Fluctuations and Flow for Granular Shearing: Results from Experiment and Simulation

We present results from both simulation and experiment on a 2D granular shear-cell. The experiments determine the position of disks and their orientations over time, as well as the force on individual disks. We use computerized particle tracking techniques to achieve the former and photoelasticity to achieve the latter. The simulations use MD force laws and efficient algorithms to simulate as closely as possible the experimental system. We measure the radial dependence of velocities and their distributions. In particular we find that the azimuthal velocity decays exponentially to some background level within a distance of about 7 disk diameters from the shearing wheel. Experimentally the distribution of azimuthal velocities is found to have a complex, roughly bimodal distribution close the the shearing wheel which is indicative of a complex combination of slip, no-slip, and rolling processes at the boundary, and a more exponential distribution away from the shearing surface. The distribution of stresses shows a falloff which is approximately exponential at large forces, although it is probably not possible to determine which among competing models for force distributions best fits these results. The model can capture most but not all of the features seen in the experiment. The mean velocity profile, the qualitative nature of force chains and the distribution of velocities far from the shearing surface are well captured in the simulations. The velocity distribution near the shearing surface and the force distributions differ considerably between theory and experiment.