# Impact Performance of Helmholtz Self-Excited Oscillation Waterjets Used for Underground Mining
## 3 Results and Discussion
### Erosion Pattern on Specim Surface  
The cavitation damage caused by the HSEOW nozzle on the specimen was mainly the jet cavitation cloud under submerged conditions (see Figure 9) [23; 24]. The circular honeycomb dense holes took shape in a radial distribution near the center. The core area of jets was concentrated in the circular area at the center, and the bubble cloud carried by jets collapsed on the target surface. Numerous dense micro-jets repeatedly impacted on the surface so that dense holes were formed. In the vicinity of the central region, the damage was distributed radially due to the stress wave effect. The density of holes and the damage intensity weakened with the increase of distance. There was cavitation inside the Helmholtz cavity, which also had a great impact on outflow field of the jets [25]. The effects of submerged and non-submerged jets on the specimens were quite different. When the bubble cloud of the non-submerged jets inside the cavity entered into the atmosphere with the jet flow, it collapsed quickly due to the effect of atmosphere pressure. When HSEOW jets flowed into the submerged environment, a strong cavitation effect would be produced. The variations of working parameters and structural parameters affected the flow characteristics and working performance of the cavitation jets [26; 27; 28]. Figure 8: (**a**) Non-dimensional velocity magnitude at different axial locations (**b**) Non-dimensional velocity magnitude at different radial locations. Figure 9: Macroscopic appearance of eroded specimen (\(L_{\mathrm{c}}/d_{1}=3\), \(D_{\mathrm{c}}/d_{1}=10\), \(\alpha=120^{\circ}\), \(P_{\mathrm{o}}=10\) MPa). When \(L_{\rm c}/d_{1}=4\), \(D_{\rm c}/d_{1}=12\), \(\alpha=120^{\circ}\), \(P_{\rm o}=10\) MPa, comparison of erosion results between conical nozzle and HSEOW nozzle is shown in Figure 10. In addition to the impact of high-pressure water flow, the specimen destruction of submerged jets was also affected by the erosion of cavitation. Under the same conditions, the cavitation effect produced by the HSEOW nozzle had a much greater destructive effect than that of conical nozzle. At the same time, it also showed that cavitation was the major factor in the destructive mechanism of the target. The 400-times magnification, photos also indicated that the surface appearance hit by HSEOW nozzle was quite different from that of conical nozzle. Pulsed nozzles could form large pits due to strong cavitation, while conical nozzles had relatively uniform surfaces. To further understand the cavitation generation and evolution of HSEOW, the time evolution of water vapor volume fraction is shown in Figure 11. The cavitation occurs at the shear layer in the chamber in Figure 11a which migrates downstream to the nozzle and grows up. The cavitation in the interior of the cavity leads to larger cavitation in the external field near the downstream nozzle outlet as shown in Figure 11b,c. The shedding of external cavitation continues until reaching the target wall in Figure 11d,e. The bubbles impact on the wall surface and collapse, which will form high energy and then dissipate gradually along the wall surface as shown in Figure 11f. The cavitation effect of the conical nozzle was far less than that of the Helmholtz nozzle. The cavitation formed due to the strong shear action in the Helmholtz cavity was ejected with the jets, in the process of migration downstream, it developed and became cloud cavitation [29]. At the end of the cavitation cloud, when the cavitation was close to the point of collapse, many small bubbles formed a fog-like cavitation composed of numerous single bubbles. The collapse process of a single spherical bubble is shown in Figure 12.  
Figure 10: Comparison of erosion results between conical nozzle and HSEOW nozzle. (**a**) Conical nozzle (400-times magnification); (**b**) HSEOW nozzle (400-times magnification). The bubble was spherical at the beginning, when it was close to the wall surface, the far end from the wall began to be disturbed and deformed.