Vertical transports produced by double-diffusive plumes in a confined homogeneous environment

Abstract

Double-diffusive plumes, injected into surroundings of nearly the same density, are observed to separate near the source and to produce both upward and downward convection, which is enhanced as layers form at the top and bottom of the experimental tank. This process has been quantified in this paper for both sugar plumes in homogeneous salt solution and salt plumes in sugar solution, over a range of small density differences on either side of zero. The tank was fitted with a partition at mid-depth, which could be closed at the end of a known period of input, so that the concentration changes above and below the source could be deduced. The density of the input and tank fluids, and that of the upper and lower layers, was measured in each case, as well as a second property, conductivity for the salt and optical rotation for the sugar inputs. These measurements were motivated by, and shed light on, the experimental results of Turner & Veronis (2000). There are large differences between the upward and downward transports produced by the two types of plume, because of the different rates of diffusion of salt and sugar in and out of the plumes and the subsequent transports through the interfaces that form as the plumes spread out along the top and bottom boundaries. One result is that at a density ratio close to 1 a salt plume produces equal upward and downward transports of salt, whereas at the same density ratio a sugar plume leads to a much larger downward flux of sugar. This is consistent with the ‘diffusive’ final state observed by Turner & Veronis (2000), and this conclusion is supported by a more detailed analysis of the redistribution of the tank fluid.