainment. Based on this argument, a simple theory, that predicts the interfacial-layer thickness, the diffusive heat and salt fluxes across the density interface, and the time variation of the temperature and salt concentrations in the convecting layers, is proposed for the case in which the convection is driven by a constant heat flux supplied to the lower layer. During a certain time interval, the theory and experiment agree well, but thereafter distinct differences can be seen. Measurements suggest that these differences may be due to the distortion of the density interface at low interfacial stabilities by turbulent eddies, which leads to a change in the buoyancy transfer mechanism. When the Richardson number falls below a critical value Riv, the interface was found to migrate slowly upwards and the mechanism of entrainment was the detachment of thin sheets of fluid by eddies scouring the interface.