Flow structures and vertical transport in tilting salt finger with a background shear

Abstract: In this work we study the fingering double diffusive convection, namely the buoyancy-driven convection flow within a fluid layer experiencing an unstable salinity gradient and a stable thermal gradient. Especially, we investigate the influences from a background shear with uniform strength. Linear stability analysis indicates that the unstable modes shift from a circular shape to a sheet-like shape as the shear becomes stronger. Three dimensional direction numerical simulations are conducted for five groups of cases, each of which has the same combination of thermal and salinity gradients (measured by corresponding Rayleigh numbers), and gradually increasing shear strength. Simulation results reveal that a very weak shear organizes the salt fingers into a very regular pattern, which can enhance the salinity flux. This enhancement effect, however, reduces as the Rayleigh number increases. As the shear further strengthens, the dominant structures change from salt fingers to salt sheets, and the coherence length scale increases in the streamwise direction. Meanwhile, salinity and heat fluxes decrease. These results suggest that even a weak shear can notably alter the morphology and transport properties of fingering double diffusive convection.