Analysis of simulation
The effect of texture arrays of different shapes
Figure3a shows the dimensionless bearing capacity of a texture array of different shapes at a speed of 6m/s, the specimen with a parallelogram texture array has the maximum bearing capacity. Figure4 shows the pressure distribution on the upper wall of the texture array with different shapes. From Fig. 4a,b, it can be concluded that due to the confluence role of textures, the maximum pressure of the convergent trapezoidal texture array is greater than that of the divergent trapezoidal texture array, but the area of the boost zone is much smaller than that of the divergent trapezoidal texture array, which eventually leads to lower total bearing capacity for the sample with convergent trapezoidal texture array.The reason is that for the texture array the mutual influence of adjacent textures perpendicular to the motion direction needs to be considered. Therefore, it can be concluded from Fig. 4c, d, the highest pressure of the diamond array texture is greater than that of the parallelogram, but the high pressure peak area and low pressure cavitation area of the parallelogram have both shifted. So the inhibition between the front and rear microtextures is reduced, and the overall bearing capacity is enhanced, which is better than that of the rhombic texture array, which is the same as the simulation result of single texture.
Influence of parallelogram textured array distribution mode
As shown in Fig.3b, the dimensionless bearing capacity of five different distribution patterns of micro-textured arrays at speed of 6m/s is compared.From the figure,it can be seen from the figure that the matching mode between the micro-textures has a stronger influence on the flow field and pressure field in the numerical simulation than simply changing the shape of the textures.
Figure4e–h show the pressure distribution of the square micro-textured array and the three different pattern arrays. Different patterns of parallelogram texture arrays will not only change the pressure field of the fluid domain but also affect the flow field. From the streamline diagram of the square texture array, it can be seen that the streamlines are only partially bent near the texture entrance and exit, and the adjacent textures will not affect their respective flow fields. In the single-symmetric and pair-symmetric texture arrays, the inclination of the parallelogram texture makes the flow field appear obvious confluence effect, which is conducive to the production of higher oil film pressure. The streamlines in the texture array with single orientation are obviously inclined, but there is no effect of confluence, and the lateral flow of the fluid will cause energy dissipation, weaken the dynamic pressure effect of the texture, and make it the lowest carrying capacity.Similar to the parallelogram texture in the single-texture simulation, the texture arrays of other different distribution patterns also have the effect of reducing the suppression effect between the front and back textures, because of the high-pressure peak area and the low-pressure cavitation area have shifted. The high-pressure area of the outlet can better extend to the direction of fluid flow.
Influence of the tilt angle of the parallelogram texture array
Figure5a shows the pressure distribution of the texture array with a single symmetry orientation. As shown in Fig.5b, in a single symmetry orientation texture array, as the tilt angle increases, the pressure on the center line e of the upper wall increases significantly. With the increase of the inclination angle of the microtexture, the outlet pressurization zone of the second and third rows of microtexture will be closer, which improves the oil film pressure. However, if the inclination angle of the micro-texture is too large, it will reduce the range of the pressurized zone and affect the Oil film pressure.
Figure5c shows the pressure distribution of a multi-symmetrical orientation texture array. As shown in Fig.5d, the oil film pressure on the straight line f between the first and second rows of textures will gradually increase as the tilt angle increases. In addition, it can be seen from Fig.5e that the increase in the inclination angle of the textures will reduce the pressure on the center line g of the upper wall surface.
Figure5f shows the pressure distribution on the upper wall of the textured array with a single orientation when the tilt angle is 45°. It can be concluded that a texture array with a single orientation cannot converge pressure.Take the straight line h at the exit of the third row of textures to analyze the pressure distribution ,as shown in Fig.5g. As the tilt angle increases, the pressure peak at the exit of the parallelogram texture shifts to the tilt direction, and the pressure also reduce accordingly at the same time. Therefore, it is believed that parallelogram micro-textured array with single orientation has no effect on improving the bearing capacity compared to a square texture, and the larger the tilt angle, and the increase of the tilt angle will weaken the dynamic pressure effect of the texture.
Influence of the speed of the upper wall
In28, the existence of eddy current was found by analyzing the velocity field, and the following explanation was given: the increase of the depth of the micro-dimples will enhance the wedge effect and improve the hydrodynamic pressure performance, but on the other hand, the eddy current phenomenon will lead to energy dissipated, impairing the hydrodynamic bearing capacity. Therefore, there will be an optimal value for the texture depth, so that the dynamic pressure bearing performance of the micro-pit unit is the strongest.This indicates that the weakening of the hydrodynamic performance with the increase of texture depth is due to the combined effect of the reduction of the cavitation zone and the increase of the vortex zone.
Figure6a shows the dimensionless bearing capacity of the square texture array and three parallelogram texture arrays with an inclination of 26.6°. With the increase in speed, the dimensionless bearing capacity of all pattern texture arrays gradually increases, and the difference in bearing performance between different pattern arrays is obvious. Within the speed range studied in this paper, the bearing capacity of the single-symmetric orientation texture array is always the best.
