Salinity effects on double-diffusive instability evolution: Finger growth, mixing, and transport

Salt-driven double-diffusive instability (DDI) is recognized as a canonical pathway through which opposing temperature and salinity gradients cause unstable fingers to transition into fully mixed layers in both natural and engineered environments. However, quantitative experimental data and characterization of the mixing transition within the finger-growth regime remain limited. In this study, laboratory experiments were conducted in a 20 cm tall, closed acrylic tank. A fluorinated ethylene propylene (FEP) tube at the tank's base introduced fluid with thermal and salinity differences (ΔT = 5 K; ΔS = 350, 450, or 550 ppm), creating an unstable configuration of cold, fresh water beneath warm, salty water. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) simultaneously captured time-resolved velocity and concentration fields, respectively, of the ascending fingers. Finger-tip heights were ensemble-averaged across more than 30 fingers for each salinity treatment, and the finger-tip height was found to collapse onto a single master non-dimensionalized curve. Agreement between linear stability theory and experimental data was observed within 2%. The mixed-material area and vertical salt flux were computed and analyzed. After time non-dimensionalization, a slight decrease in mixing efficiency per unit travel distance was observed with increasing salinity contrast, which is attributed to the prolonged preservation of sharp scalar gradients. Vertical scalar flux does not collapse onto a master non-dimensional curve because the stronger buoyancy contrast simultaneously amplifies finger velocities and the concentration of the fluid, and the flux scales as the product of these two factors.