Comparison of ring shear cell simulations in 2d/3d with experiments

We present experiments together with corresponding molecular dynamics (MD) simulations of a two-dimensional (2D) and a three-dimensional (3D) granular material in Couette shear cells undergoing slow shearing. In 2D, the grains are disks confined between an inner, rotating wheel and a fixed outer ring. In 3D, the particles are disks, and a part of the bottom is rotating with the inner ring. The simulation results are compared to experimental studies and quantitative agreement is found within about 80-90 per-cent. Tracking the positions and orientations of individual particles allows us to obtain also density distributions, velocity - and rotation-rates in the system. The key issue is to show in how far quantitative agreement between an experiment and MD simulations is possible, besides the fact that many differences in model-details and the experiment exist. We discuss the quantitative agreement/disagreement, give possible reasons, and outline further research perspectives. Especially the issue of a micro-macro transition will be detailed: Stress, strain, and other continuum quantities can be obtained from the microscopic simulations, showing that the granular material is compressive, anisotropic and micro-polar - all at the same time.