Figure6b–f show the dimensionless bearing capacity of a asymmetrically oriented texture array with different tilt angles under different speed conditions.It can be seen from the figure that under various speed conditions, the asymmetric orientation array has an optimal tilt angle to make the dimensionless bearing capacity the highest. As the speed increases, the optimal tilt angle tends to increase. When the speed is 18m/s, the optimal tilt angle of the asymmetrical array increases from 26.6° to 36.9°. This is because the increase in speed will increase the suppression of the dynamic pressure effect between the front and rear textures, and the greater the inclination angle of the parallelogram, the more conducive to staggering the low- pressure zone at the entrance of the texture and the high-pressure peak zone at the exit in the flow direction. The high-pressure zone of the texture outlet extends backward in the direction of flow, thereby increasing the range of the high-pressure zone.
Analysis of experiments results
The interaction between textures can change the pressure distribution and flow field of the fluid domain to a greater extent, which is the main reason for the aggravation of cavitation. Different microtexture depth, arrangement, angle, shape and other factors will change its pressure distribution and flow field.
Formation principle of cavitation bubbles
In17, the cavitation of lubricating oil is considered to be divided into gas cavitation and vapor cavitation. Typically, lubricating oils contain dissolved air with a saturation pressure at atmospheric levels. When the local liquid pressure is lower than the saturation pressure, the dissolved gas will escape from the solution, forming an air cavity. If the liquid pressure continues to decrease to the vapor pressure, the lubricating oil may boil at ambient temperature, forming vapor cavitation. Therefore, to further explore the cavitation type and bubble composition of texture-induced cavitation, the state of cavitation bubbles was continued to be observed after the experiment is stopped, as shown in Fig.7.
It can be seen from the figure that when the rotating table stops, the bubbles in the texture do not disappear instantly, but gradually separate from the texture entrance, and converge into a whole bubble suspended inside the texture. This shows that under the conditions of this experiment, the main reason for the generation of cavitation bubbles is gas cavitation because the oil vapor generated by steam cavitation can be absorbed instantaneously, while the gas separated from the lubricating oil takes a long time to dissolve.
The behavior of texture-induced cavitation bubbles is a dynamic equilibrium process. First, the rotation of the lower sample makes a local pressure drop zone formed at the entrance of the texture, and the air precipitated in the lubricating oil is continuously accumulated after the formation of cavitation gas nuclei, and the air bubbles visible to the naked eye are gradually formed. Then, due to the shearing and dragging effect of the rotation of the lower sample on the lubricating oil and the air bubbles, the cavitation bubbles gradually extend downstream, and the flow of the lubricating oil will bring more dissolved air, and the continuously precipitated gas makes the air bubbles big. Finally, when the precipitation of lubricating oil gas and the dissolution of gas reach equilibrium, stable cavitation bubbles are formed.
Experiments on micro-textured arrays of different shapes
The fixed load is 10N, and the experiment is carried out under different speed conditions to compare the friction coefficient of the non-textured sample and five kinds of texture samples of different shapes. The five texture arrays of different shapes can improve the lubricating performance of the friction pair compared with the non-textured samples, and their friction coefficients are all reduced.
When the speed is 6 r/min (0.019m/s), the friction coefficient of samples are not much different. When the speed is low, the oil film of the friction pair has not been fully formed, and the texture has little effect on the improvement of the oil film bearing capacity. When the speed is greater than 18 r/min (0.057m/s), the lubricating performance of the sample with the convergent trapezoidal texture is always the smallest. When the speed is 24 r/min(0.076m/s), the cavitation pictures of texture arrays with different shapes are shown in Fig.7. It can be inferred from the Figure that cavitation has occurred on the surface of the textured samples of various shapes. However, the different texture distribution positions will cause the pressure distribution around the texture to be different, and the cavitation bubbles caused by the texture will also be significantly different. The textures located downstream of the texture array are more prone to drag out cavitation bubbles. The reason is that these textures are closer to the pressure outlet and the surrounding oil film pressure is relatively low. In addition, comparing the cavitation bubbles induced by texture arrays of different shapes, it can be seen that the convergent trapezoidal texture array produces the least bubbles, while the divergent trapezoid texture induces the most cavitation bubbles.
Experiments on micro-textured arrays with different distribution patterns
Figure8 shows the cavitation state of three different parallelogram texture array samples at speed of 24 r/min. It can be seen from the figure that only a small part of the textures in asymmetric orientation texture array hascavitation bubbles overflowing. In a texture array with multiple symmetrical orientations, there are certain cavitation bubbles at the rear end of the outlet. The cavitation effect induced by the single orientation texture array is the most severe, which is due to the convergence of the cavitation bubbles of the upstream and downstream textures, forming a larger cavitation area. Comparing the pressure distribution of the texture array with different modes in the previous simulation, it can be seen that the area where the oil film pressure is higher is less likely to form cavitation bubbles.
Experiment on the oil film thickness of different pattern micro-textured arrays
In order to further analyze the mechanism of the improvement of tribological performance by texture arrays of different patterns, the laser displacement sensor is used to estimate the oil film thickness. It can be seen from Fig.9 that compared with the non-textured sample, the oil film thickness of other texture modes all have increased. Combining the previous simulation analysis, combined with the previous simulation analysis, it is found that the oil film bearing capacity of the textured array is directly proportional to the oil film thickness. This conclusion is consistent with the literature. The increase of the oil film thickness will reduce the friction coefficient of the sample and enhance the lubricating performance. Thus, the increase in the thickness of the oil film means that the direct contact area of the upper and lower sample is reduced, which makes the lubrication more sufficient.