## 4 Results and Discussions
### Influence of the Dimensionless Standoff Distance on the Rock-Breaking Effect  
Figure 17 shows the crushing ability of jet erosion for granite under different dimensionless standoff distances, and the influence of jet target distance on the erosion ability was analyzed. It can be clearly observed from the figure that for the CWJ, with an increase in the dimensionless standoff distance, the crushing ability gradually decreases, and the depth, area, and volume loss of the crushing pit are negatively correlated. Moreover, the crushing ability of the SPWJ first increases and then decreases with an increase in the dimensionless standoff distance. For both the CWJ and SPWJ, the area, depth, and erosion ability of the jet-crushing rock are closely related to the dimensionless standoff distance of the jet, and all of them depend on the characteristics of the jet's external flow field. As can  
Figure 16: Characteristics of granite erosion by water jet under different output pressures: (**a**) erosion pit area; (**b**) depth of erosion pit; (**c**) erosion pit volume; (**d**) accumulated volume loss of erosion pits; (**e**) unit input pressure failure volume. be clearly observed from Figure 17, when S = 100, the crushing capacity of the CWJ reaches its peak, whereas that of the SPWJ reaches its peak when S = 200. Under the same nozzle outlet conditions, there are significant differences between SPWJ and CWJ, and the optimal dimensionless standoff distance of the SPWJ is increased by 100%. Furthermore, it can be clearly observed from Figure 17a-c that, compared with the CWJ, the erosion crushing area of the SPWJ is weaker than that of the CWJ at the initial stages of S = 100 and 150. However, with an increase in the dimensionless standoff distance, the crushing volume of the SPWJ is better than that of the CWJ at S = 200. This is because the optimal dimensionless standoff distance of the CWJ is approximately 100. Additionally, the experimental results are close to the previously reported results, which proves the validity of the experimental results. Nevertheless, with the increase in the dimensionless standoff distance, the crushing capacity of the CWJ gradually decreases, whereas that of the SPWJ gradually increases. At the initial stage, the CWJ is at the optimal target distance, and granite is cracked in a large range by drilling, expansion, and cracking failure modes. In contrast, the energy in the core section of the SPWJ is not completely released, and thus the granite is broken by drilling and eroded in a small range. However, with an increase in the  
Figure 17: Characteristics of granite erosion by water jet under different dimensionless standoff distances: (**a**) erosion pit area; (**b**) depth of erosion pit; (**c**) erosion pit volume; (**d**) accumulated volume loss of erosion pits; (**e**) unit input pressure failure volume. dimensionless standoff distance, the SPWJ gradually reaches the optimal target distance. Owing to the increase in the target distance and the intensification of mixing with air, the shearing effect of air on the jet is enhanced, the crushing ability of the jet is obviously weakened, and the effective length of crushing is further reduced. When the dimensionless standoff distance increases from 100 to 200, the erosion area caused by the SPWJ decreases from less than 17.6 times to 53.6% of the area caused by the CWJ. Similarly, the erosion depth caused by the SPWJ changes from less than 3.57 times to better than 1.34 times the depth caused by the CWJ. Additionally, the spalling volume of granite caused by the SPWJ decreases from less than 48.3 times to better than 1.22 times the volume caused by the CWJ. With a further increase in the dimensionless standoff distance, the crushing capacity of the SPWJ and CWJ begins to decline because, with the increase in the target distance, the energy interaction between the jet and environment becomes more intense, the core segment of the jet begins to dissipate, and the axial jet tends to atomize.