Air-core vortex formation and tail breakup in tidal-energy intakes: Effects of inlet geometry and flow mechanisms

Air-core vortices (ACVs) in horizontal tidal-energy intakes can induce air entrainment, deteriorate inflow uniformity, and threaten downstream hydraulic components. Experiments and high-fidelity numerical simulations were conducted to investigate the effects of a straight pipe inlet (SPI) and a bell-mouth inlet (BMI) on ACV formation and tail breakup. The BMI suppresses ACV formation by mitigating inlet flow separation and weakening vortex stretching, reducing the maximum normalized vertical vorticity near the free surface by approximately 40%–45% compared with the SPI. By contrast, the separation-induced low-pressure region in the SPI promotes air-core expansion and destabilization, causing tail breakup within approximately one pipe diameter ( D ), whereas the continuous air core in the BMI persists beyond 6.6 D . Based on these mechanisms, a Venturi-type pipe inlet (VPI) is proposed. The VPI retains the formation-suppression capability of the BMI while promoting effective air-core breakup within 1 D under multiple operating conditions. Pressure and velocity also recover to stable downstream states within approximately 1 D . Compared with the SPI, the VPI reduces the static-pressure recovery distance by at least 60% and the velocity-recovery distance by approximately 56%. These findings clarify geometry-induced ACV dynamics and provide guidance for improving tidal-energy intake reliability.