Entrainment of single particles in a turbulent open-channel flow: a numerical study

This paper investigates erosion events in a turbulent open channel flow laden with monodisperse spherical particles. The data were generated in a previous study using direct numerical simulations with a phase-resolving immersed boundary method. The particles have a mobility below their nominal threshold of incipient motion and settle onto the rough bed that consists of a hexagonally packed layer of spheres with the same size. Conditioned averaging is employed to extract the characteristic features of an erosion event, defining a suitable criterion for detection and accounting for possible asymmetry of flow structures. The highly resolved dataset provides detailed insight into the key-mechanisms of erosion. The results show that a particle collision, together with a subsequent sweep event on time scales of several bulk units, is responsible for the erosion.