Mixing of a granular layer falling through a fluid

International audienceWe analyze the granular Rayleigh-Taylor instability of densely packed grains immersed in a compressible or an incompressible fluid using numerical simulations and two types of experiments. The simulations are based on a two-dimensional ͑2D͒ molecular dynamics model and the experiments have been carried out in systems of grains immersed in water/glycerol ͑incompressible fluid͒ and in air ͑compressible fluid͒. The variation of the interstitial fluid is shown to generate different dynamical patterns and mixing properties of the granular systems. The results have been quantified using 2D autocorrelation functions, the power spectrum of the velocity field and velocity field histograms. Excellent agreement is found between the numerical simulations and the experiments