RECENT EXPERIMENTAL STUDIES ON THE SALTATING SAND PARTICLE TRANSPORT AND WIND-SAND INTERACTION IN SALTATION
Authors: Wei Zhang and Sang Joon Lee
Abstract: Saltation is the primary transport mode of wind-blown sand particles, accounting for about 75% of total sand transport through saltation, suspension and surface creep. It provides momentum that drives the other two sand transport modes. The impact event of saltating sand particles is the major cause of wind erosion and damage to the ground surface. Therefore, it is essential to understand the basic physics in saltation process for the prevention of wind erosion. The mechanism involved in saltation has not been entirely clarified, which prevents establishing accurate theoretical models for predicting saltating sand transport. In particular, complex interactions among the saltating sand particles, the particles on the ground surface and the turbulent air-flow have not been yet fully understood owing to lack of experimental data measured simultaneously for the wind and wind-blown sand. This chapter presents our recent experimental studies on the fundamental issues of sand particle transport and the wind-sand interaction in saltation. Various state-of-the-art flow measurement techniques were applied to comprehensively examine three different types of natural sand; two samples collected from the Pohang beach (d=200-300 μm and 300-500 μm) in South Korea and the other from the Taklimakan desert (d=100-125 μm) in China. Firstly, high-speed photography was used to capture images of the saltating sand particles at 2000 fps (frames per second), which resolved the particle motion adjacent to the surface. The saltating particle trajectories were reconstructed and the physical quantities characterizing the ejection and impact events were statistically analyzed. Secondly, instantaneous velocities of the saltating sand particles were obtained using the particle tracking velocimetry (PTV) method. The particle resultant velocity, concentration and the streamwise mass flux were evaluated as a function of height. Finally, the velocity fields of wind and wind-blown sand particles were simultaneously measured based on a digital phase mask technique. The PTV and particle imaging velocimetry (PIV) were used to extract the velocity field information of dispersed sand particles and the surrounding wind flow, respectively. With the state-of-art advanced flow measurement techniques, this systematic experimental study shed new lights on the complicated saltation motion near the sand bed surface, and will be helpful in enhancing formulation of theoretical models and development of effective control measures of wind erosion.