On the mixing models for stratified flows subjected to concomitant stable and unstable stratifications

Abstract

ABSTRACTThe concurrence of stable and unstable stratification in stratified flows leads to dramatically different features of turbulent mixing. This unique flow is experimentally studied by introducing a horizontal jet of dense fluid into a pool of light fluid. The buoyancy flux from simultaneous velocity–density measurements is an indicator for competition between a stabilising mechanism and another destabilising mechanism. The difference of mixing efficiency, quantified by flux Richardson number Rif, between the (un)stable stratification is mainly contributed by the large-scale mixing. The behaviour of Rif can be modelled by the gradient Richardson number Rig linearly in the low-Ri case and nonlinearly in the high-Ri case (especially in a region where the counter-gradient flux emerges). The turbulent diapycnal diffusivity, quantifying the combined effect of reversible and irreversible mixing processes, increases as the buoyancy Reynolds number Reb increases only when Reb is large. The irreversible mixing diffusivity, which quantifies the sole irreversible mixing process, increases linearly as the turbulent Péclet number with the data points from the (un)stable stratification overlapped. The turbulent Prandtl number approaches 0.75 as Rig approaches zero, but does not show clear dependence on Rig in the examined regime.