# Research on Cavitation Characteristics of Two-Throat Nozzle Submerged Jet
## 3 Results and Discussion
### Macroscopic Evaluation of Cavitation Characteristics  
The mass loss method is a standard method for the evaluation of the cavitation intensity of cavitation jets. In this paper, the cavitation performance of the submerged cavitation jet was studied under different target distances and impact times. Figure 7 shows the surface morphology change of the samples after impact by the submerged cavitation jet under different target distances when the inlet pressure was 20 MPa and the impact time was 30 min. As can be seen from the figure, the surface morphology change of the sample  
Figure 6: Dynamic change in the cavitation cloud in a single period under an inlet pressure of 10 MPa: (**a**) high-speed photography image of the cavitation cloud; (**b**) FDM diagram. was mainly deep pits formed by high-speed impinging jets when the target distances were 15 mm and 20 mm. However, the cavitation effect only caused slight erosion along the periphery of the deep pits, as shown in Figure 7a,b. This phenomenon is consistent with the distribution of cavitation bubbles in the cavitation jet obtained by [PERSON] et al. [31] using high-speed photography. However, this is not what we expected. For the submerged cavitation jets, the collapse of the cavitation cloud occurred in the shear layer around the jet, as mentioned above. Therefore, the typical cavitation characteristic of the submerged cavitation jet was annular pits formed on the surface of the sample with the jet impact point as the center. It can be seen from Figure 7c,h, that the depth and diameter of the central pit gradually decreased and finally disappeared with the increase in target distance. In addition, the outside diameter of the annular pits formed on the surface of the sample increased first and then decreased. It reached the maximum when the target distance was 35 mm. Figure 8 shows the mass loss of the samples under different target distances. It can be seen from the figure that there were two peaks of mass loss when the target distance increased from 15 mm to 50 mm. Moreover, the two peaks of mass loss are the unique feature of the cavitation jet [32; 33; 34]. The first peak of mass loss occurred when the target distance was 15 mm. As the target distance increased, the mass loss decreased from the first peak, rose to the second peak, and decreased again. The reason for the first peak is that there was a large central hole, as shown in Figure 7a, made by the high-speed impinging jet, which made up almost all of the mass loss. The second peak was caused by cavitation erosion. The cavitation bubble did not have enough time to grow and collapse on the surface of the sample when the target distance was short. The collapsed cavitation bubbles only left many dense shallow pits on the surface of the sample, resulting in less mass loss. When the target distance was 30 mm, the cavitation cloud could fully develop and collapse just as it reached the surface of the sample. The tiny cavitation bubbles were locally broken instantly to form a vast destructive ability, causing dense honeycomb-shaped  
Figure 7: Surface morphology change of the samples after impact by the submerged cavitation jet under different target distances (Unit in mm): (**a**) target distance of 15 mm; (**b**) target distance of 20 mm; (**c**) target distance of 25 mm; (**d**) target distance of 30 mm; (**e**) target distance of 35 mm; (**f**) target distance of 40 mm; (**g**) target distance of 40 mm; (**h**) target distance of 50 mm. pits on the surface. At this time, the collapse intensity, bubble concentration balance, and cavitation erosion were the most significant. Therefore, the mass loss of the samples caused by cavitation erosion was the largest. When the target distance was more than 30 mm, most of the cavitation bubbles collapsed and rebounded several times before reaching the surface of the sample. This will reduce the number of bubbles that can reach the surface of the sample, leading to a reduction in the cavitation effect.