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Velocity fluctuations in dense granular flows.

We use simulations to investigate velocity fluctuations in dry granular flow. Our system is comprised of mono- and polydisperse sets of spherical grains falling down a vertical chute under the influence of gravity. We find three different classes of velocity distributions depending on factors such as the local density. The class of the velocity distribution depends on whether the grains are in a free-fall, fluid, or glassy state. The analytic form of the distributions match those that have been found by other authors in fairly diverse systems. Here, we have all three present in a single system in steady state. Power-law tails that match recent experiments are also found but in a transition area suggesting they may be an artifact of crossover from one class of velocity distribution to another. We find evidence that the transition from one class to another may correspond to a second order dynamical phase transition in the limit that the vertical flow speed goes to zero.