Influence of slope gradient on the behavior of saltating sand particles in a wind tunnel

Abstract

Current theoretical and experimental studies of windblown sand movement are mostly conducted on flat sand surfaces. However, sand dunes, basic forms of desert landscapes, have slope gradients that greatly influence the transportation of sand particles. In this study, sand velocity on the windward and leeward slopes of barchans dunes was measured using a laser Phase Doppler Particle Analyzer in a wind tunnel. Our results measured from slope are quite different from existing data of flat surface. The average velocity of sand particles increased from bottom to top of the windward slope. And the particle velocity on leeward slope is not greatly impact by position and wind speed. Unlike exponential decay on flat bed, the lift-off angle on windward slope shows left-skewed distribution with a single peak, and the distribution of impact and lift-off angle on leeward slope are entirely different from those obtained on flat bed. On the leeward slope, impact and lift-off angle were affected remarkably by the position on the slope. On the leeward slope, the number of particles with wind direction horizontal velocity is basically the same as those of particles with headwind direction horizontal velocity. The vertical and horizontal turbulence intensity reached the maximum when the particles were located 20mm from the bed surface near the bottom of leeward slope; and the minimum at 60mm from the bed surface. Both vertical and horizontal turbulence intensity of particles increased with increasing wind velocity. The increase in the horizontal turbulence intensity was greater than the increase in the vertical turbulence intensity. The findings from this study would provide a better understanding of movement of blown sands over sand dunes and mechanism underlying the formation of sand dunes.