SHEAR-INDUCED PARTICLE DIFFUSION AND LONGITUDINAL VELOCITY FLUCTUATIONS IN A GRANULAR-FLOW MIXING LAYER

In flows of granular material, collisions between individual particles result in the movement of particles in directions transverse to the b ulk motion. If the particles were distinguishable, a macroscopic overv iew of the transverse motions of the particles would resemble a self-d iffusion of molecules as occurs in a gas. The present granular-flow st udy includes measurements of the self-diffusion process, and of the co rresponding profiles of the average velocity and of the streamwise com ponent of the fluctuating velocity. The experimental facility consists of a vertical channel fed by an entrance hopper that is divided by a splitter plate. Using differently-coloured but otherwise identical gla ss spheres to visualize the diffusion process, the flow resembles a cl assic mixing-layer experiment. Unlike molecular motions, the local par ticle movements result from shearing of the flow; hence, the diffusion experiments were performed for different shear rates by changing the sidewall conditions of the test section, and by varying the flow rate and the channel width. In addition, experiments were also conducted us ing different sizes of glass beads to examine the scaling of the diffu sion process. A simple analysis based on the diffusion equation shows that the thickness of the mixing layer increases with the square-root of downstream distance and depends on the magnitude of the velocity fl uctuations relative to the mean velocity. The results are also consist ent with other studies that suggest that the diffusion coefficient is proportional to the particle diameter and the square-root of the granu lar temperature.