Numerical analysis of flow structures and bed entrainment in turbulent open-channel flow
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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.
Authors
Ma, JianminCollections
- Theses [4114]