## 3 Results and Discussion
### Flow Field Analysis
At t* = 0, the stream velocity under the lower surface is higher than that at the 4\({}^{\circ}\) angle of attack. As can be seen from the comparison of Figures 9 and 10, the position of the stagnation point of the leading edge at a 0\({}^{\circ}\) angle of attack is higher than that at the 4\({}^{\circ}\) angle of attack. At the same time, the leading edge of the airfoil at the angle of attack of 0\({}^{\circ}\) is closer to the ground. These two factors lead to the larger shrinkage of the cross section near the inlet of the flow channel, which leads to the more significant acceleration effect of the airflow in the flow channel. In Figure 11, it can be seen that there is a negative pressure peak on the lower surface, which is caused by the acceleration of the airflow. The negative pressure peak is even smaller than the pressure peak on the upper surface. In the period of t* = 0-0.25, the distance between the leading edge of the airfoil and the ground increases, the shrinkage of the flow channel decreases, and the acceleration effect of the airflow in the flow channel weakens. The airflow deceleration effect caused by the shrinkage of the flow channel exceeds the airflow acceleration effect caused by the expansion of the flow channel. Therefore, the airflow decelerates, and the pressure on the lower surface increases, resulting in an increase in the lift coefficient. The deceleration effect is more significant near the leading edge of the airfoil, and the pressure increment on the lower surface is concentrated near the leading edge, which leads to an increase in the moment coefficient. In the period of t* = 0.25-0.5, the distance between the leading edge of the airfoil and the ground decreases, the blocking effect on the airflow increases, and the shrinkage of the flow channel near the leading edge does not change significantly. Therefore, the airflow velocity in the flow channel decreases, the pressure increases, and the lift coefficient increases. In the period of t* = 0.5-0.84, the distance between the leading edge of the airfoil and the ground decreases, and the shrinkage of the flow channel near the leading edge becomes significant again. The significant shrinkage of the flow channel results in an increase in the stream velocity, a decrease in the pressure near the leading edge of the airfoil, and a decrease in the lift coefficient. The decrease in the pressure peak on the lower surface can be seen in Figure 11. Although the pressure peak in front of the moment reference point is significantly reduced, the backward shift of the pressure peak causes a weakening of the effect of changing the moment. At the same time, there is also a significant pressure reduction behind the moment reference point, and the arm of force based on the reference point is large. Therefore, the contribution of the pressure reduction behind the reference point to the increase in the moment coefficient exceeds the contribution of the pressure peak reduction to the decrease in the moment coefficient, and the moment coefficient increases. In the period of t* = 0.84-1.0, the distance between the leading edge of the airfoil and the ground begins to increase, the shrinkage of the flow channel decreases, the acceleration effect weakens, the stream velocity decreases, and the pressure increases, resulting in an increase in the lift coefficient. The pressure reduction in front of the moment reference point is significant. However, the arm of force is small, the effect on the moment is less than that on the pressure increment behind the moment reference point, which leads to a decrease in the moment coefficient. It can be concluded that the flow mechanism of the airfoil moving over wavy ground at a small ride height at a 0\({}^{\circ}\) angle of attack is different from that at other angles of attack. There is not only the acceleration and deceleration of the airflow caused by the expansion and contraction of the flow channel, but also the change in the airflow velocity caused by the change in the shrinkage at the inlet of the flow channel.