Study on the coupling characteristics between cavitation flow and turbine runner considering the change of acoustic speed

With the appearance of cavitation, the two-phase flow of water and vapor around the turbine runner has non-homogeneous characteristics, and the propagation mode of the acoustic pressure wave in water changes significantly, leading to changes in the coupling mode between the structure and the surrounding water and affecting the vibration characteristics of the runner. The paper analyzes the effect of acoustic speed changing from 0<cc/c0<0.5 on the structure and acoustic-fluid modes of a prototype hydraulic turbine through numerical simulations. The results show that the coupling characteristics between cavitation flow and turbine runner can be classified into three variations. The first type is acoustic-fluid dominated coupling modes, which present the same modal characteristics as in pure water. The acoustic-fluid modes ffc1-ffc13 induce runner modes fsc1-fsc13 with lower frequencies and smaller modal displacement. This may cause acoustic resonance. The second type is the new acoustic-fluid coupling modes generated by the appearance of cavitation, leading to the mode transition phenomenon of the runner's 0ND (Zero Nodal Diameter), 1ND (One Nodal Diameter), 2ND (Two Nodal Diameter), and 3ND (Three Nodal Diameter) at low acoustic speed. The third type is the runner dominated coupling modes, which refer to the 0ND-3ND modes of the runner, and they induce the corresponding modes of the surrounding acoustic-fluid. The pressure distribution of the acoustic-fluid modes changes under cavitation, which results in different modal displacements at the trailing edge of the blade. The research results provide a scientific basis for the accurate judgment of the resonance phenomenon in hydraulic machinery.