Study On Hydraulic Characteristics Of Qimo Vortex Drop Shaft Flood Drainage Tunnel In Guizhou

The vortex drop shaft is a promising technique for energy dissipation in tunnels due to its simple desing,esay construction,high energy dissipation rate and eco-friendliness.It is a main-trend research topic in the field of tunnel energy dissipation,and has great advantages in terms of economic benefits and environmental protection.Typically,the throat is considered the flow control section of the vortex shaft,but in the downstream tunnel project of Qimo vortex drop shaft flood drainage tunnel in Guizhou,the section size is relatively small in comparision to the large flow rate.Therefore,this section has become a bottleneck for controlling the flow capacity of large flow capacity.To improve the discharge capacity,it is necessary to increase the submerged depth of the shaft,which will increase the downstream flow rate and ensure that the flow rate of the entire project meets the design requirements.The hydraulic mechanism of the vortex drop shaft under high submerged depth differs from that under normal conditions.Therefore,it is necessary to explore the hydraulic characteristics of the shaft under high submerged depths to understand its behavior in such conditions.This paper employs a combination of hydraulic model testing and numerical simulation to investigate the flow mechanism and energy dissipation mechanism of the vortex chamber and the high submerged depth of the shaft.The following are the primary findings of this research:(1)After conducting the hydraulic model test,it was found that the discharge capacity of the flood discharge tunnel could meet the design requirements even under high submergence of the shaft,and the energy dissipation effect was significant.Under the annular hydraulic jump,there is an unstable swirling motion within a certain range of elevation that can be referred to as "submerged swirl".This range of submerged swirl increases with the increase of flow rate.The wall pressure distribution of the shaft and the backwater tunnel was reasonable,with the wall pressure of the cavity swirl decreasing along the path and having small values,While the wall pressure of the submerged swirl part increased almost linearly.(2)The test revealed issues such as the alternation of open full flow in the withdrawal tunnel,as well as serious sputtering and erosion of the downstream outlet energy dissipating flow on the left bank slope.To solve these problems,vent holes were added to adjust the outlet energy dissipating shape,ensuring the stability of the flow pattern in the tunnel and a reasonable downstream river flow rate,thereby addressing the erosion problem.(3)The study employed the RNG k-ε turbulence model to conduct three-dimensional numerical simulations of the gas-liquid two-phase flow in the vortex chamber.The results indicate that the wall pressure increases linearly along the flow path,while the section pressure shows a concentric circle distribution with the center of the swirl chamber as the center.The axial velocity exhibits a decreasing trend along the radius r,whereas the tangential velocity initially increases and then decreases,revealing a combined vortex distribution.The combined vortex index n ranges from-0.4 to 1 and increases with the the radius,indicating a shift from quasi-forced vortex dominated to quasi-free vortex dominated.The main flow movements in the vortex chamber are tangential and axial,with the former gradually transitioning to the latter as one moves along the chamber,in agreement with the changes in swirl angle α.(4)Numerical simulations indicate that in the vortex shaft with three flow modes(i.e.,cavity swirl,submerged swirl,and annular hydraulic jump)simultaneously,the pressure distribution in submerged swirl section is more concentric than that of the cavity swirl.The axial direction is the main movement of the cavity swirl,while the submergence swirl’s main movement is tangential.The velocity near the wall of submergence swirl region displays some stability.The flow velocity in the section conforms to the combined vortex characteristics.The cavity vortex is dominated by the forced vortex,while the submerged vortex has both free and the forced vortex ranges that are nearly equal.The simulated values of the radial differential pressure formula derived from the combined vortex distribution are consistent with theoretical values.Additionally,the turbulent kinetic energy dissipation rate of the annular hydraulic jump is significantly higher than that of the swirl region,indicating that this area is the primary energy dissipation zone of the shaft.Overall,these research results reveal the energy dissipation mechanism of the vortex drop shaft energy dissipator and can provide valuable reference for similar vortex drop shaft engineering design.