The 3-D spread of saltation sand over a flat bed surface in aeolian sand transport

This paper investigated the 3-D motion of saltation sand by high-speed photography and stereo particle image velocimetry (SPIV). By the high-speed camera, the sand particle trajectories in the transverse plane near bed surface have been obtained. It could be found that the collision between the particle and the bed surface would in principle cause the transverse motion of the particle regardless of the change of the wind direction. Based on SPIV, The three-dimensional velocities of the sand particles in the wind–sand flow have been obtained by combining the velocity data from double CCD cameras. The three-dimensional velocity of the sand particle was resolved into three component velocities in the paper, i.e. the streamwise velocity u, the vertical velocity v and the transverse velocity w. The distribution of the transverse velocities w of the sand particles approximated symmetrical. The peak value of the PDF (probability density function) of velocity w fell down obviously with the increase of the wind speed when the sand sizes were equal to or less than 125 μm. When the sand sizes were larger than 125 μm, the peak value of the PDF of w was almost constant with the change of the wind speed. Although the increments of v with the increase of the wind speed were larger than that of w, the velocity w of the sand particle appeared to be much larger than its velocity v in general. Moreover, it was near one fourth probability that the quantity of the particle velocity w was one order higher than that of its velocity v. The inclination angle between w and u of the sand would be less than 60° and the inclination angle between the v and u would be less than 20° integrally.