Experimental Characterization of Bimodal Granular Flow

Abstract

Solid–liquid flows are encountered in various industrial and natural environments. The internal structure of such flows is highly sensitive to the grading of the solid particles present. In this experimental study, an extended stereometric method is employed to assess the distributions of velocity of particles of different fractions, distinguished by different colors, in vertical and nearly horizontal granular flows. In the vertical flow experiments, mixtures comprising three fractions of lightweight particles, characterized by a very similar density, size, and shape, were tested. The results affirmed the method’s ability to discern particle velocity differences on the order of millimeters per second, establishing its suitability for characterizing nearly horizontal open-channel flows with bimodal mixtures that are stratified and exhibit more complex velocity distributions. Tilting flume experiments, incorporating additional measurements of water velocity distribution, allowed for the evaluation of local slip between water and particles, as well as between particles of the two fractions in the flow. The results indicated that, although the local slip velocity was relatively small, the average slip velocity between the carrying water and transported particles was significantly larger. This factor must be taken into consideration when evaluating bed friction or bed erosion for granular flow in a channel with an erodible bed.