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

用日本商品化的反射镜对激波聚焦和诱导空化现象进行了连续的详细观察。我们特别注意到反射镜的出口结构对椭球反射镜第二焦点处空化现象的影响。结果，建议在反射器的出射表面不是突然改变而是逐渐弯曲形状的情况下相对抑制空化，从而导致在第二焦点处的膨胀波的弱聚焦。另一方面，为了理解反射器第二焦点附近出现的复杂现象，已经进行了一些初步的研究，例如冲击波/气泡相互作用和气泡/气泡相互作用。还进行了一项实验，其中附着在明胶壁上的气泡受到冲击波的撞击。因此，形成更高的液体微射流，刺入明胶层。此外，气泡/气泡的相互作用进行了检查，使用两个由激光产生的气泡。结果发现，单个气泡可以采取几种模式的气泡运动，这取决于气泡的大小，两个气泡之间的距离和气泡生成的时间差。当两个气泡相互作用时，出现了高速射流的形成、气泡的破碎和涡环的形成等有趣的现象。所取得的大部分成果已在国内外学术研讨会上发表，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.