**5. Verification Test of Micro Texture Drag Reduction Characteristics**

To verify the drag reduction effect of the streamwise micro-texture, half of the bearing bush is made of brass material, and the other half of the smooth bearing bush is used as the test reference, as shown in Figure 19, and the bearing test bench is on the right.

**Figure 19.** Bearing bush and test bench with streamwise texture.

The inner radius of the bearing bush is 30 mm, the total axial length is 72 mm, the axial length of the micro texture is 52 mm, the width of the micro texture is 200 µm, the depth is 100 µm, and the micro texture envelope angle is 22.5◦ and is set at the outlet of the divergent wedge according to the loading conditions.

The experimental loads are 650 N, 750 N and 1150 N, respectively, and the average speed of the bearing speed is 50 r/min, 75 r/min, 100 r/min, 125 r/min, 150 r/min, 175 r/min and 200 r/min, respectively. The lubricating oil used in the experiment is 45 # lubricating oil.

Theoretically, the viscosity changes with the change of pressure and temperature. However, because half of the bearing bush is used in the experiment, and the other half of the journal is immersed in an open oil groove, the temperature change is small, and the dynamic viscosity is approximately a constant, which is taken as *η* = 0.34 Pa · s. Figure 20 shows the measured friction coefficient of smooth bearing shells under different loads and different rotating speeds.

**Figure 20.** Comparison between friction coefficient and simulation results under smooth bearing test conditions.

Since the friction coefficient is the ratio of shear force to a corresponding load, the variation law of the shear force and friction coefficient is basically the same. With the increase of rotating speed, the wall friction coefficient tend to increase; in addition, increasing the load will increase the wall shear force and friction coefficient. Figure 21 shows the friction coefficient measured by the streamwise textured bearing bush under different loads and different rotating speeds.

**Figure 21.** Comparison between friction coefficient and simulation results under test conditions of axial bearing with flow direction texture.

It can be seen from the test data that the shear force and friction coefficient of the bearing bush with local streamwise micro-texture are smaller than those of the smooth bearing. Taking the smooth bearing test data as a reference, the dimensionless shear force is calculated as shown in Figure 22.

From Figure 22, the dimensionless shear force in the whole region is less than 1, that is, the textured bearing bush can obtain a wall shear force lower than that of the smooth bearing bush, and the drag reduction rate is better than that of the high-speed state at low speed. However, the relationship with the load did not show up.

**Figure 22.** Comparison between dimensionless shear force and simulation results in different working conditions.

Compared with the simulation results, the friction force and friction coefficient of the surface texture are slightly lower in the experimental results, because the ideal airtight state cannot be achieved due to the limitation of experimental conditions, and part of the friction loss is carried away by the oil. Additionally, shear force is higher than the experimental measurement results, this is because the calculation of the shear stress is associated with speed, without considering energy loss and under the influence of external factors, with the increase of rotational speed, the simulation for the shear stress calculation results is slightly higher than that of the experimental results, but the overall trend, which verifies the correctness and effectiveness of the simulation method.

### **6. Discussion**

By analyzing the LCC and dimensionless shear force of convergent wedge and divergent wedge with rectangular cross-section micro texture, it can be basically determined that in general, the use of a streamwise texture has a better drag reduction effect, and the possible reason is that the collapse position of bubbles is changed. The effect of local microtextured surfaces is better than that of full micro-textured surfaces. From Figures 17 and 18, when the micro-texture is arranged at the exit of the converging wedge or the entrance of the spreading wedge, it will lead to the reduction of the load capacity and the increase of the wall shear force, which proves that the arrangement of the micro-texture at the minimum oil film thickness of the journal bearing has no benefit to the performance improvement. The experimental results also prove the correctness of the numerical simulation. In the analysis of the divergent wedge considering cavitation, it is found that there is a region where the pressure almost does not change. The pressure in this region may be modified by the micro-textured surface to meet the requirements of the bearing performance.

There is no obvious direct relationship between load and dimensionless shear force from the experiment, because only a kind of micro-textured surface is used for experimental research, and it is impossible to draw a conclusion. However, further research may explore the optimal corresponding relationship between micro-textures for different loads and different speeds.

The initial convergence wedge analysis does not consider the cavitation model, so it cannot be directly determined that micro texture technology cannot be used in the convergence wedge to improve the LCC under special conditions and reduce the shear force.

When the high-speed fluid flows through the micro-textured surface, it will produce a bubble generation and collapse process, which will produce a periodic impact on the moving wall. There is no effective means to analyze the impact process on the rotation accuracy of the bearing. At present, most of the research content focuses on the use of CFD software for simulation, and the research on the residence time of bubbles on the micro-textured surface and the research on the boundary slip length are relatively few. For high-precision hydrostatic bearings, further research is needed on the effect of the microtextured surface on the axial drift.

**Author Contributions:** Conceptualization, Y.S. and K.C.; methodology, Y.S.; software, Y.S.; validation, Q.B., S.C. and K.C.; formal analysis, S.C.; investigation, Y.S.; resources, Q.B.; data curation, Q.B.; writing—original draft preparation, Y.S.; writing review and editing, Y.S.; visualization, S.C.; supervision, K.C.; project administration, K.C.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
