Low-frequency oscillations and convective phenomena in a density-inverted vibrofluidised granular system


Nicolas Rivas (University of Twente)


We present a simulational, theoretical and experimental study of a vertically vibrated granular media. The systems considered are confined in narrow quasi-two-dimensional (narrow) and quasi-one-dimensional (column) geometries, where the height of the container is much larger than both horizontal lengths. The additional geometric constrain present in the column setup frustrates the convection state that is normally observed in the narrow geometry. This makes it possible to study collective oscillations of the grains with a characteristic frequency that is much lower than the frequency of energy injection. The frequency and amplitude of these oscillations is studied as a function of the energy input parameters and the size of the container. We observe that, in the quasi-two-dimensional setup, low-frequency oscillations are present even in the convective regime. This suggests that they may play a determinant role in the transition from a density inverted state to convection. A model is also presented that, considering Cauchy’s equations, is able to predict with high accuracy the frequency of the particles collective motion. The model shows that a sufficient condition for the existence of the low-frequency mode is an inverted density profile with distinct low and high density regions, a condition that may apply to other systems. The experimentally acquired results show a close qualitative and quantitative agreement with both theory and simulations across the range of parameters tested. We also present evidence of two new phenomena, previously unobserved experimentally or theoretically: a near-crystalline state in which particles display circulatory motion through the system over an extended time period, and a Leidenfrost-like state in which the dense upper region displays an unusual ‘inverse’ thermal convection.

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