Experimental Study On The Formation And Rheological Characteristics Of Flocculated Fluid Mud Layer In Large Reservoirs

Reservoir fluid mud is a layer of highly sandy water that exists near the bottom of the reservoir and is mainly formed by the flocculation and deposition of cohesive fine particles of sediment.Its physical and chemical properties are different from ordinary low concentration sandy water bodies,and it has strong fluidity,which can directly affect the reservoir sedimentation morphology and sediment discharge.Previous studies have paid more attention to the characteristics of fluid mud in harbors,waterways,and coasts,but few have focused on the formation and characteristics of reservoir fluid mud.Research on the formation of reservoir fluid mud layer is a prerequisite for understanding the laws of reservoir fluid mud movement,and can provide theoretical support for reducing sedimentation and optimizing reservoir operation.In this thesis,considering the sediment gradation and mineral characteristics in the Three Gorges Reservoir area,model sand was configured.Starting from the key process of the formation of fluid mud in the reservoir,cohesive sediment flocculation experiments in turbulent water bodies and floc deposition experiments under low turbulence conditions were carried out.The effects of different factors on sediment flocculation were analyzed,and a formula for sediment floc sedimentation velocity in turbulent water bodies considering fractal dimensions was constructed.By Collecting flocculated sediment,conducting rheological tests,a rheological equation of fluid mud considering the impact of flocculation had constructed.The main work and results are as follows:（1）The effects of turbulent shear and initial sediment concentration on the flocculation development of cohesive sediment were studied.A flocculation experiment of cohesive sediment in turbulent water was conducted,and the morphological evolution of cohesive sediment flocs was photographed using a PTV particle tracking system.The results show that there is an optimal turbulent shear rate that allowed the development of the flocs to reach a peak,and the optimal turbulent shear rate ranges from G=12s^-1to 18s^-1.When the optimal turbulent shear rate is lower,the median particle size of the flocs increases with the increase of the shear rate.When the optimal turbulent shear rate is higher,the median particle size of the flocs decreases with the increase of the shear rate.On the one hand,under low turbulence conditions,the median particle size of flocs first increases and then decreases with the increase of initial sediment concentration.while on the other hand,under high turbulence conditions,the median particle size of flocs decreases with the increase of initial sediment concentration.Scanning electron microscopy（SEM）images of the flocs during flocculation equilibrium show that flocculation can cause the basic particles of sediment to stack or overlap to form a strip like or ellipsoidal floc network.The fractal dimension of the flocs obtained using fractal theory ranges from 2.70 to 2.82.（2）The formula for settling velocity of sediment flocs in turbulent water was derived.Based on the fractal theory of flocs and the principle of force balance for single particle Sediment flocs,the resistance of particles was refined into viscous and inertial forces.The floc density equation was introduced to modify the particle gravity term,and a formula for settling velocity of sediment flocs in turbulent water was derived.The accuracy of the floc settling velocity formula was verified based on the characteristic parameters of flocs in turbulent water.The effects of turbulent shear strength,initial sediment concentration,and fractal dimension on the settling velocity of flocs were analyzed.（3）Rheological tests were conducted on cohesive sediment formed in weakly turbulent water bodies.The gravity characteristics of flocculated sediment formed under different conditions were measured,and the results showed that the density range of sediment is 1038 kg/m³to 1249 kg/m³.The relationship between the variation curve of sediment flow and viscosity curve was analyzed through rheological experiments,and the solid liquid two-phase transformation process in the presence of flocculated sediment was clarified.Using Bingham model and power law model to fit the rheological curve,the rheological equation of reservoir fluid mud was constructed.