2025 CSDMS meeting-096
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Large-Eddy Simulation of Bedform Evolution Driven by Turbulent Coherent Structures in an Oscillatory Flow
Jiaye Zhang,
University of Delaware Newark Delaware, United States. jiaye@udel.edu
In the nearshore environment, bedforms are prominent small-scale morphological features controlling hydrodynamic dissipation and sediment transport. Vortices are typically regarded as the primary mechanism driving sediment transport over vortex ripples with a steepness larger than 0.1 due to boundary layer separation. As these bedforms induced vortices must cascade into small turbulent coherent structures and eventually get dissipated, a research gap exists concerning the role of coherent structures on bedform evolutions. Here, we investigate the mechanism driving bedform evolution from an initially flat bed, for medium and fine sand ripples in oscillatory flows. Due to much lower steepness, the ripple-induced vortices are much weaker, and we hypothesize that turbulent coherent structures play a crucial role in sediment transport to initiate small bed features. In this numerical study, the Eulerian two-phase flow model, SedFoam, is utilized. The laboratory experimental scenarios from Perillo et al. (2014, Sedimentology) are modeled for medium sand ripple (d_50=0.25 mm) at mobility numbers varied from 10 to 60. By conducting a three-dimensional (3D) large-eddy simulation (LES), the generation and evolution of energy-containing turbulent coherent structures are resolved, as well as their effect on sediment transport. The simulation results demonstrate that the turbulent coherent structures, generated by shear flow in the turbulent boundary layer, are the dominant mechanism driving the initial formation of 3D small bed features, which eventually evolve into more organized symmetrical small ripples (SSR). The simulated time-dependent bedform characteristics, including ripple length and height, are further validated against measurements.
