Dissipative Particles Dynamics Simulation For Flocculation Dynamics Of Cohesive Sediment

In the estuary area where rivers and oceans meet,there is a large amount of cohesive sediment,about 25% of the coastline of China's mainland is a silty coast.The estuaries of the Yangtze,Yellow,and Haihe rivers are also dominated by cohesive sediments.The sedimentation and transport process of sand flocculation has an important impact on the process of scouring and silting of the port channel and the evolution of the estuary coast.Therefore,the research on the flocculation mechanism and sedimentation law of cohesive sediment is better for solving river regulation and maintenance,reservoir and port channel.siltation,the protection and treatment of aquatic ecosystems and the development of tidal flats and other issues of economic development have important guiding significance.Based on the self-designed cohesive sediment sedimentation test platform,using the image processing and programming software,combined with the adsorption isotherm equation,the formula of the cohesive sediment concentration rate containing the electrolyte concentration parameter is derived,and the dissipative particle dynamics method is used to the flocculation process of sediment was numerically simulated to simulate the flocculation morphology of cohesive sediment under hydrostatic and turbulent flow conditions,and the flocculation mechanism of viscous sediment under different factors was explored.The research contents and conclusions of this paper are as follows:(1)Electrolyte concentration is the main factor to affect the flocculation and settling velocity of cohesive sediment.Based on the sediment flocculation process analysis,the porosity and fractal dimension of flocs by defining threshold automatically and box-counting method by means of Image-Pro Plus and MATLAB programming are calculated.The porosity and fractal dimension of the flocs under the concentration of different sediment and electrolyte are determined.Moreover,a setting formula of flocculation with parameter of electrolyte concentration is derived based on isothermal adsorption equation,and it is showed that computed result by the formula in this paper is agree with the measured data.(2)Based on the dissipative particle dynamics method at mesoscale scale,the flocculation process of cohesive sediment under different charge and sediment concentrations was simulated by constructing the force field potential function,and the van der Waals force,electrostatic force and hydrogen were obtained.The formation process and three-dimensional structure of negatively charged sediment particles under the action of bond force.The results show:During the particle bonding process,the number of floc particles increases,and the non-bond energy and fractal dimension decrease.When the sediment negative charge is high,the floc fractal dimension is larger than the lower charge amount.Fractal dimension;The greater the amount of negative charge on the surface of the sediment particles,the longer the time for forming the flocs,and the greater the non-bonding energy;The change of non-bond energy inside the floc is the intrinsic cause of the change of the external shape of the floc;With the increase of cohesive sand concentration in the system,the number of flocs in the final system decreased,and the floc structure was more compact.Under different sediment concentration conditions,the non-bond energy decreased during the bonding process of cohesive sand particles.(3)Based on the dissipative particle dynamics method at mesoscale scale,a uniform isotropic turbulence model with force field potential function is established and the process of cohesive sediment flocculation and flocs morphology under different shear rate conditions are simulated.The results show :the appear time of flocculation decreases sharply at first with the increase of shear rate,then smooth and steady transition and then increases rapidly,and finally remains unchanged;When the shear rate is small,the non-bond energy decreases faster.As the shear rate increases further,the non-bond energy decreases gradually and the stability of the system gradually decreases.When shear rate is small,the number of final flocs decreases with the increase of shear rate,the morphology of flocs are more dense,and the fractal dimension increases gradually.With the further increase of shear rate,the number of flocs increased,accompanied by the appearance of multiple small flocs,the fractal dimension gradually decreased,and finally remained at about 1.12.The flocs have dynamic processes of collision,flocculation and crush in a shear condition.