  Using discrete simulations, we investigate the behavior of a model granular
material within an annular shear cell. Specifically, two-dimensional assemblies
of disks are placed between two circular walls, the inner one rotating with
prescribed angular velocity, while the outer one may expand or shrink and
maintains a constant radial pressure. Focusing on steady state flows, we
delineate in parameter space the range of applicability of the recently
introduced constitutive laws for sheared granular materials (based on the
inertial number). We discuss the two origins of the stronger strain rates
observed near the inner boundary, the vicinity of the wall and the
heteregeneous stress field in a Couette cell. Above a certain velocity, an
inertial region develops near the inner wall, to which the known constitutive
laws apply, with suitable corrections due to wall slip, for small enough stress
gradients. Away from the inner wall, slow, apparently unbounded creep takes
place in the nominally solid material, although its density and shear to normal
stress ratio are on the jammed side of the critical values. In addition to
rheological characterizations, our simulations provide microscopic information
on the contact network and velocity fluctuations that is potentially useful to
assess theoretical approaches.
