SUMMARY

Turbulence arises from non-linearity in the equations of fluid motion. When different fluids are involved, turbulence increases interfacial surface area and intensifies gradients, causing increased fluid interpenetration and diffusion. This turbulent mixing is vital to a variety of phenomenon, but is poorly understood. One such phenomenon is the Rayleigh-Taylor instability, which occurs at the unstable interface of fluids of differing densities under a gravitational field. Perturbations at the interface grow in time, ending in a fully turbulent regime where turbulent mixing occurs. The goal of this work is to perform experiments on fluid mixing in the context of Rayleigh-Taylor instability experiments at a large density ratio between fluids, approximately 7 (Air and Helium) and far into the turbulent mixing regime at Reynolds number 25000. Experiments are carried out in the Georgia Tech Gas Tunnel facility, designed to convect two fluids parallel to one another in an unstable configuration. The growth and dynamics of the RTI will be observed through two diagnostic measurements, High-Speed Particle Image Velocimetry (PIV) and High-Resolution PIV. In PIV, particle trackers are seeded into the flow and are illuminated by pulsing a thin laser sheet twice in close temporal succession. Images of the laser light scattering off the particles are captured for both pulses. By measuring the displacement between the particles images and the time between pulses, the velocity of the flow can be captured, giving information about the dynamics. Using 6 29MP cameras observing the entire width of the mixing region, we attain high-order turbulence statistics. Using a 4 kHz 1MP high-speed camera, we attain the developing structure of the flow and spectra. Finally, a combined PIV-PLIF diagnostics is employed to analyze the combined density-velocity statistics and comment on the turbulent kinetic energy budget.