Abstract: To address the vibration issues faced by a pump-turbine, it is essential to conduct an in-depth investigation into the modal characteristics of its runner, but the sound speed in the water flow in the passage exhibits significant uncertainty owing to the complexity of its operational environment. The finite element method (FEM) is used to explore the effect of sound speed on the runner’s modal characteristics and simplified counterpart (i.e., a disc). The results reveal that as the sound speed decreases, the added mass of the submerged disc in each modal order increases, resulting in a reduction in its natural frequencies. And, for the modes in higher diametrical or circumferential orders, the added mass and frequencies exhibit greater sensitivity to changes in sound speed. Generally, the effect is relatively minor for the disc tested in this study, while due to variations in sound speed, the runner has a change rate of its added mass that is typically 1-2 orders of magnitude higher. This means that in real engineering applications, fluctuations in sound speed could lead to substantial changes in the modal parameters of the runner. The findings shed some light on our understanding of the risk in assessing the resonance of pump-turbine runners.

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