thumbnail

Vortex trapping of suspended sand grains over ripples

Journal of Geophysical Research: Earth Surface
By: , , , and 
https://doi.org/10.1029/2023JF007620

Links

Abstract

Coastal hydrodynamics and morphodynamics integrate the effects of small-scale fluid-sediment interactions; yet, these small-scale processes are not well understood. To investigate sediment trapping by turbulent coherent structures or vortices, the transport of coarse sand over ripples was analyzed in a small-oscillatory flow tunnel with phase-separated Particle Image and Tracking Velocimetry. Results from one of the first direct measurements of vortex-trapped sand grains under oscillatory flows are presented. The vortices mobilized sand grains along the ripple slopes just prior to flow reversal and transported the suspended sediment grains. During several flow cycles, some sand grains were temporarily trapped in the vortex, prescribing semi-circular trajectories off-center from the vortex core in quadrants of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the horizontal sediment grain velocity with the horizontal fluid velocity yielded a linear relationship with a slope of 0.87. The vertical grain velocities also varied linearly with the vertical fluid velocity with a slope of approximately 1 and an offset of −0.08 m s−1. The offset is close to the still water settling velocity for coarse sand grains, as hypothesized during vortex trapping. Additionally, estimates of the off-center distance, between the centers of the semi-circular sediment paths and vortex cores, compared well with the ratio of the settling velocity to the radian frequency of the vortex yielding a linear regression slope of 0.99. Improved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in model predictions of large-scale coastal hydrodynamics and sediment transport.

Suggested Citation

Frank-Gilchrist, D.P., Penko, A.M., Palmsten, M.L., Calantoni, J., 2024, Vortex trapping of suspended sand grains over ripples: Journal of Geophysical Research: Earth Surface, v. 129, no. 10, e2023JF007620, 17 p., https://doi.org/10.1029/2023JF007620.

ISSN: 2169-9011 (online)

Publication type Article
Publication Subtype Journal Article
Title Vortex trapping of suspended sand grains over ripples
Series title Journal of Geophysical Research: Earth Surface
DOI 10.1029/2023JF007620
Volume 129
Issue 10
Publication Date October 17, 2024
Year Published 2024
Language English
Publisher American Geophysical Union
Contributing office(s) St. Petersburg Coastal and Marine Science Center
Description e2023JF007620, 17 p.
Additional publication details