# Numerical Simulation of Flow Field of Submerged Angular Cavitation Nozzle
## 3 Results and Discussion  
### Preliminary Analysis of Nozzle Flow Field  
In order to investigate the influence of nozzle structure on the cavitation effect of the angular cavitation nozzle, take the optimized nozzle structure parameters \(\alpha=13.5^{\circ}\), \(Ld=3d\), \(Le=4d\), and \(\beta=60^{\circ}\) for example to analyze the flow characteristics of the internal and external flow fields of the nozzle. In order to make a better presentation of the internal and external fields of the cavitation jet, the finally optimized structural parameters of the nozzle were selected as examples for analysis. The velocity cloud diagram, local velocity vector diagram, pressure cloud diagram, and vapor volume fraction cloud diagram of the angular cavitation nozzle on the z-x plane are shown in Figures 11-13. The axial velocity distribution and vapor volume fraction at \(d/2\) of the angular cavitation nozzle are shown in Figures 14 and 15, respectively. Figure 10: Cavitation process of transient simulation. Figure 9: Cavitation process of experiment. Reprinted with permission from Ref. [31]. Copyright 2020, Mechanika. Figure 11: Velocity distribution of the angular cavitation nozzle. Figure 12: Pressure distribution of the angular cavitation nozzle. Figure 13: Vapor volume fraction variation of the angular cavitation nozzle. It can be seen from the velocity contour of the angular cavitation nozzle (see Figure 11) that the water flows from the nozzle inlet first and reaches the maximum velocity at the outlet of the cylindrical section through the continuous acceleration of the contraction section and the convergence of the cylindrical section. Then, the jet mixes vigorously with the liquid in the external flow field in the expansion section and develops into an isokinetic core section. The velocity is stable at 200 m/s. At the same time, the backflow phenomenon in the expansion section can also be observed (see the enlarged view of the local velocity vector in the black box in Figure 11). Finally, the jet velocity gradually decays to 0 m/s in the outer flow field with the extension of the jet distance. According to the pressure contour of the angular cavitation nozzle (see Figure 12), the pressure decreases continuously from the nozzle inlet through the contraction section, the cylinder section, and the expansion section. In the expansion section of the nozzle, there is obviously a low-pressure area, which is lower than the atmosphere in the external flow field. With atmosphere as the reference pressure, the nozzle pressure is reduced from 20 MPa to 0 MPa and reaches \(-0.1\) MPa in the expansion section. Referring to the angular cavitation nozzle velocity contour (see Figure 11), it can be seen that the pump pressure is converted into liquid kinetic energy, and a negative pressure zone is generated in the nozzle expansion section due to the strong shear action between the jet and the external flow field. It can be seen from the vapor volume fraction contour in the angular cavitation nozzle (as shown in Figure 13) that there is no obvious generation of a cavitation cloud in the  
Figure 14: Axial velocity distribution. Figure 15: Axial vapor volume fraction distribution at \(d/2\). nozzle contraction section and the cylinder section. The cavitation cloud is generated and developed in the expansion section and then enters the outflow field with the jet until it disappears. The formation and collapse of cavitation clouds are closely related to the pressure. Since the noncondensable gas that generates cavitation bubbles mainly exists in the liquid or on the liquid-solid interface, the cavitation clouds are generated and fall off from the near wall surface of the expansion section. Based on the 3D structure of the nozzle, the distribution of cavitation clouds generally presents a vortex ring shape, with a small vapor volume fraction in the central part and a large vapor volume fraction around it. This can be seen in some research [6, 25].