Large-eddy simulation of two-dimensional dunes in a steady, unidirectional flow

Abstract

We performed large-eddy simulations 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) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The results are validated by comparison with simulations and experiments in the literature. The flow separates at the dune crest, generating a shear layer that plays a crucial role in the transport of momentum and energy, and the generation of coherent structures. The turbulent kinetic energy budgets show the importance of the turbulent transport and mean-flow advection in the bulk flow above the shear layer. In the recirculation zone and in the attached boundary layers production and dissipation are the most important terms. Large, coherent structures of various types can be observed. Spanwise vortices are generated in the separated shear layer due to the Kelvin–Helmholtz instability; as they are advected, they undergo lateral instabilities and develop into horseshoe-like structures, are tilted downward, and finally reach the surface. The ejection that occurs between the legs of the vortex creates the upwelling and downdrafting events on the free surface known as “boils.” Near-wall turbulence, after the reattachment point, is affected by large streamwise Taylor–Görtler vortices generated on the concave part of the stoss side, which affect the distribution of the near-wall streaks.

Keywords:

• large-eddy simulations
• environmental turbulent flows
• turbulent boundary layers

Acknowledgments

[Acknowledgements]

This research was supported by the Natural Sciences and Engineering Research Council (NSERC) under the Discovery Grant program. The authors thank the High Performance Computing Virtual Laboratory (HPCVL), Queen’s University site, for the computational support. MO acknowledges the partial support of NSERC under the Alexander Graham Bell Canada NSERC Scholarship Program. UP also acknowledges the support of the Canada Research Chairs Program.