Experimental study of Rayleigh-Taylor instability with a complex initial perturbation

Experiments have been performed investigating the Rayleigh-Taylor instability initialized with a complex initial perturbation. The experiments utilize a miscible fluid combination with Atwood number A approximate to 0.2. The initially stably stratified fluids are contained within a Plexiglas tank mounted to a linear rail system. The tank was then oscillated vertically to impose nearly sinusoidal three-dimensional internal waves of varying wavelength and complexity at the fluid interface. After imposing this perturbation, the tank is accelerated down the rails at a rate greater than Earth's gravity (g(0)) resulting in a body force of approximately 0.8g(0). The flow is visualized with either backlit photography or planar laser induced fluorescence. Image sequences from the experiments show bubble and spike merging, leading to a growth of length scale with time. Averaged vertical concentration distributions show self-similarity after similar to 233 ms with a total experiment time of similar to 300 ms. In addition, after this time, the square root of the mixing zone width appears to grow linearly with (Ag)(1/2)t. Values for the self-similar growth parameter, alpha, obtained by curve fitting to the linear portion of these curves yield values that are lower than those obtained in other experiments but are in good agreement with values found in computational studies initiated with perturbations similar to those used here. The measured alpha values do not show a dependence on the initial perturbation amplitude. The method for the determination of alpha using the expression alpha=h(2)/4Agh proposed by Cabot and Cook [Nat. Phys. 2, 562 (2006)] yields a value in agreement with that measured by curve fitting the h(1/2) versus Agt curves, and which is also in better agreement with computational studies.