Stability of curved chain-like bubble structures and transport behavior of gas nuclei in localized turbulent Flows

This study establishes spatially periodic acoustic pressure gradients beneath the gas-liquid interface by alternately arranging soft and hard acoustic boundaries, thereby stabilizing the aggregation of cavitation bubbles below the liquid surface. The evolution of bubble aggregation structures was recorded using a high-speed imaging system with high magnification and frame rate. The morphological characteristics of the structures and the motion properties of internal bubbles were analyzed through image processing methods, providing experimental evidence for gas-nucleus circulation. Bubbles with radii less than 10 μm originating near the liquid surface were found to be drawn into the cavitation structure by the primary Bjerknes force and structural attraction. Bubbles within the structure stabilized at a depth of λ/8below the liquid surface under the combination of primary and secondary Bjerknes forces, arranging themselves into a catenary configuration. Large bubbles float upward, driven by the primary Bjerknes force, and then break up, completing the gas-nucleus output process. The deformation and breakup patterns of large bubbles align with turbulence-induced bubble fragmentation. Statistical results indicate that a high-turbulent-energy-dissipation region exists between the cavitation structure and the liquid surface. To explain the abnormal variation in the equivalent radius of daughter bubbles during binary breakup, the bubble splitting effect was incorporated into the bubble model, accounting for phase transition and heat transfer. Excessive heat loss during the expansion phase prevents effective accumulation of ultrasonic input energy, significantly reducing the maximum expanded radius of daughter bubbles and accelerating their disappearance. The dependence of this phenomenon on parameters such as ultrasonic pressure amplitude agrees with experimental observations. Furthermore, the positional stability of this structure demonstrates its potential for patterned ultrasonic cavitation shot peening.