Experimental investigation of mass flux and transport rate of different size particles in mixed sand transport by wind

Abstract

Sand transport by wind in arid regions is a geomorphic process affected by the grain size distribution. Knowledge of the transport behavior of each size group in mixed sand transport is important for understanding the saltation dynamics in dune migration. This paper focuses on the characteristics of mass flux, mass concentration, horizontal velocity and transport rate of different particle size groups and the relation to their mass proportion in air. Experiments were carried out to analyze these relations in a wind tunnel by particle tracking velocimetry. The mass flux of different size particles usually decays exponentially with height for larger size groups and deviates from exponential decay for smaller size groups. The trend of mass concentration of different size groups with height is similar to the mass flux. The mean horizontal velocity of different size groups generally increases with increasing height and decreases with the increase of particle size at the same height. With increasing height, the ratio of mass flux of each size group to the total mass flux increases for the smaller size group but decreases for the larger size group, and the transition diameter of particles is about 0.14 mm for the 0.16 mm sand sample and about 0.21–0.27 mm for the 0.24 mm sand sample. For the smaller size group, the ratio of the mass-weighted average horizontal velocity of each size group to that of all particle sizes is >1.0, and the ratio of the transport rate of each size group to the total transport rate is usually larger than its mass proportion in air; this is the opposite for the larger size groups. The maximum deviation of mass flux proportion of each size group from its mass proportion in air is about ±15%, which is determined by the mass-weighted average horizontal velocity of each size group.