Stochastic transitions and jamming in granular pipe flow

We study a model granular suspension driven down a channel by an embedding fluid via computer simulations. We characterize the different system flow regimes and the stochastic nature of the transitions between them. For packing fractions below a threshold φm, granular flow is disordered and exhibits an Ostwald–de Waele–type power-law shear-stress constitutive relation. Above φm, two asymptotic states exist; disordered flow can persist indefinitely, yet, in a fraction of samples, the system self-organizes in an ordered form of flow where grains move in parallel ordered layers. In the latter regime, the Ostwald–de Waele relationship breaks down and a nearly solid plug appears in the center, with linear shear regions at the boundaries. Above a higher threshold φg, an abrupt jamming transition is observed if ordering is avoided.