Self-sustained double-diffusive interleaving

Abstract

The formation and evolution of double-diffusive interleaving is experimentally investigated with the purpose of analysing the influence of the convective flow structures, at different scales, on the mean flow. Recently, Krishnamurti (J. Fluid Mech., vol. 558, 2006, p. 113) has shown that, in the case of a continuous stratification experiment, the Reynolds stresses, due to convective flow patches, are able to vertically transport horizontal momentum, maintaining the mean flow. This mechanism is similar to the turbulent wind observed in thermal convection. In this study, the interleaving is produced using the classical set-up of Ruddick & Turner (Deep-Sea Res., vol. 558, 1979, p. 903). The dam-break experiments better resemble the case of oceanic fronts, where interleaving is commonly observed. The flow structures are investigated by measuring the two-dimensional flow fields using the particle image velocimetry technique. The resulting two-dimensional vector fields reveal complex fine-scale flow structures, and convective patterns are observed inside the finger-favourable layers. Vortical structures at scales comparable with the layer thickness are embedded in these regions and seem to be responsible for sustaining the horizontal mean flow against the viscous dissipations, especially in a region close to the layer nose. A spectral analysis of the flow fields suggest that the energy balance is governed by an inverse energy cascade, which implies a transfer of energy from the smaller scales to the larger scales (mean flow).