## 4 Results and Discussion  
### Analysis of the Lubrication Mechanism  
Figure 4 presents the velocity vector of the circumferential profile at the distribution circle \(d_{5}\) = 38 mm. The upper specimen is rotating clockwise around the specimen axis (sliding in the direction of the decreasing polar angle). The maximum speed appears on the upper surface of the seawater film which contacts with the undersurface of the cylinder. The speed declines step by step in the direction of the seawater film's thickness, with the direction remaining unchanged. For the smooth surface, the maximum speed only passes down layer by layer (Figure 4a). For the hemispherical-pit surface, the seawater at top of the pit first moves in the same direction, with the seawater at the bottom surface of the seawater film due to viscous force. After hitting the front half wall of the pit, the seawater turns downward and flows back along the bottom of the pit. After hitting the back half wall of the pit, the seawater turns upward and moves in the same direction as the seawater at the bottom surface of seawater film again. That is, a vortex ring is formed in the pit (Figure 4b). Figure 5 presents the pressure contour of the circumferential profile for the distribution circle \(d_{5}\) = 38 mm and the pressure curve of the seawater film's upper surface at the corresponding position. The hemispherical pit can be considered as two parts. Along the direction of the movement, the front part of the pit wall forms a convergent gap with  
Figure 4: Velocity vector of the circumferential profile at a \(d_{5}\) = 38 mm distribution circle: (**a**) smooth surface; (**b**) hemispherical-pit surface. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Friction Pair** & **Rotation Speed** & **Sliding Speed** & **Load** & **Time** & **Temperature** \\  & **n/(r/min)** & **v/(m/s)** & **P/N** & **t/s** & **T\({}^{o}\)C** \\ \hline
1 & & 1000 & 1.57 & & \\
2 & & 1250 & 1.96 & & \\
3 & & 1500 & 2.36 & & \\
4 & & 1750 & 2.75 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Working conditions and testing program. the cylinder undersurface, where the seawater flows from a large gap to a small one; that is, the flow rate across each section caused by the velocity flow goes down by degrees. On the other hand, the back part of the pit wall forms a divergent gap with the cylinder undersurface, where the seawater flows from a small gap to a large one; that is, the flow across each section caused by the velocity flow steps up by degrees. On the basis of the flow continuity conditions--that is, the mass of fluid flowing into any section should be equal to the mass of fluid flowing out of another section--the pressure distribution, which is high pressure generated at the front edge of pit and low pressure generated at the back edge of pit (Figure 5b), will be produced. It can be seen that the friction surfaces have a convergent wedge, the sliding surface has sufficient speed, and the lubricant has an appropriate viscosity, which will produce a hydrodynamic lubrication effect [32]. Figure 6 indicates the pressure contour on the upper surface of the seawater film of the computational fluid field, and obvious pressure peaks and bumps above pits are observed in Figure 6b. As the distribution circle radius increases, the linear velocity gradually increases and the hydrodynamic effect is obviously enhanced. The bearing capacity can be obtained through the pressure distribution integration on the upper surface of the seawater film. As shown in Figure 7, the bearing capacity of hemispherical-pit surface is 3.5% higher than that of the smooth surface, indicating that  
Figure 5: Pressure contour of the circumferential profile of a \(d_{5}\) = 38 mm distribution circle and the pressure curve of the seawater film’s upper surface at the corresponding position: (**a**) smooth surface; (**b**) hemispherical-pit surface. Figure 6: Pressure contour of the seawater film upper surface of the computational fluid field: (**a**) smooth surface; (**b**) hemispherical-pit surface.