Dense granular flow down an inclined plane: A comparison between the hard particle model and soft particle simulations

Abstract

The granular flow down an inclined plane is simulated using the discrete element (DE) technique to examine the extent to which the dynamics of an unconfined dense granular flow can be well described by a hard particle model. First, we examine the average coordination number for the particles in the flow down an inclined plane using the DE technique using the linear contact model with and without friction, and the Hertzian contact model with friction. The simulations show that the average coordination number decreases below 1 for values of the spring stiffness corresponding to real materials, such as sand and glass, even when the angle of inclination is only 1° larger than the angle of repose. Additional measures of correlations in the system, such as the fraction of particles with multibody contact, the force ratio (average ratio of the magnitudes of the largest and the second largest force on a particle), and the angle between the two largest forces on the particle, show no evidence of force chains or other correlated motions in the system. An analysis of the bond-orientational order parameter indicates that the flow is in the random state, as in event-driven (ED) simulations [V. Kumaran, J. Fluid Mech. 632, 107 (2009); J. Fluid Mech. 632, 145 (2009)]. The results of the two simulation techniques for the Bagnold coefficients (ratio of stress and square of the strain rate) and the granular temperature (mean square of the fluctuating velocity) are compared with the theory [V. Kumaran, J. Fluid Mech. 632, 107 (2009); J. Fluid Mech. 632, 145 (2009)] and are found to be in quantitative agreement. In addition, we also conduct a comparison of the collision frequency and the distribution of the precollisional relative velocities of particles in contact. The strong correlation effects exhibited by these two quantities in event-driven simulations [V. Kumaran, J. Fluid Mech. 632, 145 (2009)] are also found in the DE simulations.