| Aquatic vegetation is an important part of landscape river,ecological rivers and artificial wetland construction.It plays an important role in creating a suitable living environment,building a specific pattern and purifying pollutants.In recent years,with the country’s increasing emphasis on water ecological civilization and environmental protection,harmonious development between human and nature,ecological landscape river design,constructed wetlands,river flood control,ecological environment restoration and treatment are current hot research issue,the research involved the th,eory of jet flow,and the vegetation flow plays an irreplaceable role in the restoration of the ecological environment.Therefore,Study on the mechan ism of pollutant transport and diffusion under the coupling of jet and vegetation water flow,not only has important theoretical significance,it also has great application value.Based on turbulent jet,Reynolds equation and dimensional analysis theory,combined with par;ticle image velocimetry(PIV)technology and planar laser induced fluorescence(PLIF)techniques,the turbulent diffusion,flow field and concen tration field are studied under three conditions of no vegetation,rigid vegetation and flexible vegetation in this paper.The research work includes the following points:(1)Characteristics of velocity and concentration fields under transverse single jet without vegetationFor the transverse single jet without vegetation,three-dimensional average velocity distribution,jet trajectory,Reynolds stress and turbulence intensity were studied under different jet flow rates.The transport and diffusion characteristics of pollutants in the jet were studied,including the time-average concentration field,concentration trajectory of the jet centerline,the half width of the jet concentration,distribution of the jet concentration along the longitudinal section and dilution variation of the jet concentration.The results showed at the jet curvature gradually increases as the jet transitions change from laminar flow to turbulent flow;flow velocity distribution along the jet axis was in accordance with the exponential distribution law;the maximum Reynolds stress and turbulence intensity occurred near the jet orifice;and the transverse jet had obvious influence on the open channel flow,especially on the transverse flow.The concentration distribution of the jet centerline also followed the exponential distribution;the half width of the concentration abided by the linear distribution;and the concentration variation of the jet at different sections conformed to the Gauss distribution law.(2)Characteristics of velocity and concentration fields under transverse single jet with rigid vegetationThe results of transverse single-hole jet under three kinds of rigid vegetation showed that the flow velocity of the jet axis still accorded with the law of logarithmic distribution,the turbulent structure of the open channel flow was changed with the existence of vegetation.and the longitudinal velocity of jet flow in submerged rigid vegetation was divided into vegetation layer and free layer,the velocity of vegetation layer was approximately accorded with the linear distribution,and the velocity of free layer was in line with the "J" distribution;The maximum Reynolds stress and turbulence intensity are at the top of the vegetation canopy;the higher the vegetation density,the smaller the velocity of the open channel water flow and the greater the turbulence intensity.Compared with no vegetation,the half width of the jet concentration,distribution of the jet concentration along the longitudinal section were the same as those under no vegetation condition.But the dilution of the jet was significantly attenuated compared to no vegetation.Compared with the single-row and double-row conditions,the concentration decreased the fastest and the dilution was the smallest under the diamond vegetation condition;degree of intrusion in open channels was higher than that without vegetation for transverse jet,which reflected the retardation effect of vegetation on the transverse jet.(3)Characteristics of velocity and concentration fields under transverse single jet with flexible vegetationThe hydrodynamic and concentration field characteristics of a transverse single-hole jet were studied under two kinds of flexible vegetation conditions.The results showed that due to the different kinds of flexible vegetation,the velocity distribution of jet appeared vortices of different sizes in the initial,bending and diffusive sections.The longitudinal velocity distributions of the jet in the submerged flexible vegetation stream were different owing to the different heights and densities of two kinds of flexible vegetation.The longitudinal velocity distribution of vegetation layer was approximately linear and the velocity distribution above vegetation layer was logarithmic.Compared with the curvature of the jet axis without vegetation and rigid vegetation,the velocity distribution of the jet axis was in line with the exponential distribution.It was found that degree of intrusion of the jet axis was the highest under flexible vegetation,followed by the rigid vegetation and the minimum under no vegetation.The maximum Reynolds stress of the two kinds of flexible vegetation were at the top of the vegetation canopy.But the turbulent intensity of the broadleaf vegetation appeared at the water surface.The half width of the jet concentration,distribution of the jet concentration under flexible vegetation along the longitudinal section were the same with those under no vegetation and rigid vegetation,and the dilution on the jet trajectory was linear with the square of y/lm.(4)Characteristics of velocity and concentration fields under transverse multiple jets with different vegetationThe effects of vegetation on the hydrodynamic characteristics,and diffusion characteristics were systematically analyzed through the transverse multiple jets in three different arrangements of rigid vegetation and two kinds of flexible vegetation.It was found that the presence of vegetation changed the velocity distribution of multiple jets in open-channel,which increased the contact area between the jet and the ambient water on both sides of the jet,and reduced the pollution intensity.The concentration distribution of the centerline of the multiple jets was the same as that of the multiple jets axis,which was consistent with the exponential distribution.The interaction between multiple jets,vegetation,porous jet flow and the concentration of pollutants were investigated and the hydrodynamic characteristics and concentration field characteristics were compared with those of the transverse single jet.It was clarified that the intrusion degree of the multiple jets axis had no relationship with the number of jets,and the jet was independent of the crossflow velocity.It was closely related to the arrangements of vegetation and the shapes of vegetation.It revealed the mutual coupling mechanism between vegetation,multiple jets and environmental water. |
PDF Full Text Request