Experimental Study On Hydrodynamic Characteristics And Pollutant Concentration Field Of Open Channel With Flexible Vegetation

Aquatic vegetation is one of the important components of natural rivers and an indispensable part of ecological rivers.Due to the existence of vegetation,the characteristics of river flow and the diffusion of pollutants have become more complicated,and there have been many problems to be studied.In this paper,a straight channel test system is used to simulate river channels,plastic artificial water plants are used to simulate natural vegetation,and particle image velocity meter(PIV)and planar laser induced fluorescence(PLIF)techniques are used to study the effects of vegetation on open channel flow structure and pollutant concentration field.The effects of changes in vegetation density and submergence on the water level,velocity,Reynolds stress and turbulence intensity of vegetation-containing water flow are analyzed in detail.The effects of concentration half-width,etc.,mainly include the following research results:(1)The presence of vegetation affects the water level of the open channel.The water level rises in the front section of the vegetation area;the water level in the middle and rear sections of the vegetation area gradually decreases;the water level in the outlet section gradually rises due to the phenomenon of water jump.Controlling other factors unchanged,the water level increases with the increase of vegetation density and submergence,and the increase of submergence decreases the slope value of the water surface.The change of submergence degree has a greater impact on the water level.(2)The longitudinal velocity of water flow with submerged flexible vegetation can be divided into three zones along the vertical direction.Velocity zoning is related to vegetation shape.The vertical flow velocity under different vegetation densities is different from the top of the vegetation canopy(1.0z/hv).The flow velocity above the vegetation canopy increases with vegetation density,and the flow velocity below the vegetation canopy increases with vegetation density While decreasing.Controlling other factors remains unchanged,and the longitudinal flow velocity decreases with increasing submergence.(3)The peak values of Reynolds stress and turbulence intensity are located near the top of the vegetation canopy.As the vegetation density increases,the peak values of Reynolds stress and turbulence intensity increase.As the submergence degree increases,the peak value of Reynolds stress increases,while the peak value of turbulence intensity changes less.(4)The time-average concentration of pollutants on the top of the vegetation canopy is distributed symmetrically in a Gaussian shape.With the increase of submergence degree,the time-averaged concentration of pollutants decreases both horizontally and vertically,the diffusion along the direction of water flow is accelerated,and the distance required for complete mixing is shortened.With the increase of vegetation density,the time-averaged concentration of pollutants in horizontal and vertical directions decreases,the diffusion of pollutants along the flow direction accelerates,and the distance required for complete diffusion of pollutants decreases.(5)The vertical and horizontal density half-widths increase linearly along the way.As the submergence increases,the vertical concentration half-width gradually becomes larger than the lateral concentration half-width.As the submergence degree increases,the turbulent fluctuations of the vertical vegetation canopy shear layer increase,and the vertical pollutant diffusion dominates.With the increase of vegetation density,the vertical concentration half width bz1/2 and the horizontal concentration half width gradually increased by1/2.