General features of granular Couette flow and intruder dynamics

This paper reports on the behaviour of uniform granular particles of diameter d undergoing a gravity-free shear flow induced through parallel bumpy boundaries that move in opposite directions at constant velocity U. Non-equilibrium, discrete simulations are performed, in which particles are modelled as inelastic, frictional spheres. The flow is described using steady state profiles of mean velocity, granular temperature, solids fraction and normal pressure. A non-uniform local shear rate, characterized by an S-shaped mean velocity profile, produces an imbalance in the contact distribution of particles in the vicinity of the walls so that they drift toward the geometric centre of the flow. A spectral analysis of the transverse velocity provides evidence of convective cell structures in the secondary velocity field whose wavelength decreases with the effective shear rate. A typical tracer particle, whose trajectory has a power spectrum suggestive of persistent fractional Brownian motion, continually samples the entire shear region. In contrast, a large intruder , which also migrates away from energetic regions adjacent to the walls, eventually becomes trapped near the mid-plane of the flow with a speed that increases with φ and the effective shear rate . Fluctuations in the evolution of its transverse velocity component obey a power law of the form V_y^rms = Cφ^-a, which is a consequence of its greater inertia, while fluctuations in its net force vary directly with φ.