The effects of forced small-wavelength, finite-bandwidth initial perturbations and miscibility on the turbulent Rayleigh–Taylor instability

Rayleigh–Taylor instability experiments are performed using both immiscible and miscible incompressible liquid combinations having a relatively large Atwood number of $A\equiv ({\it\rho}_{2}-{\it\rho}_{1})/({\it\rho}_{2}+{\it\rho}_{1})=0.48$. The liquid-filled tank is attached to a test sled that is accelerated downwards along a vertical rail system using a system of weights and pulleys producing approximately $1g$ net acceleration. The tank is backlit and images are digitally recorded using a high-speed video camera. The experiments are either initiated with forced initial perturbations or are left unforced. The forced experiments have an initial perturbation imposed by vertically oscillating the liquid-filled tank to produce Faraday waves at the interface. The unforced experiments rely on random interfacial fluctuations, resulting from background noise, to seed the instability. The main focus of this study is to determine the effects of forced initial perturbations and the effects of miscibility on the growth parameter, ${\it\alpha}$. Measurements of the mixing-layer width, $h$, are acquired, from which ${\it\alpha}$ is determined. It is found that initial perturbations of the form used in this study do not affect measured ${\it\alpha}$ values. However, miscibility is observed to strongly affect ${\it\alpha}$, resulting in a factor of two reduction in its value, a finding not previously observed in past experiments. In addition, all measured ${\it\alpha}$ values are found to be smaller than those obtained in previous experimental studies.