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Reconsidering aeolian transport in the context of near-bed wind tunnel measurements in 3D

Patrick O'Brien, Cheryl McKenna Neuman
Trent University, Peterborough ON, Canada

Geomorphologists have long recognized that sand transport by wind is fundamentally three-dimensional (3D) in nature.  Considerable effort has been invested in studying the spatial components of the transport process, as for example, in CFD simulation of the airflow structures surrounding such bedforms, and in observation of the horizontal instability of sand streamers. At the particle scale, however, the perspective remains primarily two-dimensional. The core objective of the present study was to obtain simultaneous measurements of all three components (ux, uz, uy) of the motion of individual sand grains in a wind tunnel experiment. In order to study the near-surface saltation cloud in steady state, the entire 12 m long floor of the Trent tunnel was filled with well-sorted coarse quartz sand (median particle diameter, d = 550 μm). The free stream wind speed (U) threshold for fluid entrainment of these particles is 6.5 ms-1 (u*t = 0.30 ms-1), so with the test speed set to 8 ms-1, u*/u*t = 1.3. At the downwind location (x = 10.5 m) of the Particle Tracking Velocimetry equipment the bed elevation was unchanged throughout the experiment, confirming that the flux divergence was zero. The PTV-EPAS system consists of a 1 Watt, 532 nm Nd-Yag laser and a pco.dimax HDTM high speed camera. Grayscale images of illuminated sand particles are captured passing through a 1.5 mm thick light sheet that intersects the bed surface over a distance of ~120 mm.  In order to measure particle trajectories aligned at an angle to the mean airflow, the light sheet orientation (θ) was adjusted from 0o to 60o in 5o increments. The camera was repositioned for each angle to maintain a perpendicular line of sight. The results reveal that less than 1/8th of the sampled particles travelled directly along the path of the mean air flow.  However, 95% were contained within 45o.  The increasing spanwise angle (θ) of the flight path was found to systematically decrease the total velocity of a ‘typical' particle, as well as increase its launch/impact angle, thereby altering the relative magnitudes of ux, uz, uy.  The observed decline in the proportionate particle-borne kinetic energy with increasing θ, however, appears to be driven primarily by the waning particle count and not speed.  At high angles of departure from the mean airflow, the primary mode of transport shifts gradually from saltation to reptation (splash), and indeed there is no clear distinction between the two modes. The measurements obtained in this study have important implications for the parameterization of emerging numerical models in 3D, as well as for understanding the inception and growth of small-scale aeolian bedforms in the context of particle diffusion.