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.