# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 7 Enhancement Methods to Optimize and Increase the Efficiency of FBEHs
### Coupled Fluid Flow Phenomena
#### 7.3.2 Coupled Flutter and VIVs  
Researchers are investigating the complex phenomenon known as coupled flutter-VIV. Systems with combined flutter and VIVs can serve as a promising solution for self-powered applications. The literature so far mentioned cantilever beam-mounted flutter-induced PEHs. [PERSON] et al. (2020) proposed a composite energy harvesting method to simultaneously capture flutter and VIVs. The flutter-induced harvester connected the cantilever beam to the oscillating airfoil and the effect of several springs on the output response was demonstrated. Additionally, a cylindrical bluff body was attached to the free end of the cantilever. The airfoil underwent flutter vibration, and the bluff body (cylinder) captured the vortex excitation. This coupling effect boosted the energy harvesting performance. At a wind speed of 14.48 m/s, an output power of 154 uW was reported. A cylinder with two symmetrical airfoils on both sides was designed as a nonlinear vortex-induced flutter PEH (203), as shown in Figure 13e. The assembly of two airfoils connected with a shaft to the cylindrical bluff body was then attached to a cantilever beam, enabling the flutter-VIV coupling. This approach significantly enhanced the output of the harvester, which was 6.47 mW, whereas the maximum output of the harvester, individually based on VIVs and flutter, was only 0.05 mW and 0.26 mW, respectively. To improve the vibration and energy harvesting performance, [PERSON] et al. (2020) developed a novel kind of aeroelastic PEH. The flap was fastened to the trailing edge of the airfoil and tested two ways. Firstly, the flap was constrained by a spring rod and rotated at small angles; secondly, the flap was unconstrained by removing the spring rod and rotating freely. The change in vibration mode from flutter to VIV is an important finding from theoretical, computational and experimental examinations. The output power of 5.43 mW can be harvested at 14.39 m/s during unconstrained conditions. Table 8 summarizes different types of coupled energy harvesters based on different FIV phenomena. Based on the FEHs discussed in this section, the VIV-galloping coupling effect was achieved by varying dimensions or amending the features of a bluff body in terms of aspect ratios (200). The VIV has a limitation in terms of bandwidth, which refers to the range of frequencies over which the energy harvester can operate. The transition from VIVs to galloping offers a solution to overcome this limitation by expanding the frequency range in which the harvester can effectively operate. This increased flexibility enables better adaptability to varying fluid flow conditions, making the energy harvesting system more reliable and applicable to a wider range of applications. In some cases, an additional bluff body is introduced (201), to achieve a coupled VIV-wake galloping with two cylinders elastically connected. As a result, at low wind speeds, it can exhibit resonant oscillations when the vortex shedding frequency matches the first or second natural frequency, whereas, at relatively high wind speeds, it can engage in wake galloping and scavenge wind energy over a wide wind speed range, with both cylinders contributing to higher power output. This resonance can enhance the energy extraction efficiency, as it allows the system to synchronize with the incoming wind, effectively capturing more energy from the fluid flow. Similarly, the flutter-VIV coupled energy harvesters in (203,204) also influenced the output of the harvesters. By coupling these two instabilities, the system could take advantage of the strengths of each instability mechanism, optimizing energy extraction consistently across varying wind conditions and making the overall energy harvesting process more effective.