Unsteady hydrodynamics of cloud cavitating flows in a convergent-divergent channel

A simultaneous experiment for unsteady cavitating flow observations and hydrodynamic measurements in a convergent-divergent channel is conducted by combining a high speed visualization and a dynamic pressure measurement set ups. Both the cloud cavitation images and wall-pressure are addressed experimently. Spectral analysis is conducted for pressure signals, and the correlation between the unsteady cavity behaviors and the pressure fluctuations is discussed. The results show that a typical quasi-periodic process of cloud cavitation development is characterized by the process of attached cavity growth, attached cavity shedding, and the collapse of cloud cavities. The pressure signal's dominant frequency is 21 Hz, which corresponds to the quasi-periodic cycle of growth and shedding of the attached cavities. The pressure signal's secondary frequency is about 42 Hz, which corresponds to the events of growth and collapse of small cloud cavity in the rear of the attached cavity. The correlation between cavity behavior and pressure fluctuation is summarized as follows. During the attached cavity growth stage, pressure fluctuation in the region covered by stable attached cavity is small, while the pressure fluctuation at the cavity closure region is strong. When attached cavity obtains its maximum length, it is rear part becomes quite unstable, and the pressure fluctuation in the rear region of attached cavity becomes more stronger. After that, the attached cavity is broken up with the development of re-entrant jet, and the pressure fluctuation becomes complex due to the growth and collapse of the small cloud cavities. Finally, during the process of its convection to the downstream, a large cloud cavity is formed by the coalescence and growth of small cloud cavities, while a decrease in pressure fluctuation is detected.