Dynamics of multilayer Rayleigh-Taylor mixing at moderately high Atwood numbers: an experimental study using simultaneous PIV-PLIF*

Statistically stationary gas tunnel experiments are performed to study multilayer Rayleigh-Taylor instability (RTI). Mixing between three gas streams are studied where the top and bottom streams comprise of air, and the middle stream is air-helium mixture. Experiments are at two Atwood numbers 0.3 and 0.6. We find the late-time mixing width to grow linearly. Dynamics of the flow is investigated using simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF). Measures of molecular mixing indicate very high degree of molecular mixing at late times. Vertical turbulent mass flux a_y is calculated and in addition to its negative values for buoyancy-affected flows, we observe positive values of a_y due to entrainment at the lower edge of the mixing region. Scaling analysis suggests that vertical turbulent mass flux scales better with mixing width growth rate than vertical velocity fluctuations. It's found that majority of potential energy released has been dissipated due to viscous effects, and a mixing efficiency ~ 60% is observed. These experiments are of immense significance for atmospheric-oceanic flows, and for developing and validating turbulence models.