Hydrodynamic Characteristics And Pollutant Convection-diffusion For Open Channel Flow With Submerged Vegetation

The existence of vegetation in natural rivers not only enhances the self-regulation ability of aquatic ecosystem,purifies water quality,protects water source and reduces soil erosion,but also forms flow resistance,rises water level and reduces discharge capacity.To study the influence of different types of vegetation on flowing water and material transportation,this paper studies the hydrodynamic characteristics of flow through submerged vegetation with tree-like canopy,which combines theoretical analysis with flume experiment.Based on the velocity model of tree-like vegetation,the velocity distribution of submerged truncated rigid vegetation flow or submerged elliptical flexible vegetation flow is further studied.And the pollutant convection-diffusion for open channel flow with submerged rigid vegetation is analyzed with previous experimental data.The flow through submerged vegetation with tree-like canopy whose ecological features change vertically,generates a large number of coherent structures.These vortex structures affect flow structure.The submerged tree-like canopy flow is vertically split into a stem layer,a canopy layer and a free water layer.The stem layer is subdivided into a near-bed area and a mixing area.Based on the flow resistance formula,the analytical velocity model of the stem layer and the canopy layer can be obtained from the corresponding hydraulics equilibrium equation.The velocity in the free water layer follows classical logarithmic law.In the flume experiment,plastic crown and round bar are used to simulate the tree-like canopy model,and distribution of velocity under different working conditions is measured by three-dimensional Acoustic Doppler Velocimeter.The results show that the experimental data and the calculated results are in good agreement,and the model is reasonable and effective.From the experimental results,the vertical profiles of velocity near the bed in the submerged tree-like canopy flow is close to a fixed value.The blending zone which is adjacent to the canopy layer,has strong turbulence intensity and its velocity reduces as the water depth increases.The velocity of the canopy layer is negatively correlated with vegetation diameter.The velocity in the free water layer accords with logarithmic distribution.Reynolds stress has a bulge near the flat bottom and is nearly zero in the near-bed area.The maximum appears in the canopy layer or at vegetation-water interface,which means that turbulence and energy exchange of the two parts is intense.The turbulence intensity and Reynolds stress in this flow have the same tendency,and its distribution is approximate to "3" type.Based on the velocity model of tree vegetation,the velocity structure of submerged truncated rigid vegetation flow or submerged elliptical flexible vegetation flow is analyzed systematically.The interaction range of vortices and turbulence characteristics in submerged vegetation flow are studied.Results prove that the velocity expression of the canopy layer can be used for the vegetation layer in these flows,while the free water layer conforms to the logarithmic law.By using a series of experimental data,the good agreements show that velocity in these flows are "s" curve distribution,and the velocity of the vegetation layer increases uniformly,the velocity of the free water layer satisfies logarithmic distribution.And the boundary between K-H vortex and wake vortex is obvious in the vegetation layer of the submerged truncated vegetation flow,so the vegetation layer is divided into a wake layer and a mixing layer.The vertical turbulent diffusion and longitudinal dispersion characteristics of pollutants in the flow of submerged cylindrical rigid vegetation are studied.Based on the velocity characteristics of the submerged rigid vegetation flow,a three-zone mathematical model of vertical turbulent diffusion coefficient and longitudinal dispersion coefficient are established.By comparing and analyzing the existing experimental results,it is proved that the two models can reflect the reality well.