The late nonlinear phase of the Rayleigh-Taylor instability is characterized by the self-similar expansion of the instability mixing layer given at late times by h approximate to alpha Agt(2). In this paper, we present a new model of this mixing layer, based on a piecewise step function approximation where the main constraint imposed is conservation of mass. This model is used to predict the structuring of the mean density of the layer and the asymmetry of the layer for a given Atwood number. By comparing experimental data and simulation results, we confirmed the predictions of the model for the asymmetry of the a values. Our model leads to a simple correction to the formulation of the expansion of the mixing layer which is consistent with alpha for a given system that is independent of the density difference for both immiscible fluids and miscible fluids with low mass diffusion. As the model predicts the mean density profile, it can be used to state the energy released by the instability. Published under license by AIP Publishing.
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