## 4 Results and Discussion
### Flow Characteristics of Sheet Cavitation
#### 4.1.2 Unsteady Characteristics  
The unsteady periodic variation of sheet cavitation is the main reason for the pressure fluctuations on the hydrofoil surface. The shearing action generated by the re-entrant jet is the root cause of the change in the structure of the bubble tail. This section analyzes the unsteady characteristics of sheet cavitation based on the numerical results, and evaluates the prediction performance of three cavitation models at the same time.  
Figure 6 shows the time evolution of sheet cavitation for numerical results and the experimental data of a whole cycle. At the time of 0 \(T\), a groove in the cavity is formed by the action of the re-entrant jet at the cavity tail. The re-entrant jet continuously moves forward over time, and until 0.7 \(T\), the wall separation of the re-entrant jet due to the adverse pressure gradient shears the cavity tail, which leads to the shedding of microscale bubbles at the time of 0.9 \(T\). From 0.9 to 1.0 \(T\), the attached bubble becomes large, and the separation point between the cavity tail and the wall moves back to complete a periodic variation. It can be found from Figure 6 that the Zwart model cannot predict the unsteady characteristics of the cavity attached to the suction side; it does not simulate the shedding of the microscale bubbles in the cavity tail. The full cavitation model (FCM) fails to predict the cavity structure of sheet cavitation. At the end of the cavity, the full cavitation model (FCM) predicts the shedding of bubbles whose scale is large. It indicates that the FCM overpredicts the shedding of bubbles, which is inconsistent with experimental phenomena. The full cavitation model (FCM) takes the turbulent energy \(k\) into account for the definition of the condensation source term. So, the correlation between the phase transition and turbulent flow field is enhanced, and the unsteady characteristics of the flow field are enhanced, which leads to the shedding of large bubbles in the cavity tail. Obviously, it is can be found that the periodic fluctuations of the cavity structure with time evolving, as predicted by the Kunz model, is consistent with experimental phenomena. Through the above analysis, the results predicted by the Kunz model are the closest to the experiment.  
Figure 7 shows the distribution of the velocity vector field predicted by the three cavitation models. We find that the Zwart model does not predict the formation of the re-entrant jet and the vortex. The attached cavity becomes narrow and long under the action of inertia force, so the Zwart model can simulate the effect of inertia force on the elongation of the bubble. Meanwhile, the full cavitation model (FCM) predicted the formation of the re-entrant jet and the vortex shedding of bubbles, but the structure of the attached cavity is inconsistent with the experimental observation.  
Figure 6: Time evolution of sheet cavitation for numerical results and experimental pictures (\(\sigma=1.075\), \(\alpha=60^{\circ}\)).  
So, the FCM results are not accurate. The Kunz model predicts the formation and movement of the re-entrant jet accurately. It predicts the shearing action on the attached cavity tail and the shedding of microscale bubbles, which is consistent with the experimental observations.