Turburence structure and coherent motion in vegetated canopy open-channel flows

Abstract

A lot of aquatic plants are observed in actual rivers and they have significant effects on hydrodynamic characteristics such as the velocity distributions, turbulence and coherent structure, as well as mass and momentum exchanges between the vegetated and non-vegetated zones. Aquatic vegetation canopies are responsible for water quality, nutrient and particulate removal, and can reduce turbidity and sediment transport significantly. In the present study, we investigated turbulence structures and coherent motion in vegetated canopy open-channel flows, on the basis of non-intrusive LDA and PIV measurements as well as LES calculations. Section 2 deals with the theoretical consideration on submerged canopy open-channel flows, in which the importance of double-averaging techniques for both time-average and space-average was highlighted. Section 3 describes experimental techniques. Section 4 deals with mean flow structure within and over the canopy, in which the dispersive properties were highlighted. Section 5 considers turbulence structure, in which Reynolds stress, turbulence intensities and turbulent kinetic energy (TKE) budget were discussed. Further, quadrant conditional analysis and two-point space-time correlation revealed the importance of coherent structures near the vegetation edge. Section 6 examines the effects of submergence depth on coherent structure and compares them with terrestrial canopy flows. The whole flow region was divided into three sub-zones, i.e., the emergent zone, the mixing-layer zone, and the log-law zone. In particular, coherent eddies such as sweeps and ejections were highlighted on the instantaneous contour plane of Reynolds stress and vorticity. Finally, Section 7 deals with some LES simulations, which predicted large-scale coherent eddies reasonably in aquatic canopy flows.