Study on Cavitation Generation and Acoustic Transient in Liquid Nitrogen

在不同的压力下，用脉冲红宝石激光聚焦到过冷液氮中，产生了单个气泡。采用双级透镜阵列调节环形激光束，成功地产生了高度球形的空化泡。当激光能量超过阈值能量时，发生光学击穿，随后发生一系列高速现象，例如等离子体形成、包括冲击波发射的声学瞬态和蒸汽泡产生。这些现象的动力学，特别是气泡的初始行为和随后的运动，通过高速摄影进行了研究。结果发现，非常弱的压力依赖性被发现的气泡生成的阈值条件。另一方面，也产生气泡，其强烈地反射自沿着光轴发生的等离子体形成。因此，初始气泡形状是非球形的，但其体积随时间的变化类似于不同的输入能量。在气泡的生长过程中，液体的惯性是主要的，而在崩溃过程中，热效应变得显着，产生的气泡运动的延迟。后一个特征主要与气泡内的饱和蒸汽压有关，并受到其他热力学效应的影响，这一点通过研究气泡运动的周期得到了证实。气泡反弹后，气泡表面立即发生相当大的不稳定性，主要是由于泰勒不稳定性与热不稳定性耦合。

A single bubble was produced by focusing a pulsed ruby laser into subcooled liquid nitrogen at various applied pressures. Highly spherical cavitation bubbles were successfully generated by using an annular laser beam adjusted through a double-stage lens array. When the laser energy exceeds a threshold energy, the optical breakdown takes place, following by a series of high-speed phenomena such as the plasma formation, acoustic transients including shock wave emission and vapor bubble generation. The dynamics of these phenomena, especially the initial behavior and subsequent motion of the bubbles, are investigated by means of high speed photography. It is found that very weak pressure dependency is found for the threshold conditions of bubble generation. On the other hands, it is also a bubble is generated strongly reflecting from the plasma formation which occurs along with the optical axis. Therefore initial bubble shape is nonspherical, but its volume varies similarly with time for different input energies. The liquid inertia is dominant during the bubble growth, whereas the thermal effect becomes significant in the collapse process, yielding the retardation of the bubble motion. The latter feature is primarily associated with the saturated vapor pressure inside a bubble and influenced by other thermodynamic effects which is confirmed by investigating the period of the bubble motion. A considerable instability takes place over the bubble surface immediately after the bubble rebound, mainly due to the Taylor instability coupled with thermal instability.