J. Fluid Mech. (1994), uol. 265, pp. 97-124 Copyright 0 1994 Cambridge University Press 97 Molecular mixing in Rayleigh-Taylor instability By P. F. LINDEN', J. M. REDONDO' AND D. L. YOUNGS3 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Silver Street, Cambridge CB3 9EW, UK Departamento Fisica Aplicada, Universitat Politecnica de Catalunya, c/Jordi Girona 3 1, Barcelona 08034, Spain Atomic Weapons Establishment, Aldermaston, Reading RG7 4PR, UK (Received 13 July 1992 and in revised form 7 October 1993) Mixing produced by Rayleigh-Taylor instability at the interface between two layers is the subject of a comparative study between laboratory and numerical experiments. The laboratory experiments consist of a layer of brine initially at rest on top of a layer of fresh water. When a horizontal barrier separating the two layers is removed, the ensuing motion and the mixing that is produced is studied by a number of diagnostic techniques. This configuration is modelled numerically using a three-dimensional code, which solves the Euler equations on a 1803 grid. A comparison of the numerical results and the experimental results is carried out with the aim of making a careful assessment of the ability of the code to reproduce the experiments. In particular, it is found that the motions are quite sensitive to the presence of large scales produced when the barrier is removed, but the amount and form of the mixing is not very sensitive to the initial conditions. The implications of this comparison for improvements in the experimental and numerical techniques are discussed. 1. Introduction Rayleigh-Taylor instability is one of the prime candidates for mixing between fluids of different densities. It occurs when the fluids are arranged so that higher pressure occurs in the less dense fluid. The simplest case to consider is that of a fluid in a gravitational field in which the density decreases in the direction of gravity. Then with the dense fluid above the less dense fluid, gravitational instability ensues in the form of Rayleigh-Taylor instability. This mechanism occurs in stably stratified fluids when density surfaces are overturned such as in a large-amplitude internal wave. Many other examples exist both in geophysical and in industrial applications when other forms of acceleration may produce the required pressure gradients. In most of these practical circumstances the instability that is produced occurs at sufficiently fast speeds that the flow develops on a range of lengthscales and rapidly becomes turbulent. If the fluids are miscible, the generation of turbulence will promote mixing at a molecular level. This paper is part of a sequence of papers which have been written to investigate these effects. The overall aim of this study has been to investigate the mixing process produced by Rayleigh-Taylor instabilities, with a view to providing an explanation for the observed mixing rates in these flows. The configuration chosen for the experimental study is the instability produced between two layers of fluid with the dense fluid above, separated by a horizontal plane boundary. The instability is initiated by removing a barrier between the two fluids and allowing the gravitationally driven instabilities to occur. The fluids chosen were salt