Laboratory models of double-diffusive layers and intrusions

Abstract

The experiments reported in this work were aimed at understanding aspects of the formation and evolution of double-diffusive layering and intrusions. The role of turbulent entrainment in the evolution of diffusive layering was examined theoretically. It was found that entrainment destabilised a system of diffusive layers, driving a layer's properties towards a state where it overturned with one of its immediate neighbours. However, despite its important role the efficiency of entrainment was considerably lower than would be expected from a direct extrapolation of the results of single-component penetrative convection experiments to a diffusive interface. Experiments on long salt fingers were performed with the aim of understanding the conditions under which the salt fingers break down to form convecting layers. The fingers were set up by emplacing a layer of sugar solution above a salt gradient. Detailed measurements of the density profile within the fingers showed that the fluxes were highly non-uniform in the vertical, both in experiments which evolved without overturning and those in which convecting layers were formed. The maximum value of Stern's number A=Fp/vpz calculated from these profiles was consistent with critical values predicted by collective instability theory. This agreement is good evidence that collective instability governs the formation of layers from long salt fingers. The effects of double-diffusion on intrusions into a linearlystratified environment and on gravity currents were studied. In both cases a simple argument based on the buoyancy conservation equation made it possible to predict when double-diffusion should become important to the flow. Experimentally, the initial effect observed was the production of secondary flows in the environment. In gravity current experiments a secondary current was formed on the lower boundary of the tank, while in the internal intrusion experiments, a secondary outflow was formed directly below the primary intrusion. In the latter experiments, as the relative strength of double-diffusion increased, the inflow broke down into multiple intruding noses. The criterion derived could be used to assess the importance of double-diffusion in a thermohaline intrusion in the ocean. Show more