Air-core vortices are a ubiquitous phenomenon in the intakes of hydropower stations. Due to the transient and instability of two-phase vorticial flow, the prediction of air-core vortex formation is challenging, and understanding of the instability mechanism remains elusive. In this study, the large eddy simulation (LES) method and a coupled level-set and volume-of-fluid (CLSVOF) method are performed to study air-core vortex formation in a benchmark reservoir with a horizontal intake pipe. The process of air-core vortex formation can be classified into an inception stage, an instability stage, and a stability stage. In the instability stage, the surface vortex repeatedly goes through the process of inception, enhancement, attenuation, and extinction. The movement of the counterrotating secondary vortices and water surface level fluctuation plays a negative role in air-core vortex formation. The additional motion generated by the counterrotating pair drives the pair to the back wall. The main vortex undergoes attenuation due to the stretching/tilting effects induced by the secondary vortex. Water level fluctuations briefly increase the submergence depth, which in turn reduces the vertical velocity gradient and vertical vorticity, destabilizing the vortex. The perturbation of the air-core vortex by water level fluctuations is present only at the beginning of the instability stage.

空心涡是水电站进水口普遍存在的现象。由于两相涡流的瞬态性和不稳定性，空心涡形成的预测具有挑战性，对不稳定机制的理解仍然难以捉摸。在这项研究中，大涡模拟 (LES) 方法和耦合的水平集和流体体积 (CLSVOF) 方法用于研究具有水平进气管的基准水库中的空心涡形成。空心涡的形成过程可分为起始阶段、不稳定阶段和稳定阶段。在不稳定阶段，地表涡旋反复经历起始、增强、衰减和消亡的过程。反向旋转的次级涡的运动和水面的波动对空心涡的形成起着不利的作用。反向旋转对产生的附加运动将其驱动到后壁。由于次级涡流引起的拉伸/倾斜效应，主涡流发生衰减。水位波动会短暂增加淹没深度，进而降低垂直速度梯度和垂直涡度，使涡旋不稳定。水位波动对空心涡的扰动仅出现在不稳定阶段的开始。由于次级涡流引起的拉伸/倾斜效应，主涡流发生衰减。水位波动会短暂增加淹没深度，进而降低垂直速度梯度和垂直涡度，使涡旋不稳定。水位波动对空心涡的扰动仅出现在不稳定阶段的开始。由于次级涡流引起的拉伸/倾斜效应，主涡流发生衰减。水位波动会短暂增加淹没深度，进而降低垂直速度梯度和垂直涡度，使涡旋不稳定。水位波动对空心涡的扰动仅出现在不稳定阶段的开始。