Pressure Waves During Granular Flows in Varying Gravity Environments

We present results of LAMMPS Molecular Dynamics simulations of 2D gravity-driven flows of 30,000 soft uniform spheres through a vertical silo. We vary the gravitational field (g), elastic modulus of the particles (E), and silo outlet diameter (D). We present results on upwards pressure waves observed in the system. We compare our results with previous work on granular acoustics. Despite the typical particle speed being substantially less than the measured wave speed, we posit these are nonlinear shock waves, as observed in other systems near jamming. We demonstrate that the wave speeds in all systems appear to follow a power law that is distinct from linear wave expectations.