Drag force on a spherical intruder in a granular medium of polyhedral particles

Intrusion of a spherical object, or “intruder,” into dense media of spherical and polyhedral particles is numerically investigated using the discrete element method. Effects of particle angular shape on drag forces exerted on the intruder and mechanical behaviors of the granular media flowing past the intruder are systematically analyzed. In volume-controlled granular media, granular flow pattern transits from inertial to elastic state when solid volume fraction ϕ increases to exceed a critical value, namely, jamming point ϕJ, and meanwhile, a surge in the drag force is observed. The jamming point decreases linearly with the increasing particle volume ratio, which characterizes the degree of particle shape deviation from a sphere. A scaling correlation is proposed to relate a normalized drag force and solid volume fraction difference (ϕJ−ϕ) in the inertial flow regime with ϕ<ϕJ. In pressure-controlled granular media, the drag force per each intruder-particle contact follows a linear relationship with intruder velocity through a dimensionless velocity. The coefficients in the drag force scaling models exhibit significant dependence on the particle shape. At last, underlying mechanism responsible for drag reduction due to the intruder rotation is explored, and it is found that the drag correlation should be modified by incorporating angular velocity of the intruder.