## 4 Aerodynamic Performance Analysis  
Due to the unsteady flow field and the periodic shedding of the wake, the aerodynamic characteristics of the airfoil also show periodic characteristics. In order to ensure the accuracy of each working condition, when the lift coefficient and drag coefficient fluctuate periodically with time, the calculation can be considered to be convergent. In this study, the lift coefficient and the drag coefficient at 0.1 s after convergence of each working condition are intercepted for comparative analysis. Figure 6 shows the aerodynamic performance distribution of the airfoil. Figure 6a,b show that, due to the symmetry of the airfoil, when \(\alpha=0^{\circ}\), there is no pressure difference between the two sides, so the lift coefficient and drag coefficient will not change; when \(\alpha\
eq 0^{\circ}\), the amplitude of aerodynamic performance increases with the increase of angle of attack, and the change of \(K\) value has little effect on the frequency of aerodynamic performance. By comparing the average lift coefficient, the average drag coefficient, and the lift-drag ratio (Figure 6c-e, respectively), it was found that the drag coefficient decreases with the increase in K-value, which indicates that the drag coefficient of the blunt trailing edge airfoil can be well reduced. When \(\alpha=0^{\circ}\), \(\alpha=4^{\circ}\), and \(\alpha=8^{\circ}\), the lift coefficient and lift-drag ratio increase with the increase in K-value; with the increase in the angle of attack from \(8^{\circ}\) to \(12^{\circ}\), compared with the original airfoil, the lift-drag ratio of the blunt tail changes from +70% to \(-7.3\%\), showing that when the angle of attack reaches a certain value, the influence of the blunt trailing edge on the drag coefficient is greater than that on the lift coefficient, the slope of the drag coefficient increases, and finally, the slope of the lift-drag ratio decreases. With the increase in the angle of attack, the slope of the lift-drag ratio will decrease more. The greater the \(K\)-value is, the more obvious this phenomenon is. The vortex shedding frequency of the airfoil was determined by analyzing the Strouhal number (\(S_{t}=f\times C/U\)), as shown in Figure 6f. It can be seen from the figure that the \(S_{t}\) number of the blunt tail is greater than that of the original airfoil at a small angle of attack. When \(\alpha=12^{\circ}\), the \(S_{t}\) number of the original airfoil is the largest, which is very similar to the distribution of the lift-drag ratio, indicating that the vortex shedding frequency is greatly related to the aerodynamic performance of the airfoil. In order to further analyze why the influence of \(K\)-value on airfoil aerodynamic performance is completely the opposite when \(\alpha=8^{\circ}\) and \(\alpha=12^{\circ}\), we assessed vorticity for \(K=0\), \(\alpha=8^{\circ}\); \(K=0.6\), \(\alpha=8^{\circ}\); \(K=0\), \(\alpha=12^{\circ}\); and \(K=0.6\), \(\alpha=12^{\circ}\) at five selected moments (0 T, 0.25 T, 0.5 T, 0.75 T, and 1 T) in a stable cycle in each of the four working conditions, as shown in Figure 7. It can be seen from the figure that when the vortex on the suction surface (clockwise) is falling off, vortices on the pressure surface (anti-clockwise) are gradually generated and the lift coefficient increases gradually. As the vortex on the suction surface completely falls off, the lift reaches the maximum. At this time, the vortex on the pressure surface begins to fall off, vortices on the suction surface begin to form, the lift coefficient decreases gradually, and as the vortex on the pressure surface completely falls off, the lift coefficient reaches the minimum. The aerodynamic performance of an airfoil is affected by the size of the vortex and the position of vortex generation/shedding.