Experimental investigation of the multi-layer Rayleigh-Taylor instability *

  Statistically stationary experiments are performed to study the effect of complex stratification pattern on the Rayleigh-Taylor instability. Experiments are conducted in a newly built, blow-down 3-layer gas tunnel facility. Mixing between three gas streams are studied, where the top and bottom streams comprise of air, and the middle stream comprises of air-helium mixture, giving Atwood numbers of the order of 0.1 at the unstable interface between the top and middle streams. There is no shear between these streams. Three experimental cases are investigated, with one middle stream thickness (6 cm) and three Atwood numbers (0.05, 0.1 and 0.25). The growth of the middle layer is measured using back-lit visualization and Mie-scattering techniques. The dynamics of the flow is investigated using particle image velocimetry (PIV) for velocity measurements and laser induced fluorescence (LIF) for density measurements. In addition to extracting quantities of statistical importance from our measurements (like density-velocity correlations), we also look at how different forces fundamentally drive variable density mixing process (using nondimensional parameters like Richardson number), and how such a mixing process reaches an asymptotic state (using the concept of mixing efficiency and molecular mixing parameter). These experiments are of immense significance for atmospheric and oceanic sciences, and variable density turbulence modeling.