Particle segregation and mixing in a dry free surface granular flow

Geophysical massive flows as snow avalanches and debris flows are characterized by a wide grain size distribution. The interactions between particles among the granulometric classes are a consequent of a such large distribution. However, most of the studies on the geophysical massive flows assume the simplifying hypothesis of a constant granulometry. The aim of this work is studying the coexistence of two granulometric classes in statistically stationary and homogeneous conditions and investigating the physical processes responsible for the particles transfer. Experimental investigations using two different grain size classes were conducted to reproduce the two-phases mixture. The granular material was recirculated in a close system and the experiments were carried out in a laboratory flume having a loose two size granular bed. A proper optical technique was innovatively improved to obtain the average and instantaneous values of the velocity and concentration from the side-walls. Through these values, the average particle profiles of velocity, concentration and granular temperature of the two solid fractions were computed. Moreover, the instantaneous values provide the average profiles of the second order correlation of the variables, such as the component of the granular temperature and the fluctuating components of the velocity and the concentration. The average distribution profiles in time of velocity and concentration prove the statistically stationary and homogeneous condition in a stretch sufficiently long of the running flow. Moreover, two types of regimes were identified: in case of low and intermediate discharges, the frictional regime nearly prevails at the free surface; for high discharges there is a coexistence across the flow depth of two regimes, the frictional regime in the intermediate flow-depth region and the collisional regime in the free surface. The existence of a vertical velocity component of both phases brings to hypothesize the presence of secondary circulations in the flow.