Phase-resolved characteristics of bubbles in cloud cavitation shedding cycles

Sheet/cloud cavitation is formed when the concentration and the number of bubbles surpass some critical level such that the flow becomes unsteady. This phenomenon is commonly observed in the flows around the hydrofoils and propellers. Under some conditions, cloud cavities are periodically detached from the hydrofoil surface, causing strong unstable hydrofoil impacts. Additionally, the shedding and collapse of cloud cavities are associated with undesirable phenomena like vibration and noise.

In most cases, cloud shedding is attributed to two main sources, namely, bubbly shock waves and re-entrant jets. Consequently, the small-scale properties of the vapor water mixtures within the bubbly cloud cavitation flows have been found to have a considerable impact on cavitation erosion and noise. However, most of the previous studies focused on large-scale instability and low-frequency shedding, with few studies on the small-scale mixture properties within cloud cavitation.

Several experimental fluid mechanics techniques have been employed in studying cloud cavitation. While these techniques can measure the flow field outside the cavity interface, they are unable to obtain the information inside the two-phase mixture, i.e., the detailed physics in the region of cloud/sheet cavities. Recently, phase-detection optical probes have emerged as promising techniques for measuring the mixture information inside the sheet cavities as well as bubbly flows.

To this note, Dr. Hao Zhang, Professor Yunqiao Liu and Professor Benlong Wang (corresponding author) from Shanghai Jiao Tong University together with Dr. Xinsheng Cheng from Zhejiang University conducted an experimental investigation of the complex physics and mixture properties inside cloud/sheet cavities using a phase-resolved optical probe. The study focused on the vapor-water mixture in the cavitating flows around a NACA 0012 hydrofoil, which can produce a variety of cavitating structures. The probe was employed to measure the bubble sizes, velocities and void fractions at different locations in the cloud cavitation region. Accompaniedly, high-speed photography was employed to record the phase-resolved characteristics of the cavitation from clouds to bubbles. Their work is currently published in the journal, Ocean Engineering.

The authors reported distinctive characteristics at different scales: the small scale of bubbles and the large scale of cavitation shedding. A combination of raw signal history and mean void fraction profiles characterized by the shedding of cloud cavities is proved to be useful in examining the behavior of large-scale cavitation. In fact, examining the raw signal history enabled the observation of the signal periodicity for most measurement locations. The signals were decomposed into several time scales with different frequencies. Low-frequency scales were the most energetic, implying primary shedding of the cloud/sheet cavity.

Local analysis of the water vapor signals was made possible. Transient void fractions appeared larger than the corresponding average void fractions. The sizes of the dispersed bubbles were described based on the experimental data. The findings corresponded to the small-scale characteristics. Moreover, the results on the probability density function of the bubble size distributions and the small-scale information on bubbles were almost similar at different measurement locations across the cloud cavitation region.

In summary, the phase-resolved characteristics of bubbles in cloud cavitation cycles were investigated. The authors proposed an empirical bubble size distribution for cloud cavitation based on the experimental data. The findings demonstrated the feasibility of fiber-optic phase-resolved detection probes and high-speed photography in studying the complex water vapor mixtures in cavitating flows. In a statement to Advances in Engineering, Professor Benlong Wang pointed out that their findings provided valuable insights that would contribute to an in-depth understanding of the complex cloud cavitation shedding around hydrofoils and propellers.