Passive and reactive scalar measurements in a transient high-Schmidt-number Rayleigh–Taylor mixing layer

An experimental study of mixing induced by Rayleigh–Taylor (RT) instability at an Atwood number (A _t ) ~7.5 × 10⁻⁴ and Schmidt number (Sc) ~1,000 has been performed. A new transient experimental facility developed on the working principles of the draw-tank facility at Cambridge (Dalziel et al. in J Fluid Mech, 399:1–48, 1999) has been established and enhanced to observe a higher (2×) Reynolds number regime. Water and brine were used to produce the RT density stratification. The evolution of the instability was studied using passive and reactive scalar techniques and quantified using optical diagnostic methods. The data were combined to estimate local and global mixing metrics representative of the mixing mechanism across the mixing layer. In comparison with parameters reported from analogous experiments, the mixing phenomenon at a high Sc shows a strong dependency on the initial conditions prevailing at the onset of the instability and the evidence of a delay in the mixing transition. Values of global and integral mixing parameters did not reach late-time asymptotic values that have been reported previously from steady-state experiments (Texas A&M Water Channel) and may be attributed to the effect of the barrier pull and the overturning mechanism that is thought to hinder the progress of the mixing layer.