Force-velocity correlations in a dense, collisional, granular flow
  We report measurements in a 2-dimensional, gravity-driven, collisional, granular flow of the normal force delivered to the wall and of particle velocity at several points in the flow. The wall force and the flow velocity are negatively correlated. This correlation falls off only slowly with distance transverse to the flow, but dies away on the scale of a few particle diameters upstream or downstream. The data support a picture of short-lived chains of frequently colliding particles that extend transverse to the flow direction, making transient load-bearing bridges that cause bulk fluctuations in the flow velocity. The time-dependence of these spatial correlation functions indicate that while the force-bearing structures are local in space, their influence extends far upstream in the flow, albeit with a time-lag. This leads to correlated velocity fluctuations, whose spatial range increases as the jamming threshold is approached. 
Emily Gardel, Ellen Keene, Sonia Dragulin, Nalini Easwar, Narayanan   Menon