工程科学与技术 (Mar 2025)
Cavitation Characteristics and Aeration Mitigation for the Sidewall of a Radial Gate in a High-head Tunnel
Abstract
Objective In high-head spillway tunnels, aeration devices are often installed at the outset of the radial gate to reduce cavitation erosion. Despite these precautions, cavitation erosion persists due to the complex flow pattern near the radial gate. Once the flow is discharged into an open channel, the pressure relief effect lead to the diffusion of high-speed flow and the flow quickly reaches the bottom and side walls of the channel, resulting clogging risk in aeration area. Up to the present, the hydraulic reason for weakening the aeration performance are still unknown, and there are little hydraulic designs to improve the aeration protection for the radial gate. Typically, the lateral and bottom aerators are installed at the pressurized outlet when the flow is directed into an open channel under high-head conditions. However, the lateral and vertical diffusions of flow complicates the flow pattern near the radial gate, influencing the effectiveness of the lateral and bottom aerators. It is significant to understand the aeration protection nullification mechanism and propose effective aerator designs for the sidewall and bottom protection. The present study focused on a given prototype hydraulic engineering featuring a radial gate in a high-head spillway tunnel , where the tunnel wall is damaged by cavitation erosion.Methods Focused on the cavitation erosion in the given prototype tunnel, the present study obtained the hydraulic characteristics based on experimental and numerical results, optimizing the lateral aerator. The physical model was designed based on Froude similarity criterion, and the numerical simulation model was established according to the prototype tunnel. The effects of aeration devices on the aeration protection efficiency were analyzed using aeration flow pattern, cavity length, flow velocity field and flow cavitation number. In the present study, the RNG k-ε turbulent model was used to simulate three-dimensional flow field of the radial gate, the VOF was used to simulate the aeration cavity, and the partial differential equations were discretized based on the control volume method, the SIMPLER model was employed for numerical simulation. The three dimensional structured grids were constructed for the spillway tunnel, and the high grid density in the radial gate, aeration cavity and wall boundary areas can enhance the accuracy of calculation.Results and Discussions The experimental results show that the three dimensional diffusion is significant when the flow is separated from the radial gate, featuring strong water-wing effect above the flow. The lateral aeration cavity is shorter than the bottom aeration cavity, resulting in weakened lateral aeration. The re-attachment flow cannot entrain enough air into the flow. Moreover, the lateral diffused flow may encroach upon the bottom cavity area, increasing the risk of cavity clogging. The numerical results show that the lateral impact and rebound motion of flow on the sidewall cause a low-pressure area when the high-speed flow discharges from the radial gate. The cavitation erosion observed in the prototype tunnel aligns with the low pressure and clear water area on the sidewall. For different gate opening tests, the above phenomenon persists due to the constrain of the radial gate on the flow diffusion. It is also the main reason for the side-wall cavitation erosion under high-speed flow and low flow cavitation coefficient. With the increase in gate opening, the restraining effect of radial gate decreases and the lateral cavity length increases. Especially under high-head and partial gate opening conditions, the adverse phenomenon is even worse. In order to prevent the cavitation erosion on sidewalls, the current study proposed two hydraulic design principles: increasing the sidewall pressure and flow cavitation coefficient and optimizing the design of shrinking side-wall aerator. Based on experimental and numerical results, the one-step and two-step sidewall aeration designs are proposed to improve the flow cavitation coefficient and eliminate the clear water area. The optimized aerators can make the flow cavitation number σ larger than 0.3 for all gate opening tests. However, for the one-step aerator, the lateral cavity length surpasses the bottom cavity for large gate opening (Fr < 6.0), leading to the weaken aeration. The ratio of lateral and bottom cavity length is smaller than 1.0 to avoid strong water-wing effects and cavity plugging. The process of lateral impact and rebound can reduce the clear water area of the sidewall, while the multi-step aeration process can improve the aeration level in the cavitation erosion dangerous area. It should be noted that the present study mainly focus on the macroscopic aeration properties, such as aeration cavity length and flow cavitation number, which can verify the availability of aeration protection. For the microscopic aeration properties, such as air concentration distribution, bubble quantity and size distributions, the effects of aerator factors on air–water flows need to be further explored.Conclusions The present study conducted a series of experiments and numerical simulations to investigate the reason of the cavitation erosion in high-head spillway tunnels, proposing effective lateral aerator designs to improve the aeration protection. The lateral flow causes the weakened aeration, resulting in a clear water area on the sidewall and low flow cavitation coefficient. For all gate opening tests, the combination of two-step lateral aerator and the bottom aerator can effectively eliminate the clear water area on the tunnel walls, and the three-dimensional aeration is sufficient with the steady air–water flow pattern. The excellent aeration performance of the two-step aerator is adaptable for all gate opening tests and can improve the safety of high-head spillway tunnels.
