# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 6 Wake Galloping Energy Harvesters
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Fluttering Mechanism** & **Cut-in Velocity** & **Optimum** & **Velocity at** & **Output Power** & **References** \\  & **(m/s)** & **Resistance** & **Max Power** & **(m/s)** & **(mW)** & **References** \\ \hline \multicolumn{6}{c}{**TE-FEHs**} \\ \hline Flag-type & 6500 & 22 & 135 mW/kg & [PERSON] et al. [119] \\ \hline Multi-layered membrane & 7000 & 4 & 0.00033 & [PERSON] et al. [120] \\ \hline Flapping-type & 18,000 & 14 & 67 & [PERSON] et al. [121] \\ \hline Parallel-structured & 4000 & 25 & 0.82 & [PERSON] et al. [122] \\ \hline Leaf-flutter & 11,000 & 7 & 17.9 & [PERSON] et al. [123] \\ \hline Flag-type & 0.5 & 198,000 & & 4.5 & [PERSON] et al. [124] \\ \hline Flag-type & 5000 & 7.5 & 0.03672 & [PERSON] et al. [125] \\ \hline Rotary-flapping type & 9000 & 10 & 3.9 & [PERSON] et al. [126] \\ \hline Wing flutter & 10,000 & 7.5 & 1.5 W/m\({}^{2}\) & [PERSON] et al. [130] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._The wind flow allowed both transverse and translational vibrations of the tip body. The outcomes demonstrated that the triangle tip body produces the optimum result from a wake-induced vibration energy harvester. [PERSON] et al. [144] utilized several cross-sectional interference cylinders illustrated in Figure 11c to boost the energy harnessing performance. For the PEH with a square prism, it generated a peak average power of up to 803.4 \(\upmu\)W at 2.36 m/s with a spacing ratio of L/D = 0.9. When compared with the energy harvester without the interference cylinder, the corresponding synchronization region was considerably boosted by 380%. [PERSON] et al. [136] verified that the steady-state bandwidth of wake galloping FEHs can be widened using a bistable restoring force, which reduces their sensitivity to changes in flow speed. A PE-FBEH was reported by [PERSON] [139]. For both the upstream and the downstream cylinders, circular cross-sections of similar diameters were considered. At a 10 m/s wind speed and the maximum voltage obtained was 38 mV. [PERSON] et al. [145] presented a double PE-FBEH system in water. The distance between two PEHs, the specific gravity, and the cylinder diameter were all performance variables, whereas the other factors were kept constant. Water traveled from left to right through the first PEH (leading) and the second PEH (following), causing both to vibrate vertically. When the spacing distance between the energy harvesters is large enough, the vortex will cause the beams to vibrate independently. When the spacing is kept small, a coupled vibration between the two energy harvesters occurs. Furthermore, the motion of the following PEH will be affected by the wake created by the leading PEH. The existence of the following PEH, on the other hand, will alter the path of the leading PEH. In this way, the performance of a double PEH is considerably improved by the wake galloping vibrations in comparison with a single PEH. The maximum open circuit RMS voltage produced was 27.65 V and the device maximum voltage output density was 77.45 V/cm\({}^{3}\). [PERSON] et al. [146] utilized crescent- and D-shaped bluff bodies inspired by the wake galloping of iced conductors. Wake galloping EHs with iced D-shape and crescent-shape bluff bodies (Figure 11d) exhibit a substantial boost in terms of captured power when compared with energy harvesters positioned in the wake of a non-iced circular bluff body. The output power could be enhanced 20 times more than a conventional galloping energy harvester, as suggested by [PERSON] et al. [147]. The primary cause of such a performance boost was due to the synergistic impact of the interference wake. [PERSON] et al. [148] developed a TEEH driven by wake galloping that can harvest breeze wind energy. At a wind speed of 1.8 m/s, a cylindrical blunt body was utilized to achieve an output power density of 149 mW/m\({}^{2}\) at 100 M\(\Omega\).