Air-Entraining Vortex Transitions and Critical Submergence in a Circular Water Distribution Well with an Asymmetric Pipe Arrangement

Circular water distribution wells with an asymmetric inlet and outlet arrangement are susceptible to air-entraining vortices, which may threaten hydraulic structures and related equipment. To clarify vortex transitions and critical conditions under the combined effects of relative submergence S/D and outlet Froude number Fr, physical model experiments and numerical simulations were conducted. The experiments covered S/D values from 2.0 to 5.0 and Fr values from 1.03 to 2.06, while the simulations reproduced ten hydraulic measurement cases. Vortex morphology, air-entrainment intermittency, and hydraulic characteristics were examined. By combining quantitative experiment–simulation validation at multiple elevations with a systematic S/D–Fr regime analysis, the vortex transitions under the asymmetric layout were further characterized. Increasing S/D progressively weakens the vortex, causing a transition from a penetrating air-entraining vortex through intermittent air entrainment to a non-air-entraining surface vortex. Increasing Fr raises the air-entrainment intermittency and promotes the downward penetration of the air core. The critical relative submergence Sc/D increases linearly with Fr, and the fitted relation has a coefficient of determination of 0.9826. The resulting critical values are generally higher than those predicted by commonly used empirical relations, which may be associated with the asymmetric arrangement and the resulting persistent background circulation. The resulting critical relation and vortex regime map provide a quantitative basis for operating-water-level assessment and vortex-risk evaluation of circular water distribution wells with similar asymmetric layouts and within comparable hydraulic conditions.