Quantitative analysis of double-diffusive instability: Growth and mixing transition of ascending fingers

Double-diffusive instabilities (DDI) significantly impact oceanic thermohaline circulation, influencing climate patterns and nutrient distributions. This study employs simultaneous PLIF/PIV measurement techniques to obtain combined, time-evolving density and velocity fields with the aim of exploring several key aspects of DDI. The growth rate of ascending freshwater fingers, the formation of mushroom-shaped structures, and the underlying mixing mechanisms are of specific attention. The experimental setup involves slowly oozing cold fresh water through a perforated tube at the bottom of a tank filled with hot salty water, which facilitates precise control of the density ratio. Flow visualization has revealed the intriguing process of ascending freshwater fingers, their transition into mushroom-shaped structures, and subsequent mixing. These qualitative observations set the stage for detailed quantitative analysis. The primary objective is to determine whether local transitions to turbulence occur at the ends of ascending finger structures. The findings of this study are expected to fill gaps in the existing literature by providing well-designed, quantitative experimental data to study DDI and the associated mixing mechanisms.