Characterization of Cavitation Instabilities in Axial Inducers by Means of High-Speed Movies

The present paper illustrates the set-up and the preliminary results of an experimental investigation of cavitation flow instabilities carried out by means of a high-speed camera on a three bladed inducer in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The brightness thresholding technique adopted for cavitation recognition is described. A semi-automatic binary algorithm has been used for processing high speed videos of the cavitating inducer. In order to test the capabilities of the algorithm, the mean frontal cavitating area has been computed under different operating conditions. The tip cavity length has also been evaluated as a function of time. Fourier analysis showed the occurrence of a cavity length oscillation at a frequency of 15.7 Hz, which corresponds to the frequency of the rotating stall instability previously detected on the same inducer by means of pressure oscillations analysis.