# Impact Performance of Helmholtz Self-Excited Oscillation Waterjets Used for Underground Mining
## 3 Results and Discussion
### Cavitation Erosion Intensity
#### 3.3.2 Influence of Cavity Diameter  
In a HSEOW nozzle, the main function of cavity diameter is to control the size of vortices on both sides of the cavity and the feedback effect of backflow. The length of cavitation cloud in the outflow field is less affected by the cavity diameter, while the main influence is the turbulence intensity of the jets. When the cavity diameter is too small, the backflow vortices on both sides of the chamber do not have enough space to be fully developed, which squeeze the main stream and form obvious interference. The energy loss is large at this time, and the working ability of the nozzle is relatively poor. When the cavity diameter is too large, the vortices in the chamber cannot form effective modulation which generates a pulsed jet of strong impact. Therefore, there is also an optimal cavity diameter to maximize the erosion capacity of the nozzle.  
Figure 14 shows the mass loss of specimen against standoff distance under different operating pressure (\(P_{\rm o}\) = 10 MPa, 15 MPa, 20 MPa, 25 MPa), while the dimensionless cavity length remained  
Figure 13: Mass loss as a function of standoff distance under different cavity length. (**a**) \(P_{\rm o}\) = 10 MPa; (**b**) \(P_{\rm o}\) = 15 MPa; (**c**) \(P_{\rm o}\) = 20 MPa; (**d**) \(P_{\rm o}\) = 25 MPa.  
constant \(L_{c}/d_{1}=2\), and the dimensionless cavity length varied from 4 to 10. The curve increased first and then decreased, and there was a maximum value, which was similar to Figure 13. Under the same operating pressure, \(D_{c}/d_{1}=8\) was the optimal cavity diameter, where the mass loss was at the highest point of parabola. The optimal standoff distance slightly decreased as the cavity length changed smaller which could be seen that the effect of cavity diameter on the optimal standoff distance was not as obvious as that of cavity length. When the dimensionless cavity diameter was 4 and 6, the chamber was dominated by energy dissipation and cannot effectively form pressure pulsation, so the optimal standoff distance and damage to the specimen was relatively low. When the dimensionless cavity diameter was 8, the jets which achieved the optimal cavity structure corresponding to the pulsation periodically acted on the specimen and formed fatigue failure.  
With the variation of operating pressure from 10 MPa to 25 MPa, the optimal standoff distance of \(D_{c}/d_{1}=8\) changed from 36 mm to 50 mm, and the corresponding optimal standoff distance of each cavity diameter also increased. The length of the cavitation cloud increased as the pressure increased, and after the mass loss curve reached the highest point, there was still a relatively high erosion mass within a certain distance. In addition, when the operating pressure was 10 MPa, the declining trend of different cavity diameters was basically coincident after reaching the highest point. However, the curves corresponding to different cavity diameters were clearly distinguished at the operating pressure of 25 MPa, while the standoff distance was larger than the optimal standoff distance. Based on the previous studies, the optimal cavity length and diameter could be determined as \(L_{c}/d_{1}=2\), \(D_{c}/d_{1}=8\) within the scope of this paper.