How the radial gap affects the runner’s hydrodynamic damping characteristic of a pump-turbine: A physical experiment on a rotating disc

Clarifying how radial gap affects the vibration characteristic of a disc-like structure is of importance in engineering applications, such as in evaluating the operational stability of a runner of a pump turbine. In the present investigation, the runner is simplified as a disc, and a physical experiment is designed on it with variable radial gaps to measure the vibration characteristics, especially by considering rotation. Two frequency peaks for the diametrical mode are generated due to the rotation, and those with lower and higher frequencies are defined as positive and negative modes, respectively. The frequency difference between positive and negative modes increases linearly with the increasing rotating speed, and a linear function is captured to describe the relationship between natural frequency and rotating speed. Regarding the radial gap, its increase causes a slight increase in the natural frequencies but results in a significant reduction in the hydrodynamic damping ratio. Especially in the smaller radial gap conditions, such as when the relative radial gap increases from 0.67% to 3.3%, the reduction in hydrodynamic damping ratio reaches 31.52%. From the perspective of suppressing the resonance amplitude, reducing the radial gap of a runner is recommended due to the mechanism of increasing hydrodynamic damping.