| Inverted siphon is a common water conveyance structure.The layout of the transition section upstream of the intake determines the flow pattern in this section to a certain extent.The layout of the transition section at the inlet of this thesis is a nonlinear contraction section.The purpose is to study and analyze the disturbance law of the water surface in the transverse and along the section when the independent variable changes with the different water depth and upstream flow in the transition section,so as to aggravate the turbulence intensity in this section,and analyze its influence on the flow conditions of the inverted siphon inlet.Therefore,this thesis selects an inverted siphon in the middle route of the South-to-North Water Transfer Project as the research object.The model meets the gravity similarity criterion and adopts a 1:30 geometric similarity normal model.The upstream inflow is controlled by the triangular weir,and the downstream water depth is controlled by the opening and closing degree of the tail gate.Through the above research,this thesis draws the following conclusions:(1)Due to the nonlinear shrinkage structure of the section and the limited flood discharge capacity of the inverted siphon,the water flows through the upstream positive slope section to the flat slope section in front of the inlet,which is prone to obvious strong flow disturbance before and after the slope change.When the water depth in the contraction section is small,the above phenomenon becomes more and more obvious with the increase of upstream flow.Even the flow characteristics similar to hydraulic jump appear before and after the slope change,and the water surface changes greatly in the transverse and along the way,resulting in the enhancement of flow turbulence in the test section.With the increase of flow,the transverse velocity of the section is positively correlated with the flow,the velocity in the middle is larger,and the velocity near the side wall decreases.With the increase of water depth in the transition section,the maximum vertical velocity moves from the surface to near the channel bottom.(2)The strong disturbance of flow upstream of inverted siphon inlet is further studied.The flow pattern in the lower transition section for strong disturbance action is mainly manifested in the significant changes of vertical velocity distribution and turbulence intensity.When the water depth in the transition section is shallow,the greater the inflow,the greater the disturbance intensity,the stronger the water surface fluctuation,and a large amount of aerated flow appears near the water surface.At the same time,if the length of the transition section is not enough to weaken the fluctuation,a large amount of aerated water will enter the pipe,and the effective cross-section will be reduced,which will have an adverse impact on the water transmission of the pipeline.Due to the increase of water depth,the blocking effect on the upstream flow increases,which relatively weakens the strong disturbance effect caused by large flow in the transition section,and the flow turbulence weakens.(3)Based on the motion equation of turbulence theory,the velocity distribution formula of the inlet transition section under the dual influence of changing slope section and side wall is analyzed.According to the measured data distribution characteristics of the transverse and vertical velocity distribution in the upstream transition section of the river inverted siphon inlet,the preliminary boundary conditions are obtained.Based on the three-dimensional turbulent Reynolds equation,the flow velocity distribution formula meeting the layout of the inlet in the test section is deduced.By comparing the theoretical and measured values,it is found that the formula can better reflect the vertical velocity distribution in the transition section under this inlet type. |
