# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 4 Galloping Energy Harvesting  
Galloping is a self-excited vibration phenomenon generated by aeroelastic instability, which occurs in structures with edges and corners usually known as prismatic structures. It is characterized by low-frequency and high-amplitude oscillations. It appears to be related to the velocity of the entering flow and the relative direction of the fluid regarding the structure [64]. When the incoming airflow speed is low, the damping of the system is positive, and the structural vibration will slowly decay to zero. When the incoming airflow speed gradually increases and exceeds a critical value, transverse oscillations perpendicular to the direction of the wind flow in structures (with low damping) occur. When there is further increase in the flow velocity, there will be further decrease in net damping, i.e., negative damping is achieved, which leads to self-excited oscillations also known as Hopf  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Harvester Type** & **Fluid Velocity** & **Output** & **Resistance** & **Output Power** & **References** \\  & **(m/s)** & **Voltage (V)** & **(k\(\Omega\))** & **(\(\mu\)W)** & **References** \\ \hline EM-Water based & 1.38 & 0.02 & 0.038 & 1.77 & [PERSON] et al. [57] \\ \hline TE-Wind induced vibration & 2.78 & 536 & 83,300 & 392.72 & [PERSON] et al. [59] \\ \hline TE-Water based & 0.5 & 174 & 1000 & 2500 & [PERSON] et al. [61] \\ \hline TE-Flag type & 6 & & 8000 & 2.9 & [PERSON] et al. [62] \\ \hline TE-Rolling based mechanism & 8 & 120 & 40,000 & 1940 & [PERSON] et al. [63] \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 5: VIV-TEEHs: (**a**) TEEH with springs reproduced with permission from [59] Elsevier, 2022; (**b**) Hydrokinetic TEEH reproduced with permission from [61] Elsevier, 2023; (**c**) Y-shaped/flag-type TEEH reproduced with permission from [62] Elsevier, 2024; (**d**) Vortex-induced rolling TEEH reproduced with permission from [63] Elsevier, 2024.  
bifurcations. Limit cycle motion takes place when the structural amplitude becomes stable, and the damping slowly increases from negative to zero. In comparison to VIVs and flutters, structural vibrations are a preferable option for galloping energy harvesting due to their larger vibration amplitude and capacity to oscillate over an infinite range of wind velocities [65]. Typical behavior of a galloping energy harvester in terms of flow velocity and the amplitude of structural vibrations is explained with the help of Figure 6a.  
Numerous fundamental investigations were carried out in the early stages of the development of galloping-based energy harvesting devices to gain a thorough understanding of their viability. For the first time, a single-degree-of-freedom model was put forth by [PERSON] et al. [26]. In this study, the potential application of transverse galloping in wind energy harvesting was theoretically examined. To further understand the underlying parameters influencing energy harvesting efficiency, a variety of experimental investigations have also been conducted using piezoelectric, electromagnetic and triboelectric transduction mechanisms.