Shubha Tewari, Michal Dichter, Bulbul Chakraborty
Many disordered systems experience a transition from a fluid-like state to a solid-like state following a sudden arrest in dynamics called jamming. We present results of numerical simulations of dense, gravity-driven, granular flow in a two-dimensional hopper. Unlike the jams that develop in spatially homogeneous systems, boundaries play a crucial role in hopper jamming. In this work, we focus on the spatial heterogeneity of the fluctuations near jamming. We find that the flow becomes increasingly intermittent as jamming is approached, alternating between periods of slow and fast flow, although a true jamming event, or a zero-flow state, is extremely rare. Periods of slow flow occur more frequently and persist longer as the average flow rate decreases. This increase in intermittency is accompanied by a change in the spatial structure of the velocity autocorrelation functions. At the highest flow rates, the flow at the center shows the longest autocorrelation times whereas for the slowest flow rate, it is the boundary regions where fluctuations relax most slowly. The distributions of the vertical component of the velocity, near jamming, are strongly non-gaussian in the boundary regions, and the distributions at the hopper opening obey a well-defined scaling form. Characterizing a rare, true jamming event, we find that space-time clusters of slow moving particles develop and grow as jamming is approached.
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http://arxiv.org/abs/1210.5805
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