# A Study of Cavitation Erosion in Artificial Submerged Water Jets
## 2 Numerical Simulation
### Effects of Dimensionless Standoff Distance between Nozzle and Workpiece
At the dimensionless standoff distance of \(S_{D}=40\), the periodic variation in vapor volume fraction distribution in the flow field is depicted in Figure 7. A comparison of the vapor volume fraction cloud diagrams within the flow field at \(T=16\Delta T\) and \(T=36\Delta T\) reveals that with a further increase in the target distance by 10 mm, the cavitation core with the highest volume fraction within the flow field progressively enlarges as it approaches the wall, exhibiting a decreasing trend. The cavitation effect may be weakened compared to the case when \(S_{D}=30\). Figure 8 illustrates the periodic variation in vapor volume fraction distribution on the workpiece surface at the dimensionless standoff distance of \(S_{D}=20\). An observation of the figure indicates that at shorter target distances, the cavitation cloud generated within the flow field continuously and intensely impacts the workpiece surface, resulting in the formation of a cavitation cloud on the workpiece surface. Consequently, a distinctive annular region is formed. However, an analysis of the vapor volume fraction distribution reveals that the volume of cavitation cloud on the workpiece surface is not significant. This suggests that, despite favorable conditions for the growth and development of the initial cavitation bubbles, it does not thrive optimally due to spatial constraints. Before reaching full development as a cavitation core with the highest vapor volume fraction, the cavitation cloud within the flow field starts contacting the workpiece surface and subsequently collapses. Figure 9 depicts the periodic variation in vapor volume fraction distribution on the workpiece surface at the dimensionless standoff distance of \(S_{D}=30\). A comparison of the vapor volume fraction cloud diagrams within the flow field at \(T=12\Delta T\) and \(T=24\Delta T\) reveals that with a 10 mm increase from the standoff distance to the workpiece surface, the cavitation cloud generated within the flow field exhibits a more pronounced periodicity on the workpiece surface. Additionally, it requires \(12\Delta T\) for the vapor volume fraction to increase from 0.2 to 0.5. Simultaneously, there is a significant increase in the annular area of the cavitation cloud at the wall, indicating an augmentation in the volume of the cavitation cloud within the flow field following a relatively robust development process. Furthermore, observation reveals that at \(T=12\Delta T\), the cavitation cloud at the workpiece surface manifests in an overlapping ring shape, while the shape of the cavitation cloud at the center resembles that at \(T=0\). This indicates that at this juncture, as the cavitation cloud on the workpiece surface spreads from the preceding cycle, a new periodic cavitation cloud emerges and interacts with the workpiece surface, perpetuating a continuous impact on the workpiece surface. Figure 7: Periodic variation in vapor volume fraction distribution in the flow field when \(S_{D}=40\). Figure 10 depicts the periodic variation in vapor volume fraction distribution on the workpiece surface at the dimensionless standoff distance of \(S_{D}=40\). An observation of the figure indicates that when the standoff distance to the workpiece surface is increased by another 10 mm, the periodicity of the cavitation cloud generated near the workpiece surface maintains a distinct pattern. However, presently, there is a reduction in both the number and volume of cavitation clouds near the workpiece surface compared to \(S_{D}=30\), along with a decrease in the diffusion strike range on the workpiece surface. A total of \(12\Delta T\) was required for the transition from the point-like cavitation cloud appearing on the workpiece surface at \(T=0\) to the annular cavitation cloud with the highest volume fraction emerging at \(T=12\Delta T\). In comparison to the conditions when \(S_{D}=30\), the period of cavitation clouds is \(12\Delta T\).