Coherent Structures and Mass Exchange Processes in Channel Flow with Spanwise Obstructions

Abstract

This chapter analyzes coherent structures and mass exchange processes in channel flow with spanwise obstructions. Large eddy simulation (LES) is used to investigate three-dimensional flow phenomena induced by a pair of spanwise vertical obstructions (groynes) in a channel with lateral vertical walls and a flat bed. The main phenomena that govern the physics of this flow include the formation of a horseshoe vortex (HV) at the base of the upstream obstruction and of detached shear layers originating at the tips of the obstructions. The structure and spectral content of coherent structures inside the HV system are investigated. It is found that the flow in the turbulent HV region is characterized by a very sharp increase in the level of turbulence fluctuations which explains the initiation of scour in the case of a loose bed. The distribution of the bed shear stress is determined and analyzed relative to the capacity of the flow to entrain sediment. The mixing processes between the main channel and the embayment are modeled using a passive scalar. The decay of scalar mass in the groynes area corresponding to the mixing between the dissolved matter present initially in the embayment and the channel is measured. The global exchange coefficients corresponding to the initial and final stages of decay are also determined.