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.