Numerical analysis of flow structures and bed entrainment in turbulent open-channel flow

Abstract

The results from a Direct Numerical Simulation (DNS) and a Large Eddy Simulation (LES) are employed to study the large-scale coherent structures and bed entrainment in the turbulent open-channel flow. The gravel bed is represented by a hexagonal arrangement of uniform spheres. The large-scale coherent structures are composed of a group of quasistreamwise vortices and asymmetric hairpin vortices. The meandering structures are shown to be longer than the length of the computational box, more than 20 times the effective flow depth in this study, and the width tends to be one order of magnitude smaller than the length. The signature of the large-scale motion is elongated local maximum of streamwise velocity. It is also found that these structures contribute substantially to both of the Reynolds Stress (RS) and the Turbulent Kinetic Energy (TKE). The entrainment of bed gravels is investigated by the three-dimensional analysis of the relationship between near-wall coherent structures and the force moments exerted on the particles. It is found that the spanwise drag moment (MD2) is of the same order of magnitude compared with the streamwise drag moment (MD1). The majority of MD2 originates from pressure whilst the viscous force plays as an important role as pressure for MD1. The contributions of the forces at different heights of the particle to MD1 and MD2 are explored. The quasi-streamwise vortices are strongly associated with MD2 and the ejections are shown to be more favorable for bed entrainment than the sweeps in this bed condition.