# Evaluation and Validation of Viscous Oil Cavitation Model Used in Torque Converter
## 4 Parameter Study Results and Discussion
### Analysis of the Influence of Significant Cavitation Parameters
#### 4.3.1 Nuclei Volume Fraction--_Nvf_  
The performance of torque converter under various _NVF_ is shown in Figure 11. It is clear that nuclei volume fraction has a great influence on the performance of the torque converter. The capacity constant, torque ratio and mass flow rate decrease with an increasing _NVF_ when \(NVF\leq 5\times 10^{-5}\). _NVF_ exhibits limited influence on the overall performance when _NVF_ exceeds \(5\times 10^{-5}\).  
The absolute pressure distribution diagram of the middle streamline on stator blade surface (Figure 12) shows that there is a big differential pressure between the suction  
Figure 11: Capacity constant, torque ratio, and mass flow rate of torque converter at stall under various nuclei volume fractions.  
Figure 10: Oil vapor volume fraction distribution near the stator blade of torque converter.  
surface and pressure surface of stator blade, which decreases gradually from head to tail, the differential pressure makes torque converter play a role in transmitting torque. In addition, the pressure decreases drastically on the pressure surface of stator blade with an increasing \(NVF\), and the pressure change near the head is more obvious, while it decreases slightly on the suction surface, which leads to the reduction of mass flow rate, capacity constant and torque ratio. However, there are also some errors caused by calculation and streamline selection (such as the high value of \(NVF=5\times 10^{-4}\)).  
Figures 13 and 14 are the oil vapor volume fraction and the bulk interphase mass transfer rate on the middle streamline of stator blade under various \(NVF\). The results reveal that with the increase of \(NVF\), the oil vapor volume fraction and the mass transfer rate of the head and tail on blade surface increase gradually, but they are always 0 in the middle of blade, indicating that cavitation mainly occurs at the leading edge of the stator suction surface, and the increase of \(NVF\) will aggravate the cavitation degree.  
Figures 15 and 16 show the 10% vapor volume fraction distribution in the stator domain and the oil vapor volume fraction distribution on the stator blades under various \(NVF\). We ignored the trend error caused by blade selection and found, the volume of oil vapor bubbles at the head of stator blade gradually increases with an increasing \(NVF\), and covers the entire head of the stator blade when \(NVF\) is greater than \(5\times 10^{-6}\). The oil vapor volume fraction on the surface of blade exhibits an increasing trend, the area with high oil vapor volume fraction gradually expands as the \(NVF\) increases.  
Figure 12: Absolute pressure on stator blade at stall under various nuclei volume fractions.  
Figure 13: Oil vapor volume fraction on stator blade at stall under various nuclei volume fractions.  
Figure 17 reveals that the volume of bubbles, the length of bubbles and the high oil vapor content area increase with an increasing \(NVF\). As \(NVF\) represents the volume fraction of the nucleation sites, higher \(NVF\) indicates more oil vapor nuclei in the fluid, which consequently leads to an increasing volume of bubbles and deteriorated performance of torque converter. In conclusion, cavitation becomes more serious with the increase of \(NVF\). The reason is that higher \(NVF\) indicates more nuclei sites, which results in heavier cavitation.