## 4 Results and Discussion  
### Influence of Water Pressure on the Coating Removal for the Metal Wall  
High pressure water has the power to remove coating using the wall-climbing robot. In this section, the influence of water pressure on the coating removal effect was investigated. Four levels of high pressure (100, 120, 140, and 160 MPa) were chosen and compared. Figure 8 shows the surface condition for the four levels with the other experimental conditions as follows: the stand-off distance is 15 mm, the travelling speed is 2.25 m/min and the inclined angle is 10 degree. It can be found that the coating removal effect is greatly influenced by the applied water pressure. When the water pressure is 100 MPa, there is still much residual coating on the metal surface, while when the water pressure is larger than 120 MPa, most of the coating can be removed quite well. After the experiments, the coating thickness gauge was used to measure the thickness of the residual coating. Figure 9 shows the measured removal rate for the four cases. So, considering both the coating removal efficiency and the coating removal rate, the optimal impacting water pressure is 120 MPa. Figure 7: Finite element model for high pressure water jet process. In order to investigate the influence of water pressure on the target surface's impact effect more clearly, the numerical simulation under various water pressures was carried out. To better understand the influence of water pressure on the target surface, the water pressure in a more wide range is investigated. For this study, the stand-off distance and the inclined angle used in the simulation is 30 mm and 0\({}^{\circ}\), respectively. Meanwhile, the impacting water pressures are 100, 150, 200, and 250 MPa. Figure 10 shows the predicted velocity distribution under various water pressure. It can be seen that the maximum velocity always occurs at the outlet of the nozzle. The slight velocity divergence near the nozzle outlet is due to the consideration of surface tension. When the high pressure water impacts the wall surface, the velocity in the center is reduced to 0 quickly while the velocity on both sides remains very huge, as shown in Figure 10.  
Figure 8: Surface condition under various water pressure levels (**a**) initial state; (**b**) 100 MPa; (**c**) 120 MPa; (**d**)140 MPa; (**e**)160 MPa. Figure 9: Coating removal rate for various water pressure levels. According to [PERSON]'s law, the theoretical water jet exit velocity can be calculated as follows:  
\[v=f(x)=w\sqrt{\frac{2P}{\rho}} \tag{3}\]  
where \(P\) is the inlet pressure of the nozzle, \(\rho\) is the water density, and the parameter \(w\) is a nozzle efficiency parameter, which was taken as =0.95 for this study. So, for the operating pressure used in this study, the corresponding jet velocity can be obtained from the equation. Figure 11 shows the predicted outlet velocity of the nozzle under various water pressures. The predicted value coincides well with that of the theoretical calculation value based on Equation (3). The maximum error between the predicted value and the theoretical value is less than 0.5%, so the reliability of the numerical simulation results is validated. Figure 10: Predicted velocity cloud chart under various inlet water pressures (**a**) 100 MPa; (**b**) 150 MPa; (**c**) 200 MPa; (**d**) 250 MPa. The predicted static pressure distribution and shear stress distribution along the target surface under various water pressures are also obtained and the case with water pressure of 100 MPa is shown in Figure 12. It can be seen that for the specific numerical condition, the maximum static pressure reacting on the target surface is at the center of the jet. With the increase of the distance from the jet center, the static pressure gradually decreases. However, the shear stress at the center of the jet is almost 0. With the increase of the distance from the jet center, the shear stress gradually increases.