Inertial drag in granular media

Like in liquids, objects moving in granular materials experience a drag force. We investigate here whether and how an object's acceleration affects this drag force. The study is based on simulations of a canonical drag test, which involves vertically uplifting a plate through a granular packing with a prescribed acceleration pattern. Depending on the plate size, plate depth, and acceleration pattern, results provide evidence of a rate-independent regime and an inertial regime where the object acceleration strongly enhances the drag force. We introduce an elasto-inertial drag force model that captures the measured drag forces in these two regimes. The model is based on observed physical processes including a gradual, elasto-inertial mobilization of grains located above the plate. These results and analysis point out fundamental differences between mobility in granular materials upon steady and unsteady loadings.