Fluctuations of grains inside a discharging two-dimensional silo
  We present experimental data corresponding to a two dimensional dense granular flow, namely, the gravity-driven discharge of grains from a small opening in a silo. We study the microscopic velocity field with the help of particle tracking techniques. From these data, the velocity profiles can be obtained and the validity of some long-standing approaches can be assessed. Moreover, the fluctuations of the velocities are taken into consideration in order to characterize the features of the advective motion (due to the gravity force) and the diffusive motion, which shows nontrivial behaviour. 
Angel Garcimart\'in, Iker Zuriguel, Alvaro Janda and Diego Maza
cond-mat.soft