Experimental Investigation of multilayer Rayleigh-Taylor instability at moderate to high Atwood numbers

The Rayleigh-Taylor instability (RTI) in a multilayer configuration is explored by advecting three stratified gas layers down a tunnel. The top and bottom layers comprise air, while a nitrogen-helium mixture forms a lighter middle layer. The effects of the lower stable interface of the middle layer on the RTI growth of its upper unstable interface are considered for moderate and high Atwood numbers. Simultaneous Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) are used to determine the velocity and density fields at different locations down the tunnel. Key findings include linear growth of the mixing layer, a high degree of molecular mixing, and the entrainment of the bottom layer into the mixing region. Turbulent characteristics of the flow are also presented. These include simultaneous velocity-density statistics, the vertical turbulent mass flux, and the density-specific volume correlation. The presented results contribute to the development and validation of turbulence models for variable density flows. They are particularly relevant for models of more complex phenomena, such as inertial confinement fusion or atmospheric-ocean flows, where more than one fluid interface may be present.