Microscopic insights into local scour around a vertical circular pile under steady current: Coupled LES-CGDEM simulations

Abstract

Local scour around pile reduces its bearing capacity, jeopardizing the superstructure safety. Previous studies on this have primarily been based on continuum mechanics. Given that the scour process involves interactions between fluid-particle and particle–particle, the methods based on the continuum mechanics struggle to adequately account for collision effects between particles, while also failing to capture the migration process of soil particles. This study employs the coupled large eddy simulation (LES) and coarse-grained discrete element method (CGDEM) to conduct a detailed simulation of the local scour evolution around a vertical circular pile under unidirectional steady flow from a microscopic perspective, in which three flow intensities are considered in the simulation. It is found that as the flow intensity increases, the difference in scour depth upstream and downstream of the pile diminishes. Initially, the scour upstream of the pile is jointly controlled by scouring at the pile toe and soil erosion on the top slope surface, while it is dominated by pile toe scour in the later stage. The pile toe scour is associated with the multiple horseshoe vortices (HVs) upstream of the pile; the “NASA wall-mounted hump” downstream of the pile restricts further migration of the deposited sediment; an “unstable particles cluster” is formed on the sides of the pile, creating a preferential scour path. Particles upstream of the pile undergo a sequence of slow motion upstream, complex flow around the pile, accelerated movement downstream, and deposition, while particles on the sides of the pile experience slowly settling, accelerated motion, and deposition. The work further deepens the understanding of scour evolution mechanism.