驻波场中环状空化泡聚集结构的稳定性分析

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

利用高速摄影和图像分析技术研究频率为40 kHz超声清洗槽中产生的环状空化泡聚集结构的演化过程.环状结构通常出现在波节附近,其半径小于1/8波长;在一个包络波周期内,结构经历建立、稳定、消失3个阶段,结构形貌具有稳定性.环状空化结构存在气泡的输运现象,且在气泡主聚集区内有小泡团形成后向环内溢出,随后消失成离散的空化泡或泡核再次进入气泡输运循环.环状结构大小和气泡聚集区随时间小幅振荡变化,且存在结构整体旋转现象,主要取决于主声场和气泡间次级辐射场的共同作用.基于双气泡模型分析环状结构形成的物理机制,发现主声场是导致气泡形成环状聚集的控制因素.基于环链状模型分析了结构稳定性,通过数值计算得到了类环形结构等效势分布,且声压越高的地方等效势越高,此处气泡越易聚集.驱动声场越强,环链结构可能越完整,但是,声压过高导致高数密度的气泡聚集也可能破坏气泡的环状聚集稳定性进而演化成其他类型泡群结构.理论分析结果与实验现象具有很好的一致性.

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 investigated.In this work,high-speed photography and image analysis techniques are used to study the evolution of ring-like cavitation bubble aggregation structure 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 a one-eighth 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 with the bubbles.Furthermore,in this work,a mathematical model of two bubbles is used to investigate the physical mechanism behind the formation of a ring.It is found that the sound field is a key factor in ring formation.The ring chain model is used to analyze the structural stability by 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 one-eighth wavelength in radius can stabilize each bubble in the potential well,and the radial distribution presents a ring-like barrier structure.The higher the sound pressure,the greater the equivalent potential,and the more the bubbles are clustered.The higher the driving sound field,the more complete the ring chain structure is.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 consistence with the experimental phenomena.