Study On Hydraulic Characteristics Of Spillway Unilateral Gradual Expansion Stilling Basin Of Pianyan Hydropower Station

According to the side wall layout,common stilling basins can be divided into equal width stilling basin,sudden expansion stilling basin and bilateral gradual expansion stilling basin.The research results of the above three kinds of stilling basins are abundant,but some projects can’t build conventional stilling basins due to topographic and geological conditions,or conventional stilling basins can’t meet the energy dissipation requirements,so the stilling basins are built into unilateral gradual expansion type.At present,there are few projects at home and abroad that adopt one-side gradually expanding stilling basin,and there is a lack of systematic research on it.Therefore,based on the spillway project of Pianyan Hydropower Station,this thesis systematically studies the hydraulic characteristics of one-side gradually expanding stilling basin at different diffusion angles.The main research results are as follows:(1)Based on the 1: 40 hydraulic model test,it is concluded that the original design size can not form hydraulic jump in the stilling basin under large flood conditions,and it can not meet the energy dissipation requirements of all working conditions because of the small size of the stilling basin.Therefore,the original design scheme needs to be optimized as follows:the inclined tailgate is changed into an upright tailgate,the stilling basin is deepened by 2.5m and lengthened by 20 m,and discontinuous trapezoidal piers with a height of 2.5m and a width of 1.5m are set on the tailgate,and the intervals between the piers are 1.5 m..After the above shape optimization,the flow pattern of stilling basin and downstream river is good,and the flow velocity,pressure,water surface line and fluctuating pressure in stilling basin all meet the design requirements,so the optimized shape of stilling basin meets the design operating conditions.(2)In order to explore the difference of hydraulic characteristics between one-side gradually expanding stilling basin and other stilling basins in this project,three types of stilling basins are introduced: sudden expansion stilling basin,two-side gradually expanding stilling basin and equal width stilling basin.The simulation results show that compared with equal width stilling basin,one-side gradually expanding stilling basin can reduce the water level in stilling basin,reduce the outlet velocity of stilling basin and optimize the flow pattern.The turbulent kinetic energy on the diffusion side at the head of the unilateral gradually expanding stilling basin has a single peak,and there are double peaks on both sides of the sudden expansion stilling basin,but there are no obvious peaks in the uniform width and diffusion stilling basin.With the increase of diffusion angle,the energy dissipation of one-sided gradually expanding stilling basin first increases and then decreases.When the diffusion angle is greater than 6,the energy dissipation rate is little affected by the diffusion angle,while the energy dissipation rate of sudden expansion stilling basin and bilateral gradually expanding stilling basin increases with the decrease of diffusion ratio,but with the further decrease of diffusion ratio,the change of energy dissipation rate is no longer obvious.Considering the energy dissipation effect of various stilling basins,combined with the topographic conditions of this project,the one-sided gradually expanding stilling basin is the best.(3)Because of the asymmetric structure of the stilling basin,vertical axis vortex is formed on the diffusion side of the one-sided gradually expanding stilling basin,and its migration characteristics and strength characteristics show the following changes with the change of diffusion angle: when the diffusion angle is less than 3,there is no vortex on the diffusion side;when the diffusion angle increases to 4.77,the vortex migration is strong and discontinuous,and with the diffusion angle increasing,the vortex position on the diffusion side gradually becomes stable.Under different diffusion angles,the vortex intensity shows the distribution law of "small at two ends and big in the middle",and when the diffusion angle is greater than 6,the maximum value of vortex intensity increases greatly,and with the diffusion angle increasing,the vortex intensity change is no longer obvious.In addition,the vortex velocity in the one-side gradually expanding stilling basin is basically linearly distributed,which has the same characteristics as the vortex on both sides of the sudden expanding stilling basin.Under the influence of vertical vortex,the velocity distribution on the diffusion side of the one-sided gradually expanding stilling basin is disordered,and the velocity near the bottom of the stilling basin on the diffusion side is obviously smaller than that on the non-diffusion side.Asymmetrical hydraulic jump is formed in the one-side gradually expanding stilling basin,and the shape of the hydraulic jump tail is V-shaped.The hydraulic jump length is less affected by the diffusion angle.When the diffusion angle is less than 12,the hydraulic jump length is basically the same as that of the constant width stilling basin.When calculating the hydraulic jump length of the one-side gradually expanding stilling basin,it can be approximately calculated according to the constant width stilling basin.Considering the vortex characteristics and energy dissipation effect in the one-sided gradually expanding stilling basin,it is finally determined that the diffusion angle of the one-sided gradually expanding stilling basin should be 3 ～ 6,which can provide reference for similar engineering design.