Turbulent Mass Exchange in a Stratified Fluid and the Conditions of Its Fine Structure Layering

Abstract

The basic aim of the investigation is to study the regularities of fine structure layering of the stratified fluid (i.e., the oceanic pycnocline) under vertically homogeneous turbulent stirring. By means of physical (laboratory) modeling turbulent exchange through the density (salinity) interface, as well as the structure of this interface between two quasi-homogeneous layers of different salinity is investigated. The layers are continuously stirred by a system of horizontally oscillating vertical rods, passing through both layers. The rods produce vertically homogeneous turbulent stirring of a two-layered fluid media. During every experimental run the salinity in the upper layer is measured with the four electrode conductometer. In a part of experiments vertical profiles of salinity are periodically measured using a single-electrode conductivity microsensor. These measurements are used to calculate the turbulent flux of salt (mass) between layers and to relate these measurements with the structure of the density interface, as well as to determine the dependences of the mass flux and thickness of the interface on the defining dimensionless parameter—the Richardson number. It is shown that the interface with a decrease in the Richardson number first sharpens and then expands (diffuses). In this case, the mass flux between the layers reaches a maximum at a sharpened interface between the layers just before the interface begin to sharpen.