Research on velocity profiles of saltating particles based on their digital trajectories
Velocity serves as a crucial parameter in the study of saltation motion.We utilized high-speed photog-raphy technology to acquire the vertical profiles of the average velocities of saltating sand grains on a loose sand surface during the ascending and descending phases within a wind tunnel.The outcomes reveal that the particle turbulence in the vertical direction is one to two orders of magnitude greater than that in the horizontal direction,inducing the particle velocities to conform well to a normal distribution in the horizontal direction.Meanwhile,they are more dispersed and have a narrower range(approximately 0-1 m·s-1)in the vertical direction.At the identical height,the average horizontal velocity in the descending phase is approximately 1.6-1.8 times that in the ascending phase,whereas the vertical velocity in the descending phase is marginally smaller than that in the ascending phase.The relative average horizontal velocities(vx/√gd,where g represents the gravitational acceler-ation and d denotes the particle diameter)in both phases are proportional to the square root of the relative height(z/Z,where z is the height from the bed surface and Z is the thickness of the boundary layer)and the relative wind speed(u*/u*ti,where u*stands for the friction wind speed and u*ti is the impact threshold).The average verti-cal velocities in both the ascending and descending phases typically exhibit a slightly ascending trend with height.The average vertical velocity in the ascending phase is slightly higher than that in the descending phase.The un-derlying mechanisms of the average vertical velocity distribution patterns and the characteristics of variation with height necessitate further investigation for clarification.
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