Investigation of the multilayer Rayleigh-Taylor instability with simultaneous velocity and three-layer volume fraction measurements

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 two-tracer PLIF technique was developed to overcome the limitations of previous studies, which were limited by single-tracer PLIF measurements, only resolving the volume fraction of one fluid layer at a time. Velocity and volume-fraction profiles are presented for Atwood numbers ranging from 0.3 to 0.5. With two-tracer PLIF, the interactions and mixing between different fluid layers are examined using quantitative parameters such as molecular mixedness. The relationship between fluid layer interactions and the velocity field is also discussed. 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.