Large Eddy Simulation of a sediment particle under entrainment conditions.

A Large Eddy Simulation of a fully developed turbulent open-channel flow with a Reynolds number of R e = 71,755 was performed to study dynamics of coherent structures in the vicinity of a stationary sediment particle. The sediment particle was positioned at the bottom on a smooth channel bed. The impulse concept was employed by computing three-dimensional hydrodynamic forces acting on the sediment particle in order to predict the onset of particle motion. Results of quadrant analyses at high impulse events yielded mostly outward and sweep events. Auto- and cross-correlation of the hydrodynamic forces and the flow field were studied and discussed. An identification of coherent structures at high impulse events showed that large-scale, long-lived hairpin-like vortices whose bed-normal lengths are larger than ≈ 100 viscous units were resulting in an increase of lift and drag forces simultaneously, facilitating initiation of particle motion. [ABSTRACT FROM AUTHOR]