## 4 Experimental Results and Discussion
### Analysis of Coupling Effect of Vortex-Induced Flutter Excitation  
In the previous article, the output performance of the classical vortex-induced energy harvester VEH, the flutter energy harvester FEH and the composite energy harvester VFPEH were studied through experiments. The experimental results were extracted and the output voltage waveforms of the three energy harvesters at a wind velocity of 9.5 m/s were drawn (as shown in Figure 13a). Moreover, the curve diagrams of the output power of the three energy harvesters with the wind velocity when the load resistance is 140 k\(\Omega\) are also drawn (as shown in Figure 13b). As can be seen from Figure 13a, when the wind velocity is 9.5 m/s, the output performance of the VEH-30 is poor, and its output voltage is low, 2.7 V, because it only generates vortex-induced vibration under the action of air flow. Compared with VEH-30, FEH has a symmetrical flutter structure of two airfoils, and the vibration of the maneuvering airfoil improves its output performance to a certain extent. When the wind velocity is 9.5 m/s, the FEH output voltage is 5.2 V. The output voltage of VFPEH-30 is better than the superposition of VEH-30 and FEH. When the wind velocity is 9.5 m/s, its output voltage is 54.3 V, which is 20.1 times and 10.4 times that of VEH-30 and FEH, respectively. The key data of the three energy harvesters in Figure 13b were extracted to draw Table 6. In Table 6, \(V_{0}\) is the starting wind velocity of the energy harvester, \(V_{\text{max}}\) is the wind velocity corresponding to the maximum output voltage of the energy harvester within the experimental test wind velocity range, \(U_{\text{max}}\) and \(P_{\text{max}}\) are the maximum output voltage and maximum output power (matching resistance is 140 k\(\Omega\)). Figure 13: Output characteristics of three energy harvesters: (**a**) Output voltage; (**b**) Output power. From Figure 13b and Table 4, the following results can be obtained: (1) The starting wind velocity of VFEH-30 is 4.8 m/s, which is much lower than that of VEH-30 and FEH of the same size: 7.1 m/s and 8.5 m/s. (2) In the experimental test wind velocity range, the peak wind velocity corresponding to the maximum output voltage of VFEH-30 is 9.0 m/s, which is lower than the peak wind velocities of VEH-30 and FEH of 12.5 m/s and 14.5 m/s, it means that VFEH-30 can obtain higher output voltage at lower wind velocity. (3) In the experimental test wind velocity range, the maximum output voltage of VFEH-30 is 60.2 V and the maximum output power is 6.47 mW, which is 16.8 and 6.3 times higher than the maximum output voltage of the VEH (3.59 V, 0.05 mW) and FEH (9.53 V, 0.26 mW), respectively. The cylindrical bluff body and airfoil of VFEH-30 are hollow structures, while the cylindrical bluff body of VEH-30 is a solid structure. The mass of VEH-30 cylindrical bluff body is close to the sum of the cylindrical bluff body mass, the two airfoils and the long axis mass of VFEH-30. When the incoming wind velocity is low, it needs more wind energy to overcome the bluff weight of the VEH-30 cylinder to make it vibrate. For the VFEH-30, on the one hand, the windward surface of which is larger than that of the VEH-30, and on the other hand, the airfoil mass is small. When the energy of the incoming flow exceeds the gravitational energy of the airfoil, it can make the airfoil vibrate, and then induce the vibration of the cylindrical bluff body, so the starting wind velocity of the VFEH-30 is much lower than that of the VEH-30. The mass element of FEH is symmetrically distributed airfoil and long axis. Its mass is small, but the windward surface is also small. When the air flows, the harvested wind energy per unit time is small, so its starting wind velocity is also large.