## 4 Experimental Results and Discussion
### Influence of the Key Parameters of the VFPEH on Its Output Performance
#### 4.1.1 Diameter of Cylindrical Bluff Body  
By changing the diameter of the cylindrical bluff body, the influence of the bluff body on the output performance of the VFPEH is experimentally studied, and the optimal size of the cylinder is determined to achieve a better energy harvesting effect. When we make experimental prototypes of different specifications, other dimensions are the same, and the diameter of cylindrical bluff body is 20 mm, 25 mm, 30 mm and 35 mm, respectively, which are, respectively, recorded as the prototypes VFPEH-20, VFPEH-25, VFPEH-30 and VFPEH-35. We study the output performance of each prototype with the wind velocity. Through multiple experiments and data collection, the curves of output voltage and output power of the VFPEH-20, VFPEH-25, VFPEH-30 and VFPEH-35 with wind velocity are drawn by using the obtained experimental data, as shown in Figure 3. \begin{table}
\begin{tabular}{c c} \hline
**Properties** & **Airfoil** \\ \hline Materials & PLA \\ Density (kg/m\({}^{3}\)) & 1250 \\ Young’s modulus (GPa) & 3.5 \\ VFPEH captive energy mass (g) & 72.4 \\ Cantilever beam mass (substrate and PZT) (g) & 20.1 \\ airfoil mass (g) & 8.4 \\ Connecting shaft mass (shaft and bearing) (g) & 9.9 \\ Cylindrical mass (g) & 50.5 \\ Span (mm) & 50 \\ Half chord (mm) & 18 \\ Airfoil axis position (mm) & 15 \\ \hline \end{tabular}
\end{table} 
Table 2: NACA0012 wing dimensional parameters. From Figure 3, that the trend of output voltage and output power with the wind velocity is basically the same for the prototypes with different diameters of cylindrical bluff bodies. When the wind velocity is less than 4.8 m/s, the VFPEH output voltage and output power is small and unstable. When the wind velocity exceeds the starting velocity of 4.8 m/s (the starting wind velocity of the VFPEH with four diameters is around 4.8 m/s), the output voltage and output power of the VFPEH increases rapidly and reaches its maximum value (wind velocity between 8.0 m/s and 9.0 m/s), and then decreases slowly with the increase in wind velocity. When the wind velocity is greater than 14 m/s, the output voltage and output power changes slightly with the increase in wind velocity. After reaching the maximum, the output of VFPEH gradually decreases with the increase in wind velocity because the cylindrical blunt body suppresses the occurrence of flutter during the energy harvest process. It was found that from Figure 3, for the VFPEH with different cylinder bluff body diameters, with the increase in cylinder bluff body diameter, the voltage output slightly increases when the wind velocity is 4.8 m/s. When the diameter of the cylindrical bluff body is 20 mm, the maximum output voltage of VFPEH is 3.1 V, while the output voltage of VFPEH-35 reaches 5.7 V, an increase of 83.9%. When the wind velocity is greater than 4.8 m/s, with the increase in the wind velocity, the voltage output effect of VFPEH-30 with a cylindrical bluff body diameter of 30 mm is better than the other three VFPEH. It can be seen from Figure 3 that when the wind velocity is 8.0 m/s-9.0 m/s, the output voltage and output power of the VFPEH reaches near its maximum value. Table 3 shows the maximum output voltage and maximum output power of four VFPEH with different bluff body diameters when the wind velocity is 8.0 m/s-9.0 m/s. It can be seen from Figure 3b and Table 3 that when the diameter of cylindrical bluff body is 20 mm, the maximum output voltage and maximum output power of VFPEH-20 are the smallest compared with the other three sizes. When the diameter is 30 mm, the  
Figure 3: Output performance of the VFPEH at different cylindrical bluff diameters: (**a**) Output voltage; (**b**) Output power (matching resistance is 140 k\(\Omega\)).