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

TL;DR

This study uses direct numerical simulations to analyze how finite-size heavy particles are entrained in turbulent open channel flow, highlighting the roles of shear, vortices, and particle positioning in the entrainment process.

Contribution

It provides new insights into the mechanisms of particle entrainment, emphasizing the importance of flow structures and shear effects, with detailed force decomposition and flow structure analysis.

Findings

Lift force initiates particle entrainment during high-shear events.

Quasi-streamwise vortices are crucial for particle entrainment.

Particle location relative to flow structures affects entrainment frequency.

Abstract

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. Rotation-constrained 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.