# Impact Performance of Helmholtz Self-Excited Oscillation Waterjets Used for Underground Mining
## 3 Results and Discussion
### Cavitation Erosion Intensity
#### 3.3.1 Influence of Cavity Length  
According to the law of jet impact crushing, the damage to a specimen is closely related to the operating pressure and standoff distance [31]. The distribution of axial velocity decreased with the increase of the distance, thus the kinetic energy and the ability to destroy the target were reduced. As for cavitation jets, the failure of the specimen was not only due to the impact of jets, but also the cavitation collapse. When the jets were ejected from the nozzle, cavitation cloud developed completely after a certain distance, then broke and collapsed which indicated that cavitation need enough distance to develop. If the distance between the target and the nozzle outlet was within the effective range--that is, it was within the length range of the cavitation cloud and developed completely--the cavitation bubbles hit the wall surface, broke into small bubbles, and formed a micro-jet near the wall surface with huge local energy, which acted on the surface of the target to form effective damage [32; 33]. Moreover, if the standoff distance was not enough for bubble development, the effect of the jets on the target was also affected. Therefore, there must be an optimal standoff distance which best captures the working ability of HSEOW nozzles. Figure 13 shows the mass loss of specimen against standoff distance under different operating pressure (\(P_{\rm o}\) = 10 MPa, 15 MPa, 20 MPa, 25 MPa), which is able to clarify the erosion intensities of the jets. The dimensionless cavity diameter remained constant \(D_{\rm o}\)/\(d_{1}\) = 8, and the dimensionless cavity length varied from 1 to 4. The mass loss of jet erosion increased first with the increase of standoff distance, then decreased rapidly after reaching the maximum value, and finally approached to 0 regardless of operating pressure and nozzle structure. As was mentioned above, when the standoff distance was less than the length of the cavitation cloud, the cavitation was still in the initial stage and had not been fully developed, thus the erosion mass basically grew linearly with the increase of the standoff distance. When the standoff distance of the specimen exceeded the length of the cavitation cloud, the effect of the cavitation on the specimen decreased sharply and the erosion mass decreased sharply. The location of the maximum erosion capacity was related to the intensity of cavitation and the length of the cavitation cloud. It is obvious that the optimal cavity length of self-excited oscillating jets can achieve the largest mass loss for the reason that the variation of cavity length causes the development distance of the jets in the chamber much longer and the modulation effect on the jet is also changed [34]. When the dimensionless cavity length \(L_{\rm c}\)/\(d_{1}\) were 1 and 2, the curve was very similar, since the performance of self-excited oscillation exceeded the energy loss caused by the increase of cavity length, at this time, the corresponding cavity length of the jets had the strongest working ability. The structure was conducive to the formation of jet pulse and cavitation at \(L_{\rm c}\)/\(d_{1}\) = 2, so its maximum erosion capacity was basically equal to the nozzle of \(L_{\rm c}\)/\(d_{1}\) = 1, while the optimal standoff distance was slightly smaller. If the cavity length continued to increase, the energy loss in the chamber kept growing which resultedthat both the mass loss of erosion and the optimal standoff distance were greatly reduced. While the standoff distance exceeded the corresponding length of cavity cloud, the mass loss decreased rapidly; however, the influence of structure was little. It is worth noting that at the rising stage of the curve, the smaller standoff distance meant that cavitation development distance was insufficient, on the contrary, the small cavity length had stronger erosion ability. It is also observed in Figure 13a-d that the greater the operating pressure was, the larger the optimal standoff distance was.