# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 4 Galloping Energy Harvesting
### Galloping TE-FBEHs  
When an elastic structure is exposed to fluid flow, the transverse oscillations from galloping can be converted into friction and separation processes in TEEHs. The impact behavior due to galloping was presented in [97]. The harvester can generate over 200 V at wind speeds as low as 1.4 m/s. This work marks the restriction of onset wind speeds in TEEHs. However, TEEHs still face a number of obstacles in the collection of low-speed wind energy, including significant energy loss and damaging friction wear. [PERSON] et al. [98]  
Figure 7: Galloping EM-FBEHs: **(a)** Halbach effect EH reproduced with permission from [93] Elsevier, 2019; **(b)** spring-coupling EH [94]; **(c)** (i) torsional galloping EH, (ii) modified design with beam elements reproduced with permission from [95] Elsevier, 2023; **(d)** elastic strips suspended EH reproduced with permission from [96] Elsevier, 2024.  
developed a novel TEEH based on galloping to address these issues. The TE components of the energy harvester were bundled in a bluff body to prevent environmental disturbances, eliminating the significant rotation resistance and frictional wear found in conventional designs. A TEEH inspired by fish fins was reported in [99]. The beam structure with soft materials swayed in the water flow, causing the small sphere within the bluff body to produce electrical impulses via surface friction. The energy harvester had an open-circuit voltage ranging from 200 V to 313 V at flow velocities of 0.24 to 0.89 m/s. Remarkably, even after 30 days of immersion in water, the output of the device stayed at 96.81% of its original value. This resilience to water immersion demonstrates the device's endurance, as well as its potential for long-term energy harvesting in aquatic or underwater environments. [PERSON] et al. [100] introduced a unique technique for parametric optimization and large-scale standard and customized manufacturing of TEEHs. The TEEHs were combined with a wind-induced galloping oscillator to gather omnidirectional wind energy. The design included a Y-shaped mast with a magnetic tuning system, providing a strong reaction to breeze while maintaining system resilience. Beyond wind energy, the adaptable energy packs can be used as ocean waves or human motion harvesters in network setups, powering a vast real-time monitoring sensor array. Several galloping energy harvesters reported are summarized in Table 3. The comparison was performed on the basis of cut-in velocity, shape of the bluff body or modifications in the bluff body and output power generated by the energy harvesters. The optimum resistance and the velocity at which the maximum power is generated play an important role in the efficiency of the energy harvester, whereas the choice between PE and EM depends on the frequency characteristics of the FIVs. Due to their greater power-generating capabilities at low frequencies, PE harvesters thrive in low-frequency applications. The research works [86; 87] show their ability to generate power at low wind speeds and adaptability to varying wind conditions. EM harvesters, on the other hand, are better suited for high-frequency applications. The topological design as well as the angle of attack both have a considerable impact on the aerodynamic properties. As the aerodynamic forces increase, the efficiency of the EH improves, as seen in [88]. However, the durability, practicality and resistance to harsh environments are concerning. The work conducted in [90] well explains how varying the geometric properties achieves better performance and robustness in challenging conditions. Likewise, the change in geometric properties, i.e., in bluff bodies, contributes to lowering the threshold wind speeds and achieving a broad working range. This can be clearly seen in Table 3, where the cut-in speed for each bluff body configuration is highlighted.