Frictional weakening of vibrated granular flows

We computationally study the frictional properties of sheared granular media subjected to harmonic vibration applied at the boundary. Such vibrations are thought to play an important role in weakening flows, yet the independent effects of amplitude, frequency, and pressure on the process have remained unclear. Based on a dimensional analysis and DEM simulations, we show that weakening requires that the absolute amplitude of the displacement is sufficiently large relative to the grain size in addition to sufficiently high acceleration. The analysis in terms of normalized amplitude, frequency and pressure combined provides a basis for predicting flows in more complex geometries subjected to arbitrary external vibration.