## 4 Discussion of the Obtained Results
### Peculiarities of Dynamic and Static Waterjet Stimulation
Expansion of the waterjet channel is accomplished due to the effect of the rear jets of the nozzle. Similar results were also obtained and published in [7]. Results of Determining the Optimal Values of Operating Parameters Affecting the Speed of Movement of the Waterjet Nozzle along the Created Waterjet Channel  
During this study, visual representation was made for spatial positioning of the waterjet nozzle during its movement through pipelines of different diameters; specifically, the waterjet nozzle was either pressed against the upper part of the pipeline (Figure 8a for the following modes of stimulation: Din = 38 mm and Q = 21 L/min; Din = 38 mm and Q = 10.5 L/min; Din = 48 mm and Q = 21 L/min; Din = 48 mm and Q = 10.5 L/min; Din = 62 mm and Q = 21 L/min) or against the bottom part (Figure 8b for the following modes of stimulation: Din = 62 mm and Q = 10.5 L/min; Din = 106 mm and Q = 10.5 L/min; Din = 106 mm and Q = 21 L/min). A schematic representation of this fact is shown in Figure 8.  
Taking into account the spatial position of a waterjet nozzle in pipelines with different diameters, two technological coefficients were obtained according to the results of the conducted research: coefficient of the nominal position of the waterjet nozzle, which characterizes the position of the waterjet nozzle in the center of the created channel, and the coefficient of the actual position (Equations (1) and (2)), which provide a numerical estimation of the waterjet nozzle motion:  
\[K_{nom}=\frac{2\cdot D_{nozzle}}{(D_{in}-D_{nozzle})}, \tag{1}\]  
\[K_{actual}=\frac{18.3}{H_{2}}, \tag{2}\]  
where \(K_{nom}\)--coefficient of the nominal position of the waterjet nozzle, which characterizes the position of the nozzle in the center of the created channel; \(K_{actual}\)--coefficient of the actual position of the waterjet nozzle during drilling; \(D_{nozzle}\)--nozzle outer diameter, mm; \(D_{in}\)--inner diameter of the created radial channel; \(H_{2}\)--actual value of deviation from the wall of the created radial channel, mm. The proposed coefficients were calculated for the considered modes of study, the results of calculations are shown in Table 4.  
Figure 8: Schematic location of the waterjet nozzle captured during the experiment: Din—inner diameter; H2—actual value of deviation from the wall of the created radial channel. The conducted study has shown that at mode No.3, the shortest time of passing the modeled section of the waterjet channel was recorded, and accordingly, the thrust force of the waterjet nozzle at this mode of stimulation was the greatest in the specified experimental conditions. The question of determining the optimal values of the proposed coefficients to ensure the most efficient process of waterjet drilling in different types of rock requires an additional in-depth study. However, even now can an unambiguous conclusion be drawn that under the experimental conditions it has been established that the highest efficiency is achieved with the following values of the proposed coefficients: K\({}_{\text{nom}}\geq\) 1.86, K\({}_{\text{actual}}\geq\) 0.93.