Investigation of multi-component mixing in the three-layer Rayleigh-Taylor instability

The Rayleigh-Taylor instability (RTI) in a multi-layered environment is investigated through statistically stationary experiments conducted in a blown-down three-layer gas tunnel. The top and bottom layers of the gas tunnel are air, while an air-helium mixture is used for the lighter middle layer. This setup creates a three-layer stratification with an upper RTI unstable interface between the middle and top layers, and a lower RTI stable interface between the middle and bottom layers. Simultaneous particle image velocimetry (PIV) and two-tracer planar laser induced fluorescence (PLIF) are employed to resolve the velocity and volume fraction of each fluid layer. The interaction and mixing between different fluid layers with one another and with the velocity field is considered across a range of Atwood numbers. This is done by considering the covariances and joint probability density functions between different variables. This analysis shows novel experimental results that were previously unattainable through single-tracer PLIF. Key findings include significant mixing and interactions between different layers, as well as the entrainment of the stable bottom layer into the upper RTI driven mixing region. The reported experimental results are in close agreement with recent DNS simulations of the three-layer system. This work contributes to the understanding of RTI in a multilayer configuration, which is particularly significant for inertial confinement fusion (ICF) pellet design, as well as atmospheric and oceanic flows. Additionally, it provides measurements of parameters that aid in the development and validation of turbulence closure models for variable density flows.