## 5 Results and Discussion
### Comparision and Discussion  
The performance of a wind energy harvester is mainly affected by the shape and mass of the bluff body, the material characteristics and dimensions of the cantilever beam, the characteristics of the piezoelectric transducer, the conditions of the approaching flow, etc. The harvesting efficiencies of various typical galloping experimental results are summarized in Table 3. The harvesting efficiencies were calculated according to the method from [PERSON] et al. [64]. It can be found that the efficiency of the No.1 harvester was much higher than that of this paper, because the bluff body was made from lightweight materials, with a mass of 1.8 g. The bluff body in the present research was produced by 3D printing technology, with a mass of 25 g. Three-dimensional printing technology was adopted as it replicates the shape more delicately for precise shape optimization. The power of the No.1, 2, and that in this work exceed 1 mW, which is able to power the Internet of Things (IoT) ViPSN platform for a vibration-powered sensing and transmitting system [65]. The efficiencies of No.3, 4, and 5 were much lower than that of this paper, because the characteristics of the piezoelectric transducer were poor. The results of the harvesters with the square and arc-plate shaped bluff body were compared. The output power of the WEHs with a square and arc-plate-shaped bluff body was 0.50 mW and 1.01 mW at wind velocity of 5 m/s. Meanwhile, the efficiency was improved from 0.12% to 0.25%. For a wind velocity of 7 m/s, the output power was enhanced from 1.12 mW to 2.31 mW, and the efficiency was improved from 0.10% to 0.21%. The aerodynamic shape optimization doubled both the output power and efficiency  
For the square cross-section, the vortex reattachment phenomenon often appears near the side faces, as shown in Figure 14a. This phenomenon causes up-side and down-side pressure fluctuations and, thus, reduces the total lift force of the bluff body [68]. Fortunately, the arc-plate-shaped bluff body avoids the vortex reattachment and, thus, guarantees the coordination of vortex shedding, as shown in Figure 14b. Therefore, the lift force of the arc-plate-shaped cross-section is larger than that of the square cross-section. The above comparison demonstrates the superiority of the aerodynamic shape optimization of an arc-plate-shaped bluff body. This finding has great potential to be applied in energy harvesters based on galloping. In the past few years, significant efforts have been made to develop various types of piezoelectric energy harvesters based on wind-induced vibrations as the power source for wireless sensors. The work of this study can be directly applied to this type of piezoelectric wind energy harvester, to increase the power generation significantly. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Bluff Body**} & & & & \\ \hline
**No.** & **Author** & **Shape** & **Width (cm)** & **Height (cm)** & & & **Wind Velocity** & **Efficiency (\%)** \\ \hline
1 & [PERSON] [66] & Square & 2 & 10 & 1.25 & 5 & 0.77 \\
2 & Sirohi [67] & D-shape & 3 & 23.5 & 1.14 & 4.7 & 0.24 \\
3 & [PERSON] [33] & Cylinder with rods & 4.8 & 24 & 0.053 & 5.5 & 0.004 \\
4 & Hu [7] & Square with fins & 2.4 & 24 & 0.034 & 5 & 0.007 \\
5 & [PERSON] [35] & Cylinder with plate & 4.8 & 24 & 0.014 & 5 & 0.002 \\
6 & Our work & Square & 5 & 10 & 0.50 & 5 & 0.12 \\
7 & Our work & Arc-plate & 5 & 10 & 1.01 & 5 & 0.25 \\
8 & Our work & Square & 5 & 10 & 1.12 & 7 & 0.10 \\
9 & Our work & Arc-plate & 5 & 10 & 2.31 & 7 & 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of various galloping-based wind energy harvesters. [PERSON]'s team [64] invented a device converting ocean/river current's hydrokinetic energy to electricity using a circular bluff body. Different-shaped bluff bodies were also investigated for energy harvesting, such as prism-, triangular-, and pentagon-shaped bluff bodies [69; 70].