PTV measurement of the spanwise component of aeolian transport in steady state

•EPAS-PTV development is suitable for studying airflows saturated with sand.•A majority of grains in aeolian transport are not strictly aligned with the airflow.•Particle speed drops and trajectory angles steepen with increasing spanwise angle.

This paper outlines and validates an improved particle tracking technique (PTV-EPAS) with automated trajectory detection capabilities, and then reports on a novel set of wind tunnel experiments aimed at measuring all three velocity components simultaneously. In order to study a fully adjusted particle cloud, the entire floor of the tunnel was filled with quartz sand (median diameter 550 μm) and the freestream velocity set to 8 ms−1 at an elevation of 0.35 m, above the threshold for particle entrainment at 6.5 ms−1. This produced a friction velocity (u∗) of ∼0.38 ms−1 with u∗/u∗t = 1.3. Measurement of particle trajectories aligned at a spanwise angle (θ) relative to the mean airflow along the center-line of the wind tunnel involved incrementally adjusting the light sheet orientation from 0° to 60°. Three replicate experiments were carried out for each of 13 angles. Only 12% of all 2 × 105 trajectories sampled were strictly aligned with the mean streamwise air flow, while 95% were contained within 45°. As θ increases, a greater proportion of the particle transport consists of slow moving ejecta that ascend from and then impact the bed surface at higher angles than observed for saltation.