Destructive Action of Shock Focusing-Induced Cavitation and Research on Its Suppression

使用与日本商业化的反射器相同的反射器，连续地对激波聚焦和诱导空化现象进行了详细的观察。我们特别注意到反射镜的出射结构对椭球反射镜第二焦点处的空化现象的影响。结果表明，当反射面的出射面不是突然变化而是逐渐弯曲时，空化被相对抑制，从而导致膨胀波在第二焦点处的弱聚焦。另一方面，为了了解反射镜第二焦点附近出现的复杂现象，进行了激波/气泡相互作用、气泡/气泡相互作用等初步研究。还进行了一项实验，在实验中，贴在明胶墙上的气泡被冲击波击中。因此，形成了较高的液体微喷流，穿透到明胶层中。此外，利用激光产生的两个气泡研究了气泡/气泡的相互作用。研究发现，单个气泡的运动方式与气泡的大小、两个气泡之间的距离以及气泡产生的时差有关。当两个气泡相互作用时，会出现高速射流形成、气泡崩解、涡环形成等有趣的现象。所取得的大部分成果已经在国内和国际研讨会上进行了报道，并在期刊上发表。

A detailed observation of shock focusing and induced cavitation phenomena was continiously carried out using a reflector which is one of the same type as that commercialized in Japan. We especially noticed the effect of the exit configuration of the reflector on the cavitation occurence at the 2nd focal point of the ellipsoidal reflector. As a result, it is suggested that cavitation is relatively suppressed in the case where the exit surface of the reflector changes not abruptly but gradually curved shape, resulting in weak focusing of expansion waves at the 2nd focal point. On the other hands, for the purpose of understanding complicated phenomena appearing at around 2nd focal point of the reflector several elementary researches have been conducted, such as shock wave/bubble interaction and bubble/bubble interaction. An experiment was also performed, in which a bubble attached on a gelatine wall was hit by a shock wave. Consequently a higher liquid microjet was formed, piercing into the gelatine layr. Furthermore the bubble/bubble interaction was examined using two bubbles generated by means of a laser. It was found that an individual bubble can take several modes of the bubble motion, depending on the bubble size, distance between two bubbles and time difference of bubble generations. When two bubbles are affecting each other, an interesting phenomena such as the jet formation with higher velocity, the bubble disintegration and the vortex ring formation, occurred. The most of results obtained are already reported at both national and international symposia, and also published in journals.