On velocity profiles and stresses in sheared and vibrated granular systems under variable gravity

We employ discrete element three-dimensional simulations that include realistic modeling of physical system boundaries to determine the influence of gravity on velocity profiles and stresses for frictional inelastic particles that are confined in an angular Couette cell, and sheared by a rotated upper wall. In addition to Earth gravity, we consider other gravitational fields, in particular those of the Moon and Mars. The computational techniques are based on hard-sphere simulations of polydisperse particles at relatively high volume fraction (50-55%). We find that the presence of gravity induces significant changes of the velocity profiles and stresses. One important nondimensional parameter in the problem is shown to be I_Ω=γ̇d/ √P_g/ρ_s where γ̇ is the imposed shear rate, Pg is the weight of the system per unit area due to gravity, and ρ_s is the solid density. We also consider systems that are vibrated in addition to being sheared, since vibrations are one of several important methods for agitating (e.g., fluidizing and/or unjamming) granular systems. We find that the introduction of nondimensional acceleration Γ=a(2πf)²g, where a,f,g are the amplitude and frequency of oscillations, and the acceleration of gravity, explains novel features that develop in these complex granular systems.