# Numerical Simulation of Random Cavitation Suppression Based on Variable NACA Airfoils
## 4 Results and Discussion
### Flow Field and Re-Entrant Jet Discussion  
The re-entrant jet is similar to the reflux in boundary layer separation. According to the experimental determination of reference [23], it was found that the frequency of the re-entrant jet was essentially the same as the volume change frequency of cavitation, which caused the cavitation bubbles at the back of the cavitation area to burst and collapse, eventually forming cloud cavitation. Therefore, this section analyzes the flow field and the return jet, which intercepted the velocity vector diagram of the original airfoil and the airfoil with different protruding degrees on the upper surface of the zonal 1.5C section, as shown in Figure 6a. It is obvious that there is a re-entrant jet in the original airfoil, which leads to the cavitation shedding and cloud cavitation phenomena. It can be seen that the re-entry jet is still present on the upper surface of the airfoil with a maximum variable of 1 mm. In addition, the three-dimensional flow field at 0.05C of the airfoil with the degree of bulge of 1 mm is found to be intensified, and due to the bulge in the middle, the thickness of the bulge on both sides is slightly lower than the sides, the fluid in the middle will flow to both sides, and the flow on both sides will be greater than the flow rate in the middle. This causes the fluid on both sides to flow to the middle and continuously develop to the front of the airfoil and eventually form a vortex (as shown in Figure 6b); the vortex cavitation at the edge of the airfoil (show in Figure 4a) is also caused by this phenomenon.  
With the increase in the degree of bulge, the re-entrant jet decreases gradually, and when the degree of bulge is 3 mm, the high-speed fluid is blocked, and there is no reverse flow phenomenon (as shown in Figure 7a). When the degree of bulge reaches 6 mm, reverse flow is found again near the bulge (as shown in Figure 7b). This is due to the fact that the high-speed water flow in front of the bulge is mostly blocked, and the backflow is completely blocked by the bulge, so cavitation does not occur.  
To sum up, from the point of view of cavitation suppression, the blocking effect of memory metal covering on the development of cavitation is significant. However, the design purpose of airfoil is to provide lift, so considering cavitation alone will lead to the increase in airfoil resistance, and the lift-drag ratio and cavitation resistance should be considered to ensure the mechanical properties of the airfoil. The maximum bulge deformation of the covering is controlled between 3 and 6 mm by adjusting the internal fluid pressure and thickness, which can hinder the development of cavitation under abnormal operating conditions, and it reduce the erosion damage to the wall by cavitation,  
Figure 5: Pressure graph.  
and it ensures that the normal operation of machinery does not produce negative effects such as vibration and noise. After operating conditions are improved, the memory metal is restored by heating and other methods to ensure the normal operation of machinery.  
Figure 6: Flow field on the upper surface of airfoil with maximum variable of 1 mm. (**a**) Velocity vector diagram of airfoil with maximum variable of 1 mm at the z = 1.5C section. (**b**) Velocity vector diagram of airfoil with maximum variable of 1 mm at the z = 0.05C section. (**c**) Streamline diagram.