No abstract

Stommel Retention Zones (SRZs) associated with wind-driven Langmuir circulation (LC) facilitate the subsurface retention of particles and thus affect processes such as oil spill dispersion, sediment suspension, oil-particle aggregation, and plankton distribution. Since the effect of SRZs on these processes is difficult to study in the field, we present a novel laboratory facility designed to recreate the counterrotating vortex pair flow pattern associated with small-scale SRZs of various strengths in two dimensions (2D). We then inject an oil jet into this facility to study buoyant oil droplet retention in these SRZs. 2D particle image velocimetry and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation are used to characterize the flow. Large-scale oil droplet trajectories in the SRZs are visualized, and oil droplet statistics in the downwelling region and vortex core are measured using high-magnification brightfield imaging and a droplet detection algorithm. Values of maximum velocity (7.7-32.2 mm s À1 ), turbulent kinetic energy (10 À6 to 10 À4 m 2 s À2 ), and turbulent kinetic energy dissipation rate (10 À7 to 10 À5 m 2 s À3 ) in the downwelling region of the facility fall within the range of values observed in the field. Flow field strength strongly influences oil droplet trajectories, the shape and size of the SRZ, and oil droplet statistics over time, with stronger flows retaining larger oil droplets in the downwelling region over longer periods of time. Indeed, droplets of almost 400 μm diameter are observed in the downwelling region after almost 10 min for the strongest flow. The facility may be used to study the behavior of microplastics, sediments, and zooplankton in LC in the future.

In this study, the effect of the geometry modification in the physical model (step height) on flow factors in the stilling basin is investigated numerically. The Navier-Stokes equations were used to solve the flow field; the Reynolds stress turbulence model was used to model Reynolds stresses, and the volume of fluid method was used to determine the surface flow profile. Momentum and turbulence equations have been solved by the finite volume method. The results indicate that when the height of the steps in the stilling basin increases, the flow depth increases, and eventually the flow velocity decreases. When the height of the steps decreases, the flow velocity increases, and the flow depth decreases. The results of the numerical model are in good enough agreement with experimental results.