High-velocity drag friction in dense granular media

We study drag force acting on an obstacle in granular media, focusing on a high-velocity region where only a few direct studies are available. The granular media are two-dimensional and consist of small-sphere particles. A larger-disk obstacle moves in the medium at different constant speeds. The drag force is found to be proportional to the square of the velocity. We explain the observed relations by developing original scaling arguments and by demonstrating a clear data collapse. As a result, we conclude that the friction we observed is physically different from a hydrodynamic inertial friction, which has been discussed in dilute granular flows and in impact experiments. The high-velocity drag friction observed in this study may be attributed to another mechanism, where the formation of the dynamical force chains plays a crucial role. Copyright (C) EPLA, 2010