The multi-cavitation bubble system can easily produce cavitation clouds with various structure types, including ring-like cavitation structures. Nonetheless, the evolutionary behavior of the structure and the physical mechanism of its formation are less extensively investigated. In this paper, high-speed photography and image analysis techniques are used to study the evolution of ring-like cavitation bubble aggregation structures in an ultrasonic cleaning tank with a frequency of 40 kHz. The ring-like structure usually appears near the pressure nodule, and its radius is less than one eighth of the wavelength. The structure involves establishment, stability and disappearance during an envelope wave period, and its morphology is stable. The ring-like cavitation structure exists as a bubble transport phenomenon, and the formed small bubble clusters flow to the outside of the ring and become discrete cavitation bubbles, or the bubble nuclei rejoin the cycle of bubble transport in the main accumulation area of the bubble. The size of the ring structure and the bubble accumulation area oscillate slightly, and there exists the whole structure rotation phenomenon, which depends on the interaction of the main sound field and the secondary radiation field between the bubbles. Furthermore, this paper employs a mathematical model of two bubbles to investigate the physical mechanism behind the formation of a ring. It has been found that the sound field is the key factor in ring formation. The ring chain model is used to analyze the structural stability, taking into account the time delay caused by the secondary acoustic radiation of the bubble. The numerical results show that the equivalent potential energy distribution of a ring bubble chain with a radius of one-eighth wavelength can stabilize each bubble within the potential well, and the radial distribution presents a ring-like barrier structure. The higher the sound pressure, the higher the equivalent potential, and the more clustered the bubbles. The higher the driving sound field, the more complete the ring chain structure. However, high sound pressure may cause the agglomeration of bubbles with high number density to disintegrate the stability of the ring aggregation of bubbles and evolve into other types of bubble aggregation structures. The theoretical results are in good agreement with the experimental phenomena.
