Flow distortion and vibration excitation mechanisms in the tip clearance of pump-turbines under S-shaped operating conditions

To reveal the distortion and vibration-inducing mechanisms of the impeller tip gap flow in pump-turbines under S-shaped characteristic conditions, this study investigates a pumped storage power unit using numerical simulation. The results show that the gap flow evolves from a relatively ordered state into a multi-scale turbulent structure, driven by coupled mechanisms such as shear instability, momentum accumulation, and centrifugal tearing, leading to non-uniform flow disturbances. Within the axial gap, jet–rotation interaction induces a strong shear layer, resulting in enhanced velocity peaks and vortex structure reconstruction, which ultimately causes global flow instability. The excitation mechanism of pressure pulsation in the gap region undergoes a staged transition: initially dominated by main flow disturbances, then shifting to a self-excited dominant frequency at 2.3 fn, and finally evolving into a bidirectional coupling between the main flow and the gap flow. At specific stages, the dominant gap frequency resonates with the rotor's structural modal frequency, posing a potential risk of resonance-induced failure. These findings provide a theoretical basis for understanding internal flow instability and vibration coupling mechanisms in pump-turbines under S-characteristic conditions, and offer valuable insights for future structural optimization and vibration control strategies.