High Rayleigh number convection with double diffusive fingers

Abstract

An electrodeposition cell is used to sustain a destabilizing concentration difference of copper ions in aqueous solution between the top and bottom boundaries of the cell.The resulting convecting motion is analogous to Rayleigh-Bénard convection at high Prandtl numbers. In addition, a stabilizing temperature gradient is imposed across the cell. Even for thermal buoyancy two orders of magnitude smaller than chemical buoyancy, the presence of the weak stabilizing gradient has a profound effect on the convection pattern. Double diffusive fingers appear in all cases. The size of these fingers and the flow velocities are independent of the height of the cell, but they depend on the ion concentration difference between top and bottom boundaries as well as on the imposed temperature gradient. The scaling of the mass transport is compatible with previous results on double diffusive convection.

Thermohaline convection at density ratios below one: A new regime for salt fingers

“…Thus, while theory certainly allows narrow convective cells, we lack a complete explanation of the new observations of Hage and Tilgner (2010). It seems likely that an additional factor is at play in these experiments.…”

“…Stern himself continued to make significant contributions to double-diffusive theory throughout his career (Stern et al, 2001). However, our focus here will be on a previously unknown parameter regime for salt fingers revealed by the recent experiments of Hage and Tilgner (2010). We place the discussion in the context of one of Stern's most useful contributions to salt finger theory.…”

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ABSTRACTNew experimental results on haline convection show a surprising preference for narrow fingers over large-scale convection when even a small stabilizing temperature gradient is present (Hage and Tilgner, 2010). This regime has heat/salt density ratios below one, a parameter range that has not been explored in traditional salt finger theory.

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“…Here the salt gradient is strongly destabilizing and the temperature gradient is only weakly stabilizing. The work is motivated by surprising recent experimental results on haline convection by Hage and Tilgner (2010). Pure haline-driven convection is not easily explored in the laboratory; osmotic membranes would seem to be necessary.…”

“…Figures 1 and 2 are from Hage and Tilgner (2010). In Figure 1 the red vectors represent velocities measured by particle imaging velocimetry (PIV); the light/dark pattern represents the intensity of the up-(light) or down-going (dark) fluid velocities.…”