The Nature of Large-Scale Coherent Structures as a Source of Saltation Unsteadiness
1, Bailiang Li2, Jean Ellis3, Eugene Farrell4, Robin Davidson-Arnott5, Andreas Baas6
1University of Alabama, Tuscaloosa, Alabama, USA, 2Xi'an Jiaotong-Liverpool University, Lake Higher Education Town, China, 3University of South Carolina, Columbia, South Carolina, USA, 4National University of Ireland Galway, Galway, Ireland, 5University of Guelph, Guelph, Ontario, Canada, 6Kings College London, London, UK
Unsteadiness in aeolian saltation, and its manifestation in spatial variability of sand transport, has been described in several field studies and attributed to several mechanisms (e.g., Ellis et al., 2012). These studies have focused on variability over relatively short time and length scales. Observations from a beach revealed quasi-periodic fluctuations, at periods of approximately 130 s, in spanwise flow with consequent changes in the saltation field.
Measurements were obtained from a long-fetch (200+ m), low-angle (<2⁰), unobstructed intertidal zone near Jericoacoara, Ceará, Brazil, in October 2011. Instruments included a spanwise array of four RM Young ultrasonic anemometers mounted 1.00 m above the sand, and sets of Wenglor particle sensors and miniphones installed 0.10 m above the bed. A video camera was mounted downwind of the array to record spatial variability, especially linked to streamers, in saltation. The video showed that over a 36 minute interval there were 17 instances of pronounced, spanwise wind motion, with individual events lasting more than 60 s. With the measured wind speeds of about 10 ms-1, this implies coherent structure length scales of 600 m. Spanwise lengths are estimated to be of the order of 10 m. Using an into the wind perspective, the rightmost anemometer indicated that coherent structures from the right of the array were accompanied by upward vertical velocities, +w, whereas those from the left corresponded with -w. On the left side of the array, structures from the left had +w and those from the right had –w. Near the center of the array, there was no correlated motion. These spanwise motions caused noticeable changes in saltation intensity.
We posit two explanations for this motion. First, these may be turbulent superstructures, as described by Hutchins and Marusic (2007): long, meandering coherent structures whose dimensions scale with the boundary-layer. These would be expected to display a random occurrence on the surface, a characteristic that does not fit our observations of relatively fixed locations. The second explanation is that these are vortices spun off upwind barchans. Estimates of the Strouhal Number (0.3 to 0.6) for representative dune dimensions (400 – 800 m), frequency of spanwise structures (0.079), and mean wind speed (10.35 ms-1) are large relative to values reported for bodies with simple geometry (≤ 0.2), although the former is not much so. Results for superstructures and vortices with alternative length scales will be reported.