Tip cavitation characteristics of a low-head pumped hydro energy storage unit in turbine mode based on the finite-time Lyapunov exponent

In recent years, the rapid development of low-head pumped hydro storage driven by the carbon peaking and carbon neutrality goals has significantly increased the operational demand on tubular flow units, in which tip-clearance cavitation remains a major challenge for safe and reliable operation. Although substantial research efforts have been devoted to cavitation, the formation and evolution mechanism of tip-clearance cavitation still lack systematic physical interpretation. To address this gap, this study employs the Lagrangian coherent structures determined by the finite-time Lyapunov exponent to analyze the tip vortex evolution under three representative operating conditions. The results demonstrate that repelling-type Lagrangian coherent structures can robustly capture the strongest leakage-induced stretching and clarify the mechanism for cavitation inception and collapse. Compared with conventional flow-field observation approaches, the finite-time Lyapunov exponent method enhances the identification of tip-clearance coherent structures by more than an order of magnitude and reveals clear phase-dependent evolution characteristics. These findings reinforce the physical understanding of tip-vortex-induced cavitation and provide technical insights for operational optimization and safe utilization of low-head pumped hydro energy storage units.