Desilting Channel With Swirling Flow Research On Hydraulic Characteristics And Body Shape Optimization

Water diversion through channels is one of the important ways to solve the problem of uneven distribution of water resources in China.Due to the high sediment content of some rivers in China,water diversion will lead to sediment,and then produce hydraulic mechanical erosion,channel deposition,farmland desertification and other problems.In order to realize the separation of water and sedim ent in the diversion of sediment-laden rivers,a new channel sediment discharge technology——desilting channel with swirling flow is proposed by using the characteristics of strong sediment carrying capacity of swirling flow.As a new channel sediment discharge technology,the internal flow mechanism is not clear and the rationality of the shape needs to be discussed.In order to clarify the hydraulic characteristics of desilting channel with swirling flow and improve its comprehensive performance,the flo w pattern,split ratio,flow velocity characteristics,pressure characteristics,swirl angle and sediment characteristics were studied by combining experiment and numerical simulation,and the feasibility of this technology was analyzed.Combined with orth ogonal experimental design,surrogate model and NSGA-II multi-objective optimization algorithm,the multi-objective optimization of the desilting channel with swirling flow shape was carried out,and the optimization results were verified.The main researc h results are as follows:（1）The experimental study and numerical simulation results show that the numerical simulation results of flow velocity,water surface line and other parameters are consistent with the experimental values,and the numerical simula tion can reflect the motion state of the flow in the desilting channel with swirling flow;the flow pattern of the cyclone presents the evolution sequence of free flow,critical flow and submerged flow with the increase of channel inflow.Rotary flow which is beneficial to suspended sediment can be formed in the sediment discharge tunnel under submerged flow pattern,and has little effect on the flow pattern in the diversion channel.With the increase of the water depth in the upstream channel,the diversio n ratio of the sediment transport pipe decreases in a nonlinear manner,and when the water depth in the upstream channel is H/B=0.97,the diversion ratio of the sediment transport pipe is only 6.07%.When the water depth in the upstream channel is H/B=0.21,the maximum velocity in the sediment transport pipe increases by 175%compared with the average velocity of the channel,and the interception rate of sediment with particle size of（0.075～3）mm is 93.7%.The desilting channel with swirling flow is a channel sediment discharge technology with high sediment discharge rate and low water consumption,and its sediment discharge effect is closely related to the body structure.（2）On the basis of model test and numerical simulation,combined with orthogonal experimental design,four key shape parameters b_in、b_out、h and D of the channel are used as design variables,and two key hydraulic parameters Q?andυ_sect are used as objective functions.BP neural network,RBF neural network and response surface model are selected to fit the relationship between design variables and optimization objectives,and the prediction accuracy is analyzed respectively.The determination coefficient of BP prediction mode l is 0.661,and the proportion of error of prediction results less than 5%is 42%.The prediction effect is poor.The average determination coefficients of RBF and RSM prediction models are 0.990 and 0.993,respectively.The proportion of error less than 5%of the prediction results is more than 90%,and the prediction accuracy is high.Considering the strong generalization ability of RBF neural network,it is selected as the surrogate model for body shape optimization.（3）Combined with RBF surrogate model and NSGA-II algorithm,the shape of desilting channel with swirling flow was optimized.The optimization results show that when the upstream flow rate is 30 L/s,the water consumption of the optimized shape is 15.4%lower than that of the original shape,and the average flow velocity on the section 2 of the sediment transport pipe is 18.9%higher than that of the original shape,and the flow pattern and turbulent kinetic energy are better than those of the original shape.When the upstream inflow is 14.02 L/s,the sediment interception rate is high before and after the optimization of the shape,but the sediment deposition in the optimized shape of the sediment transport pipe is reduced by 43.4%compared with that in the original shape,thus forming a hydra ulic condition more conducive to the rotation of the flow in the sediment discharge tunnel.When the upstream flow rates were 30L/s,60L/s,120L/s,the maximum velocity in the sediment transport pipe increased by 11.0%,6.1%,6.7%,and the maximum turbulen t kinetic energy in the sediment transport pipe decreased by 35.7%,41.2%,50.0%,respectively.The swirl angle of the optimized shape was greater than that of the original shape,and the swirl cavity appeared in the sediment transport pipe.In summary,the desilting channel with swirling flow is a new type of sediment discharge facility with high sediment interception rate and low water consumption,and the optimized shape further improves the comprehensive performance.The research results are conducive to enriching the water and sediment separation technology in water diversion projects,and can provide reference for the design and application of this new sediment discharge technology in projects.