LOCAL MEASUREMENTS OF VELOCITY FLUCTUATIONS AND DIFFUSION-COEFFICIENTS FOR A GRANULAR MATERIAL FLOW

Measurements were made of two components of the average and fluctuatin g velocities, and of the local self-diffusion coefficients in a flow o f granular material. The experiments were performed in a 1 m-high vert ical channel with roughened sidewalls and with polished glass plates a t the front and the back to create a two-dimensional flow. The particl es used were glass spheres with a nominal diameter of 3 mm. The flows were high density and were characterized by the presence of long-durat ion frictional contacts between particles. The velocity measurements i ndicated that the flows consisted of a central uniform regime and a sh ear regime close to the walls. The fluctuating velocities in the trans verse direction increased in magnitude from the centre towards the wal ls. A similar variation was not observed for the streamwise fluctuatio ns. The self-diffusion coefficients showed a significant dependence on the fluctuating velocities and the shear rate. The velocity fluctuati ons were highly anistropic with the streamwise components being 2 to 2 .5 times the transverse components. The self-diffusion coefficients fo r the streamwise direction were an order-of-magnitude higher than thos e for the transverse direction. The surface roughness of the particles led to a decrease in the self-diffusion coefficients.