## 3 Cavitating Jet  
### Structure of Cavitating Jet  
To show the flow pattern of the cavitating jet, Figure 4 illustrates aspects of the impinging cavitating jet taken with (a) a flash lamp of 1.1 us and (b) a shutter speed of 1/25 sec, i.e., 40 ms. Thus, Figure 4a reveals an instantaneous aspect of the jet, and Figure 4b shows a kind of time-averaged aspect of the impinging cavitating jet. In the nozzle used for the cavitation jet, the cavitator and the guide pipe were installed to enhance the aggressive intensity of the cavitating jet [39], and standoff distance was defined as the distance from the upstream corner of the nozzle to the specimen surface. As shown in Figure 4a, a cloud cavitation was observed between the nozzle and the impinging surface, and a ring vortex cavitation was observed on the surface. When the cavitating jet was observed by a normal light, as shown in Figure 4b, the cavitating jet seems to be a continuous jet from a nozzle to an impinging surface. However, cloud cavitation sheds from the nozzle to the surface, and the cloud cavitation becomes a ring vortex cavitation. To show vortex cavitation in the cavitating jet more precisely, Figure 5a reveals the aspect of the free cavitating jet through a conical nozzle, and Figure 5b shows the aspect of the impinging cavitating jet with a flash lamp [9]. As shown in Figure 5b, the PVDF transducers [41; 42] were installed  
Figure 4: Aspects of cavitating jet (cylindrical nozzle, nozzle diameter \(d=2\) mm, upstream pressure \(p_{1}=30\) MPa, downstream pressure \(p_{2}=0.1\) MPa, standoff distance \(s=222\) mm): (**a**) observation with flash lamp whose exposure time is 1.1 us; (**b**) observation with normal light and shutter speed of 40 ms.  
Figure 3: Typical aspect of vortex cavitation, which produces severe impact [63]. in the impinging surface, which the white arrow shows, and the signal from the PVDF transducer was synchronized to the flash lamp; thus, Figure 5b reveals the aspect of the jet which produces the impact on the impinging surface. As shown in Figure 5a, helical vortex cavitation is observed near the nozzle outlet, and cloud cavitation is observed downstream from the nozzle. Thus, the cloud cavitation results from merging the vortex cavitation. In the case of the impinging cavitating jet, a part of the ring cavitation collapses on the surface producing the impact, which was detected by the PVDF transducer. The ring vortex cavitation produces a severe impact on the impinging surface. In view of the practical applications of cavitation impacts generated by the cavitating jet, the collapse of the ring vortex cavitation is an important phenomenon. To reveal periodical shedding of cloud cavitation from the nozzle, as mentioned in the introduction, Figure 6 shows the aspect of the impinging cavitating jet taken by a high-speed video camera. The nozzle shown in Figure 6 was the same nozzle as in Figure 4, and it had the cavitator and the guide pipe. At \(t=0\) ms, the cavitation cloud sheds from the nozzle to the downstream, and the cloud reaches the impinging surface at \(t=1.5\) ms. A part of cloud cavitation becomes a ring vortex cavitation at \(t=1.75\) ms, and it spreads out on the surface and then collapses. Although the water jet is injected continuously into water, the vortex cavitations near nozzle shed downstream coalescing each other and they become a large cloud cavitation, thus the bubble density between clouds is reduced. At \(t=4.25\) ms, the new cavitation cloud, whose shape is similar to that at \(t=0\) ms, sheds from the nozzle. Thus, it is a periodical phenomenon, as previously reported [16; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. According to a previous report [28], as shown in Figure 7, some cloud, i.e., the 1 st cloud in Figure 7a, stays near the nozzle, and the 2 nd jet core passes through the 1 st cloud, as shown in Figure 7b,c; then, the 2 nd jet core produces the 2 nd cloud downstream of the 1 st cloud.