# Comprehensive Investigations on Fluid Flow and Cavitation Characteristics in Rotating Disk System
## 3 Simulation Analysis
### Results of Simulation Analysis  
#### 3.3.1 Effect of Texture Rate on Cavitation  
In the literature summary, cavitation rarely happens on absolutely smooth surfaces. The effect of different texture rates is investigated in this subsection. For the transient  
Figure 16: Boundary condition of flow field. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Scheme**} & \multirow{3}{*}{**Radial**} & \multicolumn{3}{c}{**Circumferential Layer**} & \multirow{3}{*}{**Axial**} & \multirow{3}{*}{**Number of Grid**} & \multirow{3}{*}{**Cavitation**} \\  & & & & & & \\ \cline{3-5}  & & **Texture** & & & & \\  & & **Area** & & & & \\ \hline
1 & 68 & 20 & 100 & 14 & 48,106 & 0.0004345 \\
2 & 68 & 20 & 100 & 28 & 105,324 & 0.0003432 \\
3 & 68 & 20 & 100 & 56 & 219,760 & Divergence \\
4 & 68 & 30 & 120 & 28 & 137,484 & 0.0005012 \\
5 & 68 & 40 & 160 & 28 & 184,384 & 0.0006212 \\
6 & 102 & 30 & 120 & 28 & 207,252 & 0.0006417 \\
7 & 136 & 30 & 120 & 28 & 277,020 & 0.0006419 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mesh segmentation strategy, number of grid cells, cavitation volume fraction. Figure 15: Schematic diagram of oil film with hexahedral texture. simulation of cavitation, the default texture rate was 3.205%, which increased exponentially, while a non-texture oil film was used as a control group. First, when the rate was 0, the cavitation volume fraction was 0, indicating that no cavitation occurred on the smooth surface. When the rate was 3.205%, the fraction was 9.66 \(\times\) 10\({}^{-4}\), and it stabilized at 0.00151 and 0.00297 when the texture rate was increased to 6.41% and 12.917%, respectively. Cavitation area expands as the texture rate increases. However, it should be noted that this conclusion applies only to the hexahedral texture studied in this research and not necessarily to other types of texture. Considering that the coverage of the hexahedral texture is proportional to the cavitation strength, when processing the hexahedral texture for hydraulic machinery such as wet clutches or mechanical seals, an optimal texture rate must be considered in order to balance the optimal film carrying capacity of the oil with a low cavitation effect. As shown in Figure 17, the cavitation volume fraction increased briefly during the simulation cycle before returning to a stable state. Although the final values varied, they all peaked within 0.015 s. In Figure 18, When the rate is 3.205%, cavitation occurs only inside the texture dimples, the spacing between adjacent textures is greater, and the hexahedral dimples resemble isolated rough elements. When oil flows into the dimples, the texture roots are subjected to greater shearing action, and cavitation is more likely to occur at the textures' downstream low-pressure position. As shown in Figures 19 and 20, as the texture rate increases, cavitation is no longer limited to the interior of the texture dimples, and the higher density of texture dimples increases the intensity of the boundary layer turbulence, causing cavitation to occur in the non- texture region at the outer warp. The CFX simulations support that the simplifying assumptions made in the Reynolds equation model, computed by a finite difference algorithm, are a sufficiently good approximation. Comparing Figures 11a and 17, the cavitation volume fraction is 0 when texture rate is 0. Comparing Figures 11b and 18, when the texture rate is low, the cavitation vapor preferentially appears inside the texture dimples at the outer diameter, which proves that cavitation is more likely to occur at larger angular velocities. Comparing Figures 12 and 18, it is found that after the texture rate increases, the cavitation vapor is still concentrated at the outer diameter. The cavitation vapor is still concentrated at the outer diameter but is no longer confined to the texture dimples and cavitation vapor starts to appear in the non-texture region. Figure 19: Cavitation morphology change with hexahedral texture rate 6.41%.