Investigation On Flow Characteristics And Fine Particles Deposition Of Open Channel Flow Within Vegetation

This dissertation mainly studied the flow dynamic characteristics in an open channel covered by the artificial vegetation and sediment deposition pattern inside the vegetation area.At present,a few scholars have focused on the vegetation flow,but the vegetation shape they consider is simple.Hence,it is necessary to study the influence of artificial vegetation with a novel shape on the flow.The research content in this dissertation includes the analytical solution of longitudinal velocity distribution in the lateral direction,turbulent structure and longitudinal dispersion coefficient in an artificial vegetated channel.Besides,the sediment deposition pattern inside the vegetation area is correlated with the flow characteristics in the open channel fully covered by the cylindrical rigid vegetation.Combining mathematical model and laboratory test,this research analyzed hydrodynamic characteristics of vegetation flow and its influence on the sediment deposition,which can be briefly divided into the following several aspects.When the artificial vegetation exists in the open channel,the longitudinal velocity in the vegetation area decreases due to the drag force of vegetation,and the longitudinal velocity in the non-vegetation area increases.Based on the depth-averaged Navier-Stokes equation,the drag force of vegetation is added into this equation.The transverse distribution of the longitudinal velocity can be obtained by solving this improved Navier-Stokes equation.According to the vegetation shape,the drag force of it is calculated by adding the force of each float.The measured velocity data is consistent with the predicted ones,validating the improved depth-average Navier-Stokes equation is efficient in predicting the velocity in vegetation flow.The artificial vegetation partially covering the open channel interacts with the flow and the flow structure develops along the river,which ultimately achieves stablility.This dissertation explores the longitudinal development process of vegetation flow,the flow structure in the lateral direction and its turbulence characteristics.At the leading edge of the vegetation area,the flow does not fully develop,with decreasing velocity inside the vegetation area and increasing velocity inside the non-vegetation area.The velocity difference in these two areas leads to the creation of the shear layer and vortex at the interface.The thickness of the shear layer and vortex structure increase gradually.After that,the flow is fully-developed,where the velocity does not change and the thickness of the shear layer keeps constant.By using the methods of spectral analysis and quadrant analysis,the dominant frequency of the vortex in the shear layer was obtained,which was a constant,and the vortex contributes to lateral momentum exchange between the vegetation and non-vegetation areas.Besides,spectral analysis demonstrates that the sizes of the dominant vortices can be classified into stem-scale(small-scale)and shear-scale(larger scale)vortices as conventional in canonical canopy flows.The length of stem-scale vortices is related to the diameter of the vegetation and the length of shear-scale vortices is related to the width of the vegetation width.The experimental findings here can provide theoretical guidance in engineering practice.The longitudinal dispersion coefficient influences the scalar transport in the flow,so it is important in chemical engineering,water pollution control,and ecological restoration.In the pipe and open channel flow,the longitudinal dispersion coefficient can be obtained by solving the convection-diffusion equation.However,it is difficult to obtain the longitudinal dispersion coefficient in the vegetation flow.The two-zone model is proposed to calculate the longitudinal dispersion coefficient in the partially covered open channel.Before using this model,the transverse distribution of longitudinal velocity needs to be known first,which can be obtained from the improved depth-average Navier-Stokes equation.In our experiment,Rhodamine concentration curves were measured and the LDC was obtained using the routing procedure.Results show that the measured LDC is consistent with the predicted one,thereby validating the accuracy and reliability of our proposed two-zone model.The interaction between the vegetation and flow tends to influence the sediment deposition or suspension.For different vegetation densities and upstream velocities,the velocity and turbulence kinetic energy in the fully covered vegetated channel are measured along the flow direction and the sediment deposition pattern inside the vegetation area also is measured.The sediment deposition pattern is correlated with the development of flow structure.Near the leading edge of the vegetation area,net deposition decreases to the minimum value.This is due to the increasing updraft and turbulence kinetic energy.Then,the updraft and turbulence kinetic energy decrease gradually,leading to the net deposition increases.In the fully developed region,the net deposition inside the vegetation area has a positive correlation with the vegetation density and has a negative correlation with the upstream velocity.A model for canopy-average turbulence kinetic energy was validated and used to explore the range of field conditions for which turbulence kinetic energy within a meadow would be reduced,relative to the bare bed.This would support the accumulation of fine organic material within the vegetation area,which is in favour of the growth of the vegetation.This model has certain guiding significance to the ecological restoration engineering using vegetation planting.