The lagrangian coherent structure and the sediment particle behavior in the lock exchange stratified flows

Abstract

Shear-induced turbulent mixing in the stratified flow is widely known as an import driving force of the circulations of coastal water and transports of small particles such as suspended sediments and aquatic microorganisms. Therefore, this work mainly studies the influence of the vertical mixing and stratification on transports of gravitationally-falling sediment particles. In particular, Lagrangian Coherent Structure (LCS) of unsteady flow, which can be determined as finite-time Lyapunov exponents (FTLE) field of fluid particles is compared with the conventional Lagrangian particle trajectories which is obtained by solving particle motion equations. The shear-induced turbulent mixing of water mass and its effect on the transport of particles are investigated by a series of the lock-exchange flow experiments using the large eddy simulation (LES). When two water bodies have sufficiently high density difference, the Kelvin-Helmholtz billows are well developed along the lateral boundary and they trap the sediment particles which were initially located close to a transient layer at the half-depth of the channel. The simulation results also show that the local stretching rate of fluid particles dominates the transport of the gravitationally falling particles. In particular, the sediment particles are trapped within the relatively low FTLE field. The effect of sediment mass dropped the particles downward but not allows particles pass through the lower FTLE fields.