Granular shear flow dynamics and forces: Experiment and continuum theory

We analyze the main features of granular shear flow through experimental measurements in a Couette geometry and a comparison to a locally Newtonian, continuum model of granular flow. The model is based on earlier hydrodynamic models, adjusted to take into account the experimentally observed coupling between fluctuations in particle motion and mean-flow properties. Experimentally, the local velocity fluctuations are found to decrease more slowly with distance from the shear surface than the velocity. This can be explained by an effective viscosity that diverges more rapidly as the random-close-packing density is approached than is predicted by Enskog theory for dense hard-sphere systems. Experiment and theory are in good agreement, especially for the following key features of granular flow: The flow is confined to a small shear band, fluctuations decay approximately exponentially away from the sheared wall, and the shear stress is approximately independent of the shear velocity. The functional forms of the velocity and fluctuation profiles predicted by the model agree with the experimental results.