## 3 Results and Discussion
### Effect of Rotation Speed on Rock Breaking by Jet Rotation Impact  
Figure 7 shows the depth, diameter, and volume of the crushed pits at different bit rotation speeds. As shown in the figure, there was an optimal rotation speed for maximum erosion.  
The data shown in Figure 7 were fitted to different jet pressures and rotation speeds (Figure 8). As shown in Figure 8, the depths, diameters, and rock-breaking volumes for the crushed pits increased quadratically with increasing rotation speed, and there was an optimum rotation speed to maximize pit depth, diameter, and volume. When the jet pressure was 6 MPa the maximum crushed pit diameter was reached at 150 rpm, but when the pressure was 12 MPa, the maximum pit diameter was reached at 200 rpm. However, there was little difference between the maximum pit diameters at those two rotation speeds, even though the crushed pit depths were quite different. The maximum crushed pit depth at 6 MPa and 150 rpm was 35 mm, whereas the pit depth at 12 MPa and 200 rpm was 60 mm. This indicates that rock-breaking depth played a dominant role in the rotary erosion of the jet flow and that the diameter of the crushed pit was subordinate. In addition, for the jet to have produced  
Figure 6: Influence of jet pressure on the depth and diameter of crushed pits by two kinds of water jets: (**a**) influence of jet pressure on the depth of crushed pits; (**b**) influence of jet pressure on the diameter of crushed pits.  
Figure 7: Influence of rotation speeds on rock-breaking efficiency.  
the maximum volume of the crushed pit, the rotation speed needed to be different for different jet pressures. Under the conditions used for our experiments, the maximum volume of the crushed pit generated by the 6 MPa jet was produced at 150 rpm, whereas the maximum rock-breaking action by the 12 MPa jet occurred at 200 rpm. This indicates that the optimum rock-breaking rotation speed for a water jet bit is positively correlated with jet pressure, and for maximum rock breaking, the higher the jet pressure is, the faster the optimal rock-breaking rotation speed is.  
The reasons for the jet rotating at the optimum speed eroding the rock are as follows. When the jet rotates and erodes the rock, it is difficult for water to collect in the bottom of the groove being cut. This reduces the water cushion effect and enhances the ability of the water jet to break the rock. In addition, according to the definition of rotation intensity [22], the faster the jet is rotated, the greater the swirling intensity is, the larger the jet diffusion angle becomes, and the more intense the turbulent mixing with the surroundings. The more the surrounding medium participates, the greater the capacity for increased exchange. This causes the jet to attenuate fast and shortens the effective rock-breaking distance. Furthermore, the shape of the jet is significantly affected by rotation; excessively rapid rotation will perturb the water jet and may deflect or atomize it, and either of those perturbations will weaken the jet's rock-breaking ability [23].  
Figure 8: The influence of rotation speed on rock breaking by two kinds of water jets: (**a**) influence of rotation speed on depth of crushed pits; (**b**) influence of rotation speed on depth of crushed pits; and (**c**) influence of rotation speed on volume of crushed pits.