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In this paper, we have designed and employed a suspended-wall silo to remove Janssen effect in order to explore directly the local pressure dependence of Granular Orifice Flow (GOF) systematically. We find that as Janssen effect is removed, the flow rate Q changes linearly with the external pressure. The slope {\alpha} of the linear change decays exponentially with the ratio of the silo size and the size of the orifice {\Phi}/D, which suggests the existence of a characteristic ratio {\lambda} (~2.4). When {\Phi}/D > {\lambda}, {\alpha} gradually decays to zero, and the effect of external pressure on the GOF becomes negligible, where the Beverloo law retrieves. Our results show that Janssen effect is not a determining factor of the constant rate of GOF, although it may contribute to shield the top load. The key parameter in GOF is {\Phi}/D. In small {\Phi}/D, the flow rate of GOF can be directly adjusted by the external pressure via our suspended-wall setup, which may be useful to the transportation of granules in microgravity environment where the gravity-driven Beverloo law is disabled.

We present an X-ray tomography study of the segregation mechanisms of tracer particles in a three-dimensional cyclically sheared bi-disperse granular medium. Big tracers are dragged by convection to rise to the top surface and then remain trapped there due to the small downward convection cross-section, which leads to segregation. Additionally, we also find that the local structural up-down asymmetry due to arching effect around big tracers will induce the tracers to have a net upward displacement against its smaller neighbors, which is another mechanism for segregation., Comment: 24 pages, 5 figures

We report a peculiar dynamic phenomenon in granular gases, chain structures of head-on collisions caused by the boundary heated mechanism form a network in an Airbus micro-gravity experiment and horizontal vibrated one in the laboratory, which differ markedly from the grazing-collision-dominant in randomly driven granular fluid. This new order property is an orientation correlation between the relative position and the relative velocity of any particle pair, which weakens the collision frequency and leads a long range boundary effect. By the histogram of the relative position and the relative velocity, we find this position-velocity correlation is not only at limits of very small relative velocities but also large ones, which means the breakdown of molecular chaos assumption is not limited to a small portion of the phase space. Through a simple anisotropic angular distribution model of the relative position and the relative velocity, we could modify classical uniform angular integration results of mean field values taking the effect of the observed collision chain structure explicitly into account.

In this paper, granular segregation in a two-compartment cell in zero gravity is studied numerically by DEM simulation. In the simulation using a virtual window method we find a non-monotonic flux, a function which governs the segregation. A parameter is used to quantify the segregation. The effect of three parameters: the total number of particles N, the excitation strength {\Gamma}, and the position of the window coupling the two compartments, on the segregation {\epsilon} and the waiting time {\tau} are investigated. It is found that the segregation observed in zero gravity exists and does not depend on the excitation strength {\Gamma}. The waiting time {\tau}, however, depends strongly on {\Gamma}: Higher the {\Gamma}, lower the waiting time {\tau}. The simulation results are important in guiding the SJ-10 satellite microgravity experiments.

Using an event-driven molecular dynamics simulation, we show that simple monodisperse granular beads confined in coupled columns may oscillate as a new type of granular clock. To trigger this oscillation, the system needs to be driven against gravity into a density-inverted state, with a high-density clustering phase supported from below by a gas-like low-density phase (Leidenfrost effect) in each column. Our analysis reveals that the density-inverted structure and the relaxation dynamics between the phases can amplify any small asymmetry between the columns, and lead to a giant oscillation. The oscillation occurs only for an intermediate range of the coupling strength, and the corresponding phase diagram can be universally described with a characteristic height of the density-inverted structure. A minimal two-phase model is proposed and linear stability analysis shows that the triggering mechanism of the oscillation can be explained as a switchable two-parameter Hopf bifurcation. Numerical solutions of the model also reproduce similar oscillatory dynamics to the simulation results., Comment: 5 figures

Vibrating walls, used to maintain the temperature in a granular gas, modify the system strongly. Most conspicuously, the usual one-peak velocity distribution splits into two, asymmetrically positioned. A surgical repair of the usual hydrodynamic description is presented that provides an account for, and an understanding of, the situation., Comment: 3 fig

We show that the van der Waals-like phase separation instability of a driven granular gas at zero gravity, previously investigated in two-dimensional settings, persists in three dimensions. We consider a monodisperse granular gas driven by a thermal wall of a three-dimensional rectangular container at zero gravity. The basic steady state of this system, as described by granular hydrodynamic equations, involves a denser and colder layer of granulate located at the wall opposite to the driving wall. When the inelastic energy loss is sufficiently high, the driven granular gas exhibits, in some range of average densities, negative compressibility in the directions parallel to the driving wall. When the lateral dimensions of the container are sufficiently large, the negative compressibility causes spontaneous symmetry breaking of the basic steady state and a phase separation instability. Event-driven molecular dynamics simulations confirm and complement our theoretical predictions., Comment: 7 pages, 7 figures, to appear in Phys. Rev. E