## 3 Cavitating Jet
### Structure of Cavitating Jet
The 3 rd jet core passes through the 1 st and 2 nd clouds and produces the 3 rd cloud downstream of the 2 nd cloud, as shown in Figure 7d. The 5 th jet core produces the 5 th cloud downstream of the 1 st cloud as shown in Figure 7e. Detailed images of the free cavitating jet observed by the high-speed video camera and the schematic cross-sectional diagrams of the progress of a cavitating jet are shown in  
Figure 5: Vortex cavitation in cavitating jet. (a) helical vortex cavitation in free cavitating jet (conical nozzle, nozzle diameter \(d=2.1\) mm, upstream pressure \(p_{1}=6\) MPa, downstream pressure \(p_{2}=0.18\) MPa); (b) ring vortex cavitation on impinging surface (cylindrical nozzle, nozzle diameter \(d=2\) mm, upstream pressure \(p_{1}=12\) MPa, downstream pressure \(p_{2}=0.36\) MPa, standoff distance \(s=40\) mm) [9]. reference [28]. In Figure 6, the cavitation cloud marked by the yellow arrow stays at nearly the same position. The cloud cavitation staying where it is means that it has a long lifetime. The jet core with a longer lifetime cavitation cloud impinges the surface with high impingement pressure, as a drag of the jet core with the larger cloud is lesser than that of a smaller cloud because of the density of water and bubbles. Thus, the aggressive intensity of the cavitating jet with a longer lifetime cavitation cloud is larger. Namely, the lifetime of the cloud is a key factor of the cavitating jet, and the longer the cloud lifetime, the more aggressive the intensity of the cavitating jet [28]. Figure 8 shows a typical aspect of a pure aluminum specimen that was exposed to the fixed cavitating jet, revealing the treatment area of the fixed cavitating jet. Plastic deformation pits are observed in a ring region, whose outer diameter is 60 mm and inner diameter is 30 mm. When the nozzle is scanned or the specimen is moved, the treatment area is uniform [67]. In Figure 8, as the nozzle throat diameter was 2 mm, the cavitating jet can treat an area 30 times wider than that of the nozzle throat. While the white region, which was impinged by the jet core, was observed at the jet center, the main treatment area is the ring region. Namely, the jet center is not treated by cavitation impacts. The understanding of the treatment area by the cavitating jet shows that a ring region is very important when using the cavitating jet for the practical applications. Figure 6: Aspects of impinging cavitating jet observed by high-speed video camera (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). Regarding the reason for the ring region, Figure 9 illustrates the schematic diagram of the impinging cavitating jet, considering the observations of the cavitating jet by the instantaneous photograph and the high-speed video. As shown in Figure 4b, when the cavitating jet is observed by an instantaneous photograph with normal light, the cavitating region seems to be a continuous jet, as shown in Figure 9a. It was previously thought that swirl cavitation in the shear region around the jet directly hits the impinging surface and that the swirl directly produces a ring treatment area [68, 69, 70, 71]. However, this is incorrect, because the aspect of the cavitating jet, as shown in Figure 4b is a kind of time-averaged cavitating region. Considering Figures 4a, 5 and 6, vortex cavitation is initiated in and near the nozzle outlet, and cloud cavitations combine each other; then, the cloud cavitation forms the ring vortex cavitation on the impinging surface. Thus, in order to simulate bubbles in the cavitating jet, the pressure hysteresis of these processes should be considered. For example, if the cavitation in  
Figure 8: Typical treatment area by a fixed cavitating jet (pure aluminum, nozzle diameter \(d=2\) mm, upstream pressure \(p_{1}=30\) MPa, downstream pressure \(p_{2}=0.1\) MPa, standoff distance \(s=262\) mm, exposure time \(t=1\) min) [4].