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Mohammad Omidyeganeh
Department of Mechanical and Materials Engineering Queen's University Kingston, ON, K7L 3N6, Canada

Ugo Piomelli
Dept. of Mechanical Engineering University of Maryland College Park, MD 20742, USA; Department of Mechanical and Materials Engineering Queen's University, Kingston (ON) K7L 3N6, Canada


We performed large eddy simulation of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900). The results are validated with simulations and experiments in the literature. A grid sensitivity analysis showed that simulations require approximately five million grid points to converge flow statistics. Large coherent structures can be observed. Large-scale streamwise vortices with sizes of the order of the flow depth are observed. These structures are generated due to the curvature of the bed, and result in re-organization of the streaky structure in the wall layer. Large horseshoe-like structures are generated as a result of the interaction between spanwise rollers and the wall. As they develop, they are tilted downward and attach to the surface by their tips. This creates the events on the free surface known as "boils" (upwelling motions observed at the water surface). The evolution of these structures towards the surface is illustrated and their signatures are studied in details.