  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 vary as a power of
the local velocity gradient. 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 shear rate. The functional forms of the velocity
and fluctuation profiles predicted by the model agree with the experimental
results.
