Experimental Study On The Effect Of Flow Turbulence On The Flocculation Settling Form And Settling Speed Of Cohesive Sediment In The Three Gorges Reservoir Area

The experimental study on the influence of flow turbulence on the morphology and settling velocity of fine sediment is helpful to deeply understand the sedimentation characteristics of fine sediment in the Three Gorges Reservoir area,which is of great significance for water conservancy regulation,safety maintenance,waterway regulation and pollution control.However,the technology of in-situ real-time measurement of cohesive sediment flocculation and settling velocity is not mature enough and needs further improvement The related laws of sediment movement can be better studied.Therefore,this paper designs an image acquisition,processing and analysis system to improve the clarity of floc separation chamber,and uses experimental simulation method to study the effect of turbulence on flocculation settlement morphology and settling velocity of fine sediment,analyzes the change law of particle morphology(represented by particle size and fractal dimension in this paper)with turbulent shear rate,and defines the variation rule of sediment floc settling velocity with turbulent shear rate Law.The results and conclusions are as follows:(1)In order to solve the problem of flocculation and settlement of viscous sediment under the condition of turbulence,this paper developed an advanced,high-definition and accurate turbulence flocculation and settlement device,PTV image acquisition system.The device and system provide isotropic uniform turbulence through the longitudinal vibration grid,and use the floc separation chamber in the settlement device to separate the sediment floc,so as to reduce the interference of background value The accuracy of PTV image acquisition system is guaranteed,and the high-definition in-situ observation of sediment flocs is realized,which overcomes the difficulty of clear observation of sediment flocs,and lays a reliable foundation for further image processing.(2)PTV(particle tracking)is designed The system is based on Image J software,using the software function and the PTV algorithm calculation program of floc particle size optimization,fractal dimension and settling velocity.The accuracy of the calculation value of the system is verified under the condition of still water and non flocculation,and the sediment morphology and sedimentation during sediment settlement can be measured and analyzed correctly Speed.(3)Based on the analysis of the change of sand particle size and fractal dimension of Changshou,Zhongxian and Fengjie samples with shear rate under the condition of stable flocculation in the flocculation sedimentation test,the conclusion is as follows:(1)the change of cohesive sediment with shear rate in the Three Gorges Reservoir area increases first,and when the shear rate increases to the critical shear rate,it is also 20s^-1-25 s^-1 The particle size will reach the maximum value between S-1 and then decrease with the increase of shear rate.(2)In Changshou,Zhongxian and Fengjie,the flocculation degree of cohesive sediment is generally between 5.7-8.2 when flocculation is stable.Meanwhile,the flocculation degree will first increase and then decrease with turbulent shear,and the critical turbulent shear rate is also 20s^-1-25s^-1.(3)The three-dimensional fractal dimension of the cohesive sediment in Changshou,Zhongxian and Fengjie will reach a certain value with the rapid increase of shear rate first,then tend to be stable or slightly decreased,and the two-dimensional fractal dimension will also increase rapidly first and then slowly.(4)The fractal dimension between flocs decreased with the increase of particle size,while the three-dimensional fractal dimension(spatial density)and two-dimensional fractal dimension(projection regularity)of large flocs decreased with the increase of particle size.(4)It is found that the average settling speed and the average settling speed of the median particle size increase with the increase of the shear rate in the flocculation stable state,and decrease with the increase of the shear rate until the critical shear rate is reached,and the critical shear rate is 20 s^-1-25 s^-1 Between.Generally speaking,this study first provides an effective measurement tool for the subsequent study of cohesive sediment flocculation settlement morphology and settling velocity.Secondly,the influence of turbulence on particle size and fractal dimension and the influence of particle size and fractal dimension on settling velocity are studied clearly.