FIGURE 4 Time histories of Cl and Cd for CA models. The flapping was set to downstroke firstly and then upstroke. Where, the downstroke is indicated by the shaded area. (a-c) the Cl tends to rise firstly, then fall and then rise. The higher the frequency, the greater the lift coefficient. (d-f) the Cd shows the trend of rising firstly, then falling, then rising and then falling.
Figure 5 shows the comparison of CA models at the same frequency. From Figure 5 (a-c), the Cl of AP1 is always the highest at all three flapping frequencies. This is even more clear when comparing the increase rates of the Cl-curves from Table 2, being 107.14% (when f = 55 Hz), 58.86% (when f =65 Hz), and 65.85% (when f =75 Hz), respectively. As shown in Figure 5(d-f), theCd of the AP1 is the highest when f = 55 Hz. With the increase of the flapping frequency, theCd averaged over the flapping cycle of AP1 decreases (see Table 2). At 65 Hz, the average Cdof AP1 is lower than AP2, the average Cd of AP2 is the highest; At 75 Hz, the average Cd of AP1 is the lowest, the average Cd of AP2 is still the highest. In addition, from Table 2, it can be seen that the average lift-drag ratio (\(\overset{\overline{}}{C_{l}/C_{d}}\)) of AP1 is always the highest at the investigated flapping frequencies.