The formation and destruction of fine-structure by double-diffusive processes

The formation of layered structure by the imposition of an unstable buoyancy flux on a region containing opposing, uniform gradients of two components is examined. Turner (Journal of Fluid Mechanics, 33, 183–200, 1968) investigated the formation of layers when a stable salinity gradient is heated from below. The present work is an extension of his, allowing for the presence of a destabilizing temperature gradient in the interior of the fluid. Experiments were carried out using two solutes (sugar and salt) as the components contributing to the density field. It is found theoretically, and confirmed by experiment, that the scale of the layers depends on the ratio of the two density gradients in the interior Gϱ. The layers were observed to form sequentially with increasing distance from the boundary across which the buoyancy flux was applied. The relative contributions of the energy provided by the boundary flux and that stored in the destabilizing component during the formation of the first layer are found to depend on Gϱ but not on the magnitude of the boundary flux. When Gϱ=0, and there is no destabilizing density gradient, all the energy comes from the imposed flux. As Gϱ→1, the energy from the destabilizing component becomes more important, providing all the energy in the limit. Some observations of destruction of fine-structure are reported. Two kinds were observed. One type was characterized by the vertical migration of an interface to an adjacent one thereby destroying the layer in between. The other kind was identified by a breakdown of an interface in situ, apparently resulting from an equalization of the density of the two layers on either side. Finally some brief comparisons with oceanic fine-structure are made.