POD-based frequency study of rotating stall in splitter pump-turbines

Rotating stall (RS) compromises pump-turbine stability by inducing strong inter-blade flow separation and low-frequency pressure/velocity fluctuations, especially in the pumping hump region. In this study, an unsteady computational fluid dynamics framework with the shear stress transport k–ω model is established for a splitter-blade pump-turbine, and the numerical performance curves are validated against experiments. Proper Orthogonal Decomposition (POD) is then employed to reduce the high-dimensional unsteady velocity field into dominant spatial modes and temporal coefficients, and the temporal coefficients are analyzed by Fast Fourier Transform (FFT) to identify RS characteristic frequencies while suppressing blade-passing harmonics. Three representative discharge ratios across the hump region (Q* = 0.68, Q* = 0.73, and Q* = 0.82) are investigated. The corresponding non-dimensional stall frequencies are F* = 0.0098, F* = 0.0074, and F* = 0.0011, respectively, indicating a clear decrease in RS frequency and intensity as discharge increases toward the upper hump boundary. Modal energy distributions further show that a small number of leading POD modes capture the principal RS structures, and the spanwise amplitude of velocity fluctuations decays from hub to shroud, consistent with the observed attenuation of stall cells. The proposed POD–FFT procedure provides a reproducible and data-efficient route for extracting RS dynamics and supports stability assessment under pumping conditions.