Correlated motion in the bulk of dense granular flows

Numerical simulations of two-dimensional stationary dense granular flows are performed. We check that the system obeys the h(stop) phenomenology. Focusing. on the spatial correlations of the instantaneous velocity fluctuations of the grains, we give evidence of the existence of correlated motion over several grain diameters in the bulk of the flow. Investigating the role of contact friction and restitution, we show that the associated typical length scale lambda is essentially independent of the grain properties. Moreover, we show that lambda is not controlled by the packing compacity. However, in agreement with previous experimental work, we observe that the correlation length decreases with the shear rate. Computing the flows inertia number I, we show a first-order dependence of lambda on I.