Structure stability of cyclic chain-like cavitation cloud in thin liquid layer

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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.

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Stability analysis of ring-like cavitation bubble cluster structure in standing wave field

Lei,

Wu,

Huang

et al. 2024

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Analysis of suppressive effect of large bubbles on oscillation of cavitation bubble in cavitation field

Huang¹,

Fan²,

Tian³

et al. 2023

In this paper, the interaction of multiple bubbles in the cavitation field is investigated in combination with the phenomenon of small bubbles hovering around large bubbles observed experimentally. A model composed of three bubbles is developed and the dynamic behavior of cavitation bubble is analyzed. By considering the time delay effect of the interaction between bubbles and the nonspherical oscillation of large bubbles, the modified bubble dynamic equations are obtained. Numerical results show that the nonspherical effect of large bubbles has little effect on the oscillation of cavitation bubble. The suppressive effect of large bubble on cavitation bubble is closely related to the radius of the large bubble. The larger the size of the large bubble, the stronger the inhibition. When the size of large bubble is close to the resonant radius, the oscillation of cavitation bubble appears coupled resonance response, and the maximum expansion radius of bubble appears resonance peak. The distribution of the secondary Bjerknes force with bubble radius and the separation distances is strongly influenced by driving frequencies or sound pressures. When the large bubble is in the submillimeter order, the strength of the secondary Bjerknes force and the acoustic response mode are different due to the different strength of the nonlinear response of the cavitation bubble. As the distance decreases, when the acoustic pressure increases to a certain value, the secondary Bjerknes force on the cavitation bubble decreases due to abnormal acoustic absorption. The secondary Bjerknes force on cavitation bubble is likely to be repulsive at different separation distances. The theoretical results accord well with experimental phenomenon.

Structural stability analysis of spherical bubble clusters in acoustic cavitation fields

Liu,

Huang,

et al. 2024

Self Cite

The upwelling growth and evolution of spherical bubble clusters appearing at one-quarter wavelength from the water surface in ultrasonic cavitation fields at frequencies of 28 kHz and 40 kHz are studied by high-speed photography. Due to the interactions of bubbles, the stable bubble aggregation occurs throughout the rise of the bubble cluster, whose vertical pressures difference leads to a more significant spreading in the upper part of the cluster in the standing-wave field. At 28kHz, the rising speed is about 0.6m/s, controlled by the primary acoustic field. After a violent collapse of the bubble clusters, the aggregating structure began to hover near the water surface. The size and stability of the structures are affected by the frequency and pressure of the primary acoustic field. If two clusters are close to each other, the clusters deviate from the spherical shape, even trailing off, and eventually merge into a single bubble cluster. By considering the influence of water-air boundary, based on the mirror principle, a spherical bubble cluster model is developed to explore the structure stability of the clusters, and the modified dynamics equations are obtained. The effects of driving acoustic pressure amplitude, bubble number density, depth from the water surface, and bubble equilibrium radius on the optimal stabilization radius of the spherical bubble cluster are numerically analyzed using the equivalent potential at 28 kHz and 40 kHz. The results show that the optimum stabilizing radius of spherical bubble cluster is in the range of 1~2 mm, and it tends to decrease slightly with the increase of the the driving acoustic pressure and bubble number density. It is worthy to note that the nonlinearity is enhanced by increasing acoustic pressure, which may promote the stability of the structures of the cluster. The smaller the unstable equilibrium radius, the easier it is to grow, and the stable size at 40 kHz is slightly smaller than that at 28 kHz. Generally, spherical clusters first appear in high-pressure zone, and then translate toward the low-pressure zone. If the acoustic pressure falls below a certain critical value, bubble clusters disappear. The theoretical analysis is in good agreement with the experimental observations. The analysis of the growth and structural stability properties of spherical bubble clusters contributes to the understanding of the behavioral modulation of bubbles.