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.