Abstract: To address the vibration issues faced by pump-turbines, it is essential to conduct an in-depth investigation into the modal characteristics of the runner. Owing to the complexity of the operational environment, the sound speed in the water within the hydraulic passage exhibits significant uncertainty. This study employs the finite element method (FEM) to explore the influence of sound speed on the modal characteristics of pump-turbine runners and their simplified counterparts (i.e., discs). The results reveal that as the sound speed decreases, the added mass corresponding to each modal order of the submerged disc increases, resulting in a reduction in natural frequency. Moreover, modes with higher diametrical or circumferential orders exhibit greater sensitivity to changes in sound speed in terms of both added mass and frequency. Overall, the effect of sound speed on the modal properties of the disc is relatively minor. In contrast, for the runner, the rate of change in added mass due to variations in sound speed is typically 1–2 orders of magnitude higher than that of the disc. This indicates that in real engineering applications, fluctuations in sound speed can lead to substantial changes in the modal parameters of the runner. The findings provide a scientific basis and theoretical reference for accurately assessing the resonance risk of pump-turbine runners.

Key words: pump-turbine, runner, disc, sound speed, natural frequency