“Transition Stages of Rayleigh-Taylor Instability Between Miscible Fluids” Andrew W. Cook and Paul E. Dimotakis Transition stages of Rayleigh-Taylor instability between miscible fluids∗

Direct Numerical Simulations (DNS) have been performed of three-dimensional, Rayleigh-Taylor instability (RTI) between two incompressible, miscible fluids, with a 3:1 density ratio. Periodic boundary conditions are imposed in the horizontal directions of a rectangular domain, with no-slip top and bottom walls. Solutions are obtained for the Navier-Stokes equations, augmented by a species transport-diffusion equation, with various initial perturbations. The DNS achieved outer-scale Reynolds numbers, based on mixing-zone height and its rate of growth, in excess of 3000. Initial growth is found to be diffusive and independent of the initial perturbations. The onset of nonlinear mixing-zone growth is not predicted by available linear-stability theory. The nonlinear growth rates are found to depend on the initial perturbations, through the end of the simulations. Mixing is found to be even more sensitive to initial conditions than growth rates. Taylor microscales and Reynolds numbers are anisotropic throughout the simulations. Improved collapse of many statistics is achieved if the height of the mixing zone, rather than time, is used as a scaling and progress variable. Mixing has dynamical consequences for this flow, since it is driven by the action of the imposed acceleration field on local density differences.