Title: The Nature of Drag in Granular Impact

Author (Talk): Ted Brzinski, University of Pennsylvania

Abstract:

A projectile dropped into a granular medium experiences a stopping force that is the sum of an inertial drag and a Coulomb-friction-like quasi-static drag. Now we have measured the forces acting on intruders moving quasi-statically through a granular medium in order to investigate the grain-scale mechanics that dominate the quasi-static component of the drag. By utilizing different intruder geometries, we demonstrate that the friction force acts normal to the intruder surface. And by altering the hydrostatic loading on the bed by generating a homogenized airflow through the bed, we demonstrate that the inter-particle contact forces that set the magnitude of the friction force are loaded hydrostatically rather than by the motion of the intruder itself. We find that the same empirical coefficient describes the magnitude of the quasi-static drag for all projectile geometries and granular media tested. Surprisingly the magnitude of this drag force is many times greater than one might expect from the Coulomb model of friction. We verify that these results hold for real impacts by comparing experimental impact data to the expectation given by the frictional forces measured in our quasistatic lowering experiments.