Large Eddy Simulation Of Turbulent Flow And Bed Morphodynamics Of Open Channel With Vegetation

Vegetation is ubiquitous in nature aquatic environment.Aquatic plants play important roles in water conservation,channel conveyance capacity,flood controlling as well as river evolution.In this paper,the in-house code Complex Geometry Large Eddy Simulation(CGLES)is used to simulate the hydrodynamic characteristics and bed morphodynamics of two common distribution patterns of aquatic vegetation.The main work and results are as follows:(1)The detailed impacts of the deflection and shape of submerged vegetation on turbulent structure are studied,the results show that the vertical velocity increases significantly in the wake zone of the bending vegetation and is 2?3 times that of erected vegetation;The deflection of vegetation plays a dominant role in determining the penetration depth of Kelvin–Helmholtz(K-H)vortices,specifically,the penetration depth increases by 50.1% after vegetation deflecting.(2)After the forming of local scour around vegetation elements,the vertical profile of averaged streamwise velocity changes from “J-shape” to “S-shape”.The existing of canopy can obviously lower the critical shields number of incipient,the critical value is around 0.5 times that of channel flow without vegetation.When the bed shear stress is high enough,the “dynamic flattening” phenomenon could be observed in sandy bed.(3)the streamwise velocity decreases first and then increases in the wake region of bank-attached vegetation patch.In the immediate downstream region of vegetation patch,the element-scale vortices are dominant,the Reynolds stress and near bed turbulent kinetic energy is lower.In the downstream of the low velocity zone,the patchscale vortices are dominant,the Reynolds stress and near bed turbulent kinetic energy is higher.(4)When the patch scale is fixed,there are some characteristics of the bed morphology:(1)evident scour zone exists in the upstream region of vegetation patch,followed with deposition zone.however,no-distinct scour is observed in the downstream region of vegetation patch.(2)The bed morphology effects significantly by the channel's aspect ratio and vegetation density.