In the research of windblown sand movement, the lift-off and incident velocities of saltating sand particles play a significant role in bridging the spatial and temporal scales from single sand particle's motion to windblown sand flux. In this paper, we achieved wind tunnel measurements of the movement of sand particles near sand bed through improving the wind tunnel experimental scheme of paticle image velocimetry (PIV) and data processing method. And then the influence of observation height on the probability distributions of lift-off and incident velocities of sand particles was analyzed. The results demonstrate that the observation height has no obvious influence on the distribution pattern of the lift-off and incident velocities of sand particles, i.e., the probability distribution of horizontal and vertical velocities of lift-off and incident sand particles follow a Gaussian distribution and a negative exponential distribution, respectively. However, it influences the center of the Gaussian distribution, the decay constant and the amplitude of the negative exponential distribution.
In this paper, the velocities of sand particles near the sand bed in the saltation cloud were measured in a wind tunnel through an improved experimental scheme of the Particle Image Velocimetry (PIV) system. The influences of the diameter of sand particles in the saltation cloud and wind velocity on the probability distribution function (PDF) of lift-off velocities of sand particles were investigated. Results demonstrate that for the sand particles saltating above the sand bed with the mean grain diameter (d m = 0.3 mm), smaller and larger ones have the same velocity distribution, and wind velocity has no obvious influence on the distribution shape of the lift-off velocities, i.e., the PDFs of the horizontal and vertical lift-off velocities both follow a lognormal distribution, but the diameter of sand particles in the saltation cloud and wind velocity have an influence on the parameters of the PDF of horizontal and vertical lift-off velocities. Eventually, we present formulas to describe the PDF of lift-off velocities of sand particles with regard to the influence of wind velocity and the diameter of sand particles in the saltation cloud above the sand bed with d m = 0.3 mm.
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