An Experimental Study of Cavity Shedding Mechanisms for Unsteady Cloud Cavitation

The objective of this work is to shed light on different mechanisms associated to the cloud cavitation instability and corresponding vibration characteristics. Results are presented for a modified NACA66 hydrofoil tested in the EPFL high-speed cavitation tunnel. The high-speed digital camera and the single point Laser Doppler Vibrometer are applied to analyze the unsteady cavitating flow structures and corresponding structural vibration characteristics. For the partial cloud cavitation, it occurs when the maximum cavity length is less than 1.0c with Strouhal number of 0.3-0.4. As for the trailing cloud cavitation, the Strouhal number decreases to 0.1-0.15 with the cavity extending to the downstream of the hydrofoil, accompanied with intense vibration and noise. Compared with the cavity shedding dynamics in the partial cloud cavitation, it has a more complex cavitating flow patterns for the trailing cloud cavitation. When the shedding cloud cavity reaches to the rear part of the hydrofoil, it collapses and induces a strong shock wave, which moves backward. As a result, the main cavity shrinks and may even entirely disappears on the suction side. After that, the cavity begins to form and develop again in the next period. Hence, a significant reduction of the shedding frequency and much larger vibration velocities can be observed for the trailing cloud cavitation.