# Research on Cavitation Characteristics of Two-Throat Nozzle Submerged Jet
## 3 Results and Discussion  
### Dynamic Change in the Cavitation Cloud in a Single Period  
Figure 4 shows the high-speed photographic images of the cavitation cloud under an inlet pressure of 20 MPa in a single period, where X is the width of the cavitation cloud, and Y is the length of the cavitation cloud. The interval between the two adjacent images was 0.1 ms. It can be seen from Figure 4a that the cavitation cloud generated by the two-throat nozzle submerged cavitation jet had obvious periodicity. In a single period, the cavitation cloud mainly went through four phases: inception, growth, shedding, and collapse. At 0 ms, the cavitation cloud was distributed near the nozzle exit and was in the inception phase. At 0.1 ms, the cavitation cloud began to fall off. During the period from 0.1 ms to 0.8 ms, the cavitation cloud was in the growth phase. The cavitation cloud kept moving downstream with the increase in length and width. At 0.8 ms, the cavitation cloud reached its maximum length and width at this moment. At 0.9 ms, the final stage of the collapse of the cavitation cloud was observed. In Figure 4b, it can be seen from the FDM diagram that the generation and disappearance of the cavitation cloud near the nozzle is mainly concentrated in the shear layers on both sides of the jet. Whereas, the generation and disappearance of the cavitation cloud in the downstream of the jet are mainly concentrated in the center of the jet. This is because the pressure fluctuation of the jet shear layer decreased. The environmental pressure replaced the jet shear layer pressure, which plays a significant role in the dynamic change in the cavitation cloud. According to the FDM diagram, the moment when the cavitation cloud entered the collapse phase can be determined more intuitively. The collapse of the cavitation cloud mainly occurred after the fracture of the cavitation cloud and is concentrated in the latter half of the cavitation cloud. The cavitation cloud of the previous period began to fall off at 0 ms. Whereas, the cavitation cloud of the new period generated and entered the grow phase, as indicated in the figure in red. At 0.8 ms, the cavitation cloud of the previous period collapsed completely. The collapse area was concentrated in the range of Y = 20-30 mm. In order to investigate the effect of inlet pressure on the dynamic change of the cavitation cloud, visualization experiments with high-speed photography were carried out under the inlet pressures of 20 MPa, 15 MPa, and 10 MPa. The experimental results are shown in Figures 4-6, respectively. It can be seen that the length, width, and area of the cavitation cloud had significant differences under different inlet pressures. Due to the injection flow rate and the intensification of cavitation within the shear layer, the size of the cavitation cloud increased with the increase in inlet pressure. Moreover, the asymmetry of the cavitation cloud increased as the inlet pressure increased. This indicates that the fluctuation of the cavitation cloud boundary is more severe with the increase in inlet pressure. The shedding frequency of the cavitation cloud determines the frequency of cavitation, which directly affects the effect of cavitation. By comparing the process of the cavitation cloud from shedding to complete collapse under different inlet pressures, it was found that the shedding phase of the cavitation cloud increased slightly with the increase in inlet pressure. The shedding phase of the cavitation cloud was about 0.6 ms, 0.7 ms, and 0.8 ms when the inlet pressures were 10 MPa, 15 MPa, and 20 MPa, respectively. This indicates that the shedding frequency of the cavitation cloud decreases with the increase in inlet pressure. This result is consistent with that obtained by [PERSON] et al. [15]. Figure 4: Dynamic change in the cavitation cloud in a single period under an inlet pressure of 20 MPa: (**a**) high-speed photography image of the cavitation cloud; (**b**) FDM diagram.