# Effects of Nozzle Exit Angle on the Pressure Characteristics of SRWJs Used for Deep-Hole Drilling
## 4 Experimental Results and Discussion  
### Axial Pressure Oscillations  
\(P_{max}\) and \(P_{a}\) against \(S\) under different exit angles are plotted in Figures 5 and 6, respectively, corresponding to the inlet pressures of \(P_{i}\) = 10 MPa and 20 MPa. More specifically, Figures 5a and 6a show the development trends of \(P_{max}\) against \(S\), and Figures 5b and 6b illustrate the trends of \(P_{a}\) along with \(S\). Figure 4: Picture of the tested organ-pipe nozzles. Figure 3: Profile of organ-pipe nozzle. As illustrated in Figures 5 and 6, the exit angle of the organ-pipe nozzle greatly influenced the axial pressure oscillations of the SRWJ, regardless of the inlet pressure. To be more specific, the exit angle of \(0^{\circ}\) resulted in the most violent pressure oscillations under the two inlet pressures, because the peaks and amplitudes were the maximum at almost all the standoff distances when compared with those under the other exit angles. Moreover, the development trends of \(P_{max}\) and \(P_{a}\) against the standoff distance for the exit angle of \(\theta=0^{\circ}\) were dramatically different from those for all the other exit angles. In more specific terms, for the case of \(0^{\circ}\), both \(P_{max}\) and \(P_{a}\) first increased and then decreased with the increase of \(S\). If the typical features of the axial pressure oscillations of SRWJs are taken into consideration [20], it can be claimed that only the organ-pipe nozzle with an exit angle of \(0^{\circ}\) can generate successful SRWJs. That is to say, the exit angle of the organ-pipe nozzle can even determine the occurrence of SRWJs. This is also experimental proof of the conjecture proposed by [PERSON] et al. [12] that the interactions between the nozzle exit and the jet significantly affect the pressure oscillations of SRWJs. Furthermore, the optimal standoff distance, which is defined as the distance where \(P_{max}\) or \(P_{a}\) reaches the maximum, was approximately 30 mm for \(P_{max}\) and approximately 60 mm for \(P_{a}\) when the exit angle was \(0^{\circ}\). Therefore, the optimal standoff distances for the maximum \(P_{max}\) and \(P_{a}\) were different, which is in good agreement with the previous experimental results [24, 33]. Another interesting phenomenon observed is that the values of \(P_{max}\) under different exit angles were very close to each other at a standoff distance of \(S=10\) mm, which was independent of the inlet pressure, and so were the values of \(P_{a}\). This indicates that the influence of the exit angle on the axial pressure oscillations of the SRWJ was rather insignificant at this standoff distance, which is most likely due to the fact that the cavitation bubbles take some time to grow into large-scale vortex rings [13, 14, 27]. In other words, strong pressure oscillation starts to occur at a certain standoff distance, because the volume changes of the vortex-ring in the shear layer are a dominant source contributing to the pressure oscillations of SRWJs [12]. In more specific terms, at an inlet pressure of \(P_{i}=10\) MPa, the \(P_{max}\) and \(P_{a}\) of SRWJs issuing from organ-pipe nozzles of the other five exit angles rapidly decreased with the increasing standoff distance,  
Figure 5: \(P_{max}\) (a) and \(P_{a}\) (b) as a function of \(S\) at \(P_{i}=10\) MPa. Figure 6: \(P_{max}\) (a) and \(P_{a}\) (b) as a function of \(S\) at \(P_{i}=20\) MPa. as shown in Figure 5. The development trends of \(P_{max}\) against \(S\) were very similar to those of a regular submerged waterjet, indicating that the organ-pipe nozzles with exit angles larger than \(0^{\circ}\) could no longer generate effective SRWJs. In addition, it was observed that different exit angles led to clearly different values of \(P_{max}\) at each standoff distance, and the order of the exit angles for obtaining greater peaks was \(0^{\circ}\), \(60^{\circ}\), \(45^{\circ}\), \(75^{\circ}\), \(30^{\circ}\), and \(90^{\circ}\), as depicted in Figure 5a.