## 3 Drag Reduction Technology Based on Fluid Lubrication
### Bionic Microtexture Dynamic Pressure Lubrication and Drag Reduction  
There is a certain initial matching gap between the cylinder tube and the piston in a high-speed servo-hydraulic system, which can usually reach 10-50 \(\upmu\)m, and the gap is filled with lubricant (hydraulic oil), so the type of friction between the moving pairs is determined as the complete fluid lubrication based on the state of fluid lubrication. When a certain bionic microtexture morphology is designed on the inner surface of the cylinder barrel, its effect on the interface of the moving pair can be regarded as the effect of a hydrodynamic bearing. When the texture morphology changes, the hydrodynamic lubrication effect will also be changed accordingly. According to the characteristic of the grid and striped textures, [PERSON] et al. [31] compared diamond-like textures and strip textures to determine the optimal textures by solving the Reynolds equation while considering the influence of multiple parameters simultaneously, including the axial length ratio, the diamond angle, and the area occupancy, as shown in Figure 10. The results show that the lift of the diamond texture is nine times greater than that of the striped texture under the same working conditions, while its friction coefficient is only one-quarter of the striped texture, which can greatly improve the friction and responsiveness of the hydraulic cylinder. It can be seen that dynamic pressure lubrication has a significant relationship with texture morphology. It depends on characteristic parameters such as the shape, depth, and distribution of the texture itself, in addition to depending on the influence of actual working parameters such as load-velocity, fluid temperature and viscosity, and contact surface roughness. [PERSON]'s group [32] took the piston as the object and established single-textured and nine-textured models to compare and study their synergy based on the classical Reynolds equation, as shown in Figure 11. The simulation results reveal that the beneficial effect of the synergy firstly increases and then decreases as the area ratio of the texture increases. This is because the synergy of the textures acts as an \"average pressure\" and causes the pressure to decrease. As the area ratio of the texture increases, the beneficial effect from the synergy gradually increases and then decreases, which implies that there is an optimum area ratio--when the depth-to-area ratio is 0.009, and the texture depth is 10 \(\upmu\)m, the synergy effect on the lubrication effect is best. Additionally, we can learn from the study that, for a hydraulic cylinder with a light load, the film thickness changes with speed, and  
Figure 10: The hydraulic cylinder with diamond-like microtextures: (**a**) structure of hydraulic cylinder; (**b**) schematic diagram of a partially microtextured hydraulic cylinder. the synergistic exists but does not result in a substantial change. Therefore, the synergy is still related to the loads, structure of microtextures, etc. Aiming at constraining the seal performance and friction of the piston, a drag reduction technology using a bionic non-smooth surface was presented by [PERSON] et al. [33]. The seal ring of the cylinder was chosen as the research object, and there were many bionic textures on its outer surface, as shown in Figure 12. The piston speed and the bionic pit diameter for the drag reduction effect were studied. The simulation results demonstrate that the effective stress of the bionic non-smooth sealing ring meets the requirement of cylinder seal tightness when the compression rate is 20%. Moreover, it is able to reach the maximum drag reduction rate under a certain speed of the piston when the pit diameter is 1.5 mm. Meanwhile, the drag reduction rate increases with an increase in initial velocity and a certain diameter of the pit. The seal ring has a stable drag reduction rate when the speed reaches 0.6 m/s.