Intermittent flow of a collection of rigid frictional disks in a vertical pipe.

We study the quasistatic flow of a collection of rigid frictional disks pushed upward (against the gravity) inside a narrow vertical pipe by a compliant mechanism. The contact dynamics method was used for the numerical simulations in combination with a friction law at disk-disk and wall-disk contacts characterized by discontinuous velocity weakening from a static threshold to a dynamic coefficient of friction. The material is sheared by the rolling of particles at the walls inducing a convective motion in the bulk. We observe a transition from constant flow to an intermittent flow when the driving velocity is reduced below a characteristic velocity that scales as k(-1/2) with the stiffness k of the pushing mechanism. The intermittent flow is composed of alternating phases of creep motion, where the pressure at the bottom of the granular column rises nonlinearly with time, and sudden slip, corresponding to a fast pressure drop. We show that the mean static pressure is correctly predicted by the Janssen model. The interplay between friction mobilization at the walls and structural changes in the bulk gives rise to a broad distribution of slip amplitudes characterized by a power law with an exponent approximately -1.7 that appears to be robust with respect to our system parameters.