## 3 Transient Cavitation Behavior in a Hydrodynamic Torque Converter  
### Validation of the CFD Model  
The detailed specifications of the pump, turbine, and stator are summarized in Table 3.  
The hydrodynamic performance of a torque converter is defined by several operating parameters that are described as follows. \[SR\ (\text{Speed ratio}):\ SR=\frac{T_{\text{Turbine}}}{N_{\text{Pump}}} \tag{13}\]  
\[TR(\text{Torque ratio}):TR=\frac{T_{\text{Turbine}}}{T_{\text{Pump}}} \tag{14}\]  
\[\eta\ (\text{Efficiency}):\ \eta=\frac{N_{\text{Turbine}}\times T_{\text{Turbine}}}{N _{\text{Pump}}\times T_{\text{Pump}}}=SR\times TR \tag{15}\]  
\[CC\ (\text{Capacity constant}):\ CC=\frac{T_{\text{Pump}}}{\rho{}g{}N_{\text{Pump }}{}^{2}D{}^{5}}, \tag{16}\]  
where \(N_{\text{Pump}}\) and \(T_{\text{Turbine}}\) are the rotational speed (rpm) and \(T_{\text{Pump}}\) and \(T_{\text{Turbine}}\) are the torque (N m) of pump and turbine, respectively. \(D\) represents the diameter of the torus. CC represents the capacity of the power transmission, \(10^{-6}\) min\({}^{2}r^{-2}\)m\({}^{-1}\). The details of the hydrodynamic performance calculation and experimental apparatus of the hydrodynamic torque converter were reported previously [26]. CFD models with and without cavitation were used. Figure 6 compares the calculated hydrodynamic performances with the test data. The calculated hydrodynamic performances were consistent at high \(SR\)s (\(SR\geq 0.3\)). However, the CFD models differed tremendously in terms of the torque ratio and capacity constant at low \(SR\)s (\(SR<0.3\)). The experimental data indicated that the capacity constant increased with increasing \(SR\) at low \(SR\)s, whereas the trend reversed at high \(SR\)s. The calculated capacity constant without cavitation did not show this trend with a maximum error of 13.23% under the stall operating condition. In contrast, the calculated capacity constant with cavitation had a maximum error of \(<\)5% at low \(SR\)s. The capacity constant considering cavitation increased with the \(SR\), which agrees with the test data. The two CFD models returned slightly higher values than the test data at high \(SR\)s, which could be attributed to the flow leakage at the shell and core of the hydrodynamic torque converter being neglected. When \(SR\geq 0.3\), the two CFD models were consistent regarding the torque ratio. When \(SR<0.3\), the CFD models had maximum errors of 4% and 8% with and without cavitation, respectively, for the torque ratio. The CFD results indicate that the development of cavitation reduces both \(TR\) and \(CC\) at low \(SR\)s (\(SR<0.3\)). At high \(SR\)s (\(SR\geq 0.3\)), the two CFD models agreed with each other, which was attributed to the disappearance of cavitation. In general, the model considering cavitation predicted the capacity and torque ratio more accurately than the model without cavitation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Element** & **Inlet Angle** & **Outlet Angle** & **Number of Blades** \\ \hline Pump & 123\({}^{\circ}\) & 64\({}^{\circ}\) & 29 \\ Turbine & 32\({}^{\circ}\) & 157\({}^{\circ}\) & 24 \\ Stator & 120\({}^{\circ}\) & 33\({}^{\circ}\) & 22 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Structural parameters of the hydrodynamic torque converter. Transient Cavitation Behavior in the Hydrodynamic Torque Converter under the Stall Operating Condition  
Heavy cavitation was predicted by the calculations in the stator under the stall operating condition, and only sheet cavitation was detected in the turbine. The fluid coming out of the turbine struck the stator blade with a large incidence angle, which resulted in a local high-velocity and low-pressure region on the suction surface near the leading edge and a large reverse flow downstream. The reattachment point of the reverse flow was located approximately 0.38 of the blade length, which was induced by a local reverse pressure gradient.