# Evaluation and Validation of Viscous Oil Cavitation Model Used in Torque Converter
## 5 Cavitation Parameters Modification and Calculation  
Based on the parameter study results and the properties of viscous oil that as the working medium of torque converter, we modified the parameters of cavitation model to improve the simulation accuracy, as shown in Table 4.  
According to the properties of fluid, the density of hydraulic transmission oil and gas are 835.2 kg/m\({}^{3}\) and 0.718 kg/m\({}^{3}\) at 90 \({}^{\circ}\)C, and the dissolved and undissolved gas in the hydraulic oil promotes the cavitation process and increase the saturated vapor pressure. Therefore, we modified the relevant parameters that \(MDR=\) 1163 and \(P_{\rm v}=\) 700 Pa.  
The maximum influencing parameter--\(NVF\)--was modified to 5 \(\times\) 10\({}^{-5}\) to correct the over-prediction of cavitation by the original cavitation model parameters, which can significantly reduce the volume of cavitation bubbles and the oil vapor volume fraction. Besides, we modified \(R_{\rm B}\) to 2 \(\times\) 10\({}^{-5}\) m which can reduce the rapid growth and collapse of bubbles caused by the excessive initial bubble radius. \(F_{\rm vap}\) was modified to 20 due to the vaporization degree and vaporization rate of viscous oil are less than that of water.  
However, we did not modify \(F_{\rm cond}\) and \(CRRF\) because our results show that these two parameters have little effect on the cavitation performance of torque converter.  
We calculated the hydrodynamic performance of torque converter under cavitation working conditions using the modified parameters to verify the accuracy of the new cavitation model. Figure 37 shows the comparison of torque ratio, efficiency and capacity constant between original model, modified model, and test at various speed ratio under cavitating conditions, and the errors of turbine torque, torque ratio, efficiency, and capacity constant between modified cavitation model and test at various speed ratio under cavitating conditions as shown in Figure 38.  
Through the comparison of calculation results before and after parameters modification, we can conclude that the results of modified cavitation model parameters are more consistent with the test, especially the capacity constant. The error of capacity constant was reduced from 6.7% to within 2% when \(SR<\) 0.4, and other performance results of torque converter maintain a higher accuracy. In conclusion, the modification of cavitation model can effectively improve the calculation accuracy of hydrodynamic torque converter cavitation performance.  
In addition, we obtained the pressure data at different positions on stator blade through the micro pressure test inside hydraulic torque converter, as shown in Table 5. Compared with the internal flow field extracted from the CFD results, we found that the error between the calculated results and the test was within 8%, which further verified the  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Parameter** & \(P_{\rm v}\)/\(\mathbf{Pa}\) & \(F_{\rm cond}\) & \(F_{\rm vap}\) & \(R_{\rm B}\)/\(\mathbf{m}\) & \(MDR\) & \(NVF\) & \(CRRF\) \\ \hline Default Value & 110 & 0.01 & 50 & 2 \(\times\) 10\({}^{-6}\) & 1000 & 5 \(\times\) 10\({}^{-4}\) & 0.25 \\ Modified Value & 700 & 0.01 & 20 & 2 \(\times\) 10\({}^{-5}\) & 1163 & 5 \(\times\) 10\({}^{-5}\) & 0.25 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameter modification on torque converter cavitation model.  
Figure 36: Oil vapor volume fraction distribution in stator domain at stall under various maximum density ratios.  
accuracy of the modified cavitation model. It is worth noting that the average pressure on the suction surface of the stator blade is obviously less than the oil supply pressure, which is caused by cavitation that occurred at this position.