Research On Hydraulic Characteristics Of Open Channel Flows With Rigid Vegetation

In this dissertation,the hydrodynamics in open channel with four forms of rigid vegetation patches are discussed in detail with flume experiments and numerical simulation.Four vegetation forms are double-layer vegetation,discontinuous vegetation patches,suspended canopy and tree-like canopy.The flume experiments of double-layer vegetation flow were based on the Particle Image Velocimetry(PIV)system.Different flow rate,water depth and dowel arrangement are considered.A schematic profile of longitudinal velocity for double-layer vegetation flow is presented.The longitudinal velocity profile is nearly constant in the lower zone of each vegetation layer and increases in the upper zone with inflection points at the top of short vegetation layer and tall vegetation layer.The individual measuring points near the bed form the velocity spike.In the layer without vegetation,the longitudinal velocity follows the log-law.For the submerged single layer cases,the maximum of vorticity occurs at the top of short vegetation layer.The vorticity peak of staggered configuration is 5 to 6 times bigger than that of linear configuration.The staggered configuration is conducive to reducing bank erosion.For the submerged double layer cases,the vorticity and Reynolds stress,not influenced by dowel arrangement,at the top of tall vegetation layer are bigger than that at the top of short vegetation layer.The linear configuration is helpful for flood control and river bed protection.For discontinuous vegetation patches flow,different patch density,Reynolds number and gap length cases are conducted in the experiments and Large Eddy Simulation method.The evolutions of longitudinal velocity and Reynolds stress are related to patch density are Reynolds number.The hydrodynamics in open channel are more stable for long gap length case,which is also benefit to the flow adjustment.The longitudinal velocity in gap zone is much smaller than that in patches zone and backflow occurs in gap zone.According to the quadrant and spectrum analysis,the sweep is dominated in vertical exchange zone at the upper part of patches and ejection is dominated in free water layer.Two vortex scales,stem-scale and shear-scale,play a key role in the patches zone.The suspended canopy flow is simulated by the Rigid 3D Vegetation model based on Delft3D software.Two experimental data sets are used to verify the numerical model.And the results of numerical model are in good agreement with the data sets.The flow adjustment is divided into three zones(diverging flow zone,developing zone and fully developed zone)in the streamwise direction.The velocity local extreme zone in the gap beneath the canopy is found around the leading edge of the developing zone.The length scale of velocity local extreme zone and fully developed zone is significantly influenced by canopy density and gap thickness.The bed boundary layer thickness is linked to gap thickness(hg)and non-dimensional flow blockage parameter(Cdahc).The experiments of tree-like canopy,based on ADV,are conducted in the Laboratory of Applied Hydraulics,NTUA.The submerged tree-like canopy flow in the vertical direction consists of stem layer,tree layer and no-canopy layer.The longitudinal velocity is almost constant in the stem layer.Then it decreases firstly then increases with increment of depth in the tree layer.Strong fluctuation of turbulence intensity and Reynolds stress is found in the tree layer.The longitudinal velocity in no-canopy layer follows logarithmic law.The quadrant analysis recovers the tree layer is the sweep-dominated area and no-canopy layer is the ejection-dominated area.