Investigating the mechanism by which finite-size heavy particles are entrained in turbulent open channel flow over a smooth surface

The dynamics of entrainment of finite-size heavy particles in a turbulent open channel flow over a smooth surface are analyzed. Three types of simulations, namely with freely moving, rotation-constrained, and spanwise-motion-constrained particles, were conducted using particle-resolved direct numerical simulations. With the aid of a relative velocity suitably defined in the vicinity of the finite-size particle, we decompose the hydrodynamic force into drag and lift contributions and evaluate the local wall-normal shear rate around the particles. By means of coherent structure eduction techniques, we investigate flow structures before and during lift-off events. Rotationconstrained simulations revealed the insignificance of particle rotation in the entrainment mechanism. Spanwise-motion-constrained simulations revealed the importance of particle location with respect to flow structures with apparent changes in entrainment frequency, duration of the entrainment process, wall-normal shear around the particles, and distance to the nearest vortical structures during lift-off. The contribution of lift to the wall-normal force is found to be responsible for the initiation of particle entrainment, which is induced by a high-shear event associated with fast-moving fluid. The presence of quasi-streamwise vortices is shown to be an important ingredient for the entrainment of particles into the bulk flow. The results show that, at marginal Shields number values, a high wall-normal shear rate and the proximity of an intense quasi-streamwise vortex are essential elements of the entrainment mechanism.