Experimental investigation of Rayleigh---Taylor mixing in gases using simultaneous PIV-PLIF

Dynamics of Rayleigh--Taylor (RT) mixing is studied using statistically stationary experiments performed in a multi-layer gas tunnel. The density ratio of air and air-helium-nitrogen mixture used results in an Atwood number ($A)$ \textasciitilde 0.13. Two types of diagnostics --- particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) --- are employed to obtain mixing width and simultaneous velocity-density data. PLIF using acetone is implemented for the first time for convective-type (flowing) statistically stationary RT experiments with gases. Velocity and density statistics, and their correlations ($u', v', \rho ', \rho 'v')$ are presented. As Atwood number for current experiments exceeds the widely accepted $A$ \textasciitilde 0.1 limit for Boussinesq approximation, non-Boussinesq-ness and anisotropy effects at this Atwood number are evaluated using metrics like higher-order moments (skewness, kurtosis) and anisotropy tensor. Results from current experiments are compared with existing turbulent RT mixing models (like BHR models). \underline {Reference:} AKULA, B. {\&} RANJAN, D. 2016 Dynamics of buoyancy-driven flows at moderately high Atwood numbers. \textit{Journal of Fluid Mechanics 795, 313--355.}