Effects of overburden pressure on granular shear flows: Multiplicity and chaos

The shear flow of granular materials is studied in the presence of overburden pressure and gravity using the discrete element method. The system comprises particles confined between parallel walls moving with constant speed in opposite directions and with an overburden pressure applied on the top wall, which is free to move in the wall-normal direction. The overburden pressures are high enough that the stored elastic energy due to compression of the particles is significant. The variation in the average velocity, solid volume fraction, and fluctuation velocity profiles with overburden pressure is shown. For most of the overburden pressures, a coexistence of a sheared region and a pluglike region is observed. As the overburden pressure increases, a flow transition is observed from a plug in the lower region to a plug in the upper region. At intermediate overburden pressures, a wide shear zone and a fluctuating profiles are observed. The transition is found to be continuous with increasing overburden pressure, and a scaling relation for the average midplane velocity variation with pressure is obtained for different wall velocities and particle stiffness values. Analysis of different configurations of the system show that the pluglike region forms near the fluctuating wall, where pressure is applied. The instantaneous velocity profile is found to vary chaotically with time at intermediate pressures. The midline velocity distribution is found to be Gaussian in most of the cases studied with a higher standard deviation at the intermediate pressures and lower wall velocities. The work highlights the nonlinear behavior of granular shear flow as a result of overburden pressure, which includes multiplicity and chaotic velocities.