Experimental investigation on effect of drag-reduced cavitation on stability of a blub turbine

Cavitation significantly influences the stability and flexibility of hydraulic turbines. However, its specific effects on vibrations and pressure fluctuations, particularly under the drag-reduced cavitation condition, remain poorly understood. In this study, the pressure fluctuation and vibration signals under the drag-reduced cavitation condition within a bulb turbine are simultaneously captured. The results indicate continuous cavity formation at the blade tip under drag-reduced cavitation. In this state, the efficiency recovery correlates with stabilized cavitation flow and attenuated rotor-stator interaction effect. Under the drag-reduced cavitation condition, the intensity of the medium-frequency component of both pressure fluctuation and vibration reaches its minimum, with distribution peaks becoming more pronounced, indicating the improved flow stability. The multifractal strength of pressure fluctuation is minimal under drag-reduced cavitation condition, with the average intensity being approximately 70 % of that under incipient cavitation condition. Correlation analysis reveals that the vibration is induced by the pressure fluctuation in the bulb turbine. The cross-correlation between the two signals exhibits multifractal characteristics. The nonlinear coupling effect and cross-correlation reach the maximum value under the drag-reduced cavitation condition. A comprehensive analysis of drag-reduced cavitation's impact on stability is crucial for improving the performance of bulb turbines.