## 3 Results and Discussion
### Flow Field Analysis  
The aerodynamic coefficient varying with time at a 0\({}^{\circ}\) angle of attack is different from that at other angles of attack, which have the same trend in a period. The lift coefficient at a 4\({}^{\circ}\) angle of attack is taken as an example. The lift coefficient reaches the minimum at t* = 0.25, and then reaches the maximum at t* = 0.75. At a 0\({}^{\circ}\) angle of attack, the lift coefficient reaches the maximum at t* = 0.5, and then reaches the minimum at t* = 0.84. Figures 9 and 10 show the velocity contours at different moments of 4\({}^{\circ}\) and 0\({}^{\circ}\) angles of attack, respectively. Figure 11 shows the pressure coefficient distribution on the surface of the airfoil at corresponding moments. The pink vertical dotted line in the figure is the relative position of the moment reference point on the airfoil. The pressure coefficient is defined as Equation (4), where \(p\) is the local pressure, \(p_{\infty}\) and \(V_{\infty}\) are the pressure and velocity of the incoming flow far ahead, and \(\rho\) is the air density. \[C_{p}=\frac{p-p_{\infty}}{1/2\rho V_{\infty}{}^{2}}. \tag{6}\]  
Figure 9: _Cont._  
It can be seen from Figure 9 that the flow channel between the airfoil and the wavy ground at a \(4^{\circ}\) angle of attack expands gradually during the period of \(\mathbf{t}^{*}=0\) to \(\mathbf{t}^{*}=0.25\), which results in the  
Figure 11: Pressure coefficient distributions on the airfoil of different moments (**a**) \(\alpha=0^{\circ}\), (**b**) \(\alpha=4^{\circ}\). weakening of the blocking effect on the airflow. Therefore, the velocity of air in the flow channel increases, the pressure on the lower surface of the airfoil decreases, and the lift coefficient on the lower surface of the airfoil decreases. It can be seen from the pressure distribution in Figure 11 that the pressure coefficient of the lower surface of the airfoil decreases, while the pressure coefficient of the upper surface changes little in the whole period. The reason for the pressure coefficient of the upper surface is that the velocity distribution around the upper surface has little change. Figure 11 also presents that the pressure reduction in the lower surface is more distributed behind the moment reference point, which leads to an increase in the moment coefficient. At t* = 0.25, the leading edge of the airfoil reaches the trough of wavy ground, which means that the distance between the leading edge of the airfoil and the wavy ground reaches a maximum. At t* = 0.75, the leading edge of the airfoil reaches the peak, and the distance between the airfoil and the ground reaches the minimum. Consequently, the flow channel under the airfoil contracts gradually in the period of t* = 0.25-0.75, which results in a decrease in the stream velocity, an increase in the pressure in the flow channel, and an increase in the lift coefficient of the airfoil. The increase in pressure on the lower surface is more distributed behind the moment reference point, thus the moment coefficient of the airfoil decreases gradually. From t* = 0.75 to t* = 1.0, the distance between the leading edge of the airfoil and the ground increases gradually, and the flow channel expands during this period. The result is that the stream velocity increases and the pressure decreases, thus the lift coefficient decreases with the increase in the moment coefficient. The trend of aerodynamic coefficients at other angles of attack except 0\({}^{\circ}\) is consistent with that at a 4\({}^{\circ}\) angle of attack, and correspondingly their flow field structures are consistent with the above analysis. It can be concluded that the change in aerodynamic coefficients is caused by the expansion or contraction of the flow channel between the lower surface of the airfoil and the wavy ground. The following is an analysis of the phenomenon where the trend of aerodynamic coefficients at a 0\({}^{\circ}\) angle of attack in a period is different from that at other angles of attack.