A two-dimensional study by Sun & Yu at a low Reynolds number of 17 indicated that the effect of the clap and fling was very small when the distance between two aerofoils was approximately 1c. Figure 13a plots the minimum distances along the wingspan between two wings at the stroke reversals in the flapping-wing system with the flapping axes distance (d = 40 mm), which is denoted by the non-clap-and-fling case. 1c at the dorsal stroke reversal (t/T = 0.00–0.50) and 1.2c at the ventral stroke reversal (t/T = 0.50–1.00). A single-winged case was simulated and used for comparing the generation of vertical forces to verify whether or not the clap-and-fling effect was still present in this case. The results revealed that the average vertical force generated by the non-clap-and-fling case (108.0 mN) was only approximately 2.0% higher than that of the single-winged case (105.9 mN). As shown in figure 13b, most of the enhanced vertical force was from the fling effect at the beginning of each stroke. Hence, this result agreed with that in the study by Sun & Yu , although the effect of the clap and fling was not fully removed in the non-clap-and-fling case. Therefore, the result in the study by Sun & Yu could be also applied to relatively high Reynolds number environments.
Figure 13. (a) Minimum distance at the stroke reversals between two wings along the wingspan in the flapping-wing mechanism in the case of d = 40 mm (denoted by the non-clap-and-fling case) and (b) vertical force enhancement due to the presence of the clap-and-fling in the non-clap-and-fling case compared with the single-wing case.
Numerical and experimental approaches were employed to investigate the effect of the clap-and-fling mechanism on the force generation
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This study proposed a hovering insect-like two-winged FW-MAV with high flapping amplitude generation, which presented a clap and fling at each stroke reversal for the first time. The estimated forces from the CFD model based on the three-dimensional deformable wing kinematics agreed well with the forces measured by a 6-axis load cell, with differences of approximately 7.5% and 7.7% for vertical (Fz) and horizontal (Fy) forces, respectively. From the measurement, the clap and flings at both stroke reversals augmented the average vertical force by approximately 16.2% when compared with that in the case in which the minimum distance between the two wings was extended to 1c. The CFD results indicated that the clap and flings enhanced the vertical force by approximately 11.5% and horizontal drag force (F?) by approximately 18.4%. Analyses of the force generation at each quarter of a flapping cycle revealed that approximately 62.6% of the enhanced vertical force was attributed to the fling effect at the beginning of the strokes, while the claps at the ends of strokes contributed to the vertical force enhancement by approximately 37.4%. With respect to the enhanced horizontal drag force, the flings contributed to approximately 71.7% of the force increase and the claps contributed to 28.3% of the force enhancement. The enhanced force could be explained by the airflow structures. That is, a strong downward jet expelled from the trailing edges during the fling as well as the clap at each stroke reversal is a mechanism of the force enhancement. Additionally, the influx of air into the low-pressure region between the wings from the leading edges during the fling phases also significantly contributed to the enhanced force. Thus, the clap and flings played a significant role in improving the vertical force in the hovering insect-like two-winged FW-MAV developed in this study. In a relatively high Reynolds number environment, it was also revealed that the effect of the clap and fling was insignificant when the minimum distance between the two wings exceeded 1.2c.