This paper applies acoustic-solid coupling numerical methods to a systematical analysis on the wet modal characteristics of the top cover of a HLBD993-LJ-407 pumped storage unit at varying fluid-structure conditions

revealing and quantifying the effects of its fluid-induced added mass. Results indicate the added mass reduces the cover's natural frequencies significantly

with the most pronounced attenuation observed in the 1ND mode (natural frequency reduction of 19.6%

corresponding to an added mass coefficient of approximately 0.55). The added mass effect peaks in the water layer within the upper crown gap of the runner

while it diminishes in the flow zones near the inner and outer surfaces of the guide vanes due to boundary connectivity. Meanwhile

it follows the non-linear superposition in the case of multiple fluid domains coexisting. The effect is offset partially by the counter-phase fluid motions in the upper crown gap and guide vane zones; A fully reflective boundary suppresses fluid motions more effectively than a fully absorbing one

increasing the added mass coefficient by more than 20%

and intensifying frequency reduction.. This study has explored the mechanism of added mass influencing the vibration characteristics of the top cover at complicated flow conditions

laying a basis for further study of vibration suppression design and operational optimization.