Numerical Simulation Of Water And Sediment Characteristics In Desilting Channel With Swirling Flow

With the rapid development of industry and agriculture in China and the increasing improvement of people’s quality of li fe,the problem of water shortage has become more and more improtant.China is rich in total water resources,but the temporal and spatial distribution is uneven,and the per capita water resources share is low,which often requires the construction of large-scale water transfer projects to allocate water resources.However,due to the high sediment content in some rivers in China,a large amount of sediment will inevitably be carried in the process of water diversion,which will lead to the reduction of th e flow capacity of the diversion channel,desertification of farmland,abrasion of hydraulic machinery and other problems.The desilting channel with a swirling flow is a new technology for separating water and sediment in the channel,which is proposed based on the strong carrying capacity of the swirling flow.In terms of hydraulic characteristics,model tests and numerical simulation have been carried out,at the meantime;RBF surrogate model and NSGA-II multi-objective optimization algorithm are used to optimize the water consumption for sediment discharge and average velocity of typical sections.To clarify its water-sediment characteristics,the water and sediment two-phase flow in the desilting channel with a swirling flow is simulated with realizable k-εmodel,VOF method and discrete phase model on the basis of experimental test.Through the comparison with the experimental results,the shape factors of sediment with different particle sizes are determined,the desilting effect of the desilting channel with a swirling flow under the condition of continuous sediment inflow is studied,the sediment movement characteristics and distribution of sediment content in the channel are analyzed,and the desilting characteristics of the channel under different velocity,water depth and channel slope is explored.The results show that:(1)At the flow rate of 50m~3/h,the same scheme of adding sediment at the top of the channel as the model test is used to conduct multiple groups of numerical simulations and compare the results of water and sediment model tests.It is determined that the shape factors of sediment between 0.075 and 3.000 mm is from0.2 to 1.0.The simulated sediment trapping efficiency of sediment with particle size of 0.075-3.000 mm in the desilting channel with a swirling flow is in good agreement with the experimental value under the condition of local inflow of sediment from the top of the channel.The relative error is only 3.5%between them,which mainly comes from 0.075-0.160 mm of fine sediment particle size and it shows that the mathematical model can accurately sim ulate the characteristics of water and sediment in the desilting channel with a swirling flow.(2)When the inflow of upstream channel water is 50 m~3/h and the inflow of sediment at the channel inlet is 60 g/s,the sediment content at the bottom of the channel rapidly decreases from 45.6 kg/m~3 to 0.40 kg/m~3 after passing through the cross-section of the desilting tunnel.The swirling flow in the sediment transport pipe has obvious sediment carrying effect,and under the strong centrifugal action,the sediment is mainly distributed near the wall of the sediment discharge tunnel,where the maximum sediment content is about 200kg/m~3.Under the condition of continuous sediment inflow,the sediment trapping efficiency of the desilting channel with a swirling flow is 95.98%for the particle size of 0.075-3.000 mm.Only a small amount of fine sediment enters the downstream channel in the form of suspended matter,and there is no large amount of sediment deposition in the channel and the sediment transport pipe.(3)Under the same inflow and sediment conditions,when only changing the bottom slope of the channel,the change of the desilting rate and internal sediment movement distribution characteristics of the discharge tunnel is relatively small.When the flow rate is changed by increasing the inlet flow velocity or increasing the inlet water depth,the desilting rate shows a decreasing trend.When the inlet flow rate and continuous sediment inflow are the same,the desilting rate has only small difference.The overall desilting rate can still reach over 86%when the flow rate increased from 50m~3/h to 110m~3/h,and the rate of decrease in the diversion ratio is much higher than the rate of decrease in desilting rate.Under the same hydraulic conditions,there is a positive correlation between desilting rate and the proportion of large sediment particles.Under different hydraulic conditions and sediment grading,the sediment in the desilting tunnel exhibits a pattern of high near the wall and low at the tunnel axis.Therefore,the desilting channel has limited requirements for inflow conditions and sediment grading,and its application range is relatively wide.The above results indicate that the desilting channel with a swirling flow has good suitability under different hydraulic conditions and sediment grading,and is a simple and reliable water sediment separation technology that can achieve efficient and continuous sand discharge.This achievement can provide useful reference for the application of desilting channel with a swirling flow in engineering and the study of water and sediment characteristics.