# 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
#### 7.3.1 Interaction of VIVs and Galloping  
In the case of coupled vortex excitation and galloping, the natural frequency of the system is close to the vortex shedding frequency. This frequency can amplify the oscillations of the structure and lead to galloping. Usually, the VIV happens first, and as the wind speed rises, galloping occurs. Additionally, combining both phenomena causes vigorous vibrations. Combining VIVs with galloping to improve wind energy harvesting performance has recently gained attention and popularity. A thorough phenomenological explanation of the VIV-galloping interaction mechanism was provided by [PERSON] et al. [192]. The study showed the coupled and decoupled situations of the VIV-galloping interaction. The fabricated prototypes fall into two primary categories. One contends that the aerodynamic properties of a circular cylinder can be changed by adding structural attachments, leading to galloping or a hybrid of VIVs and galloping rather than a pure VIV-based harvester. [PERSON] et al. [193] developed a wind-based PEH which could benefit from the interaction between \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Harvester Type** & \
\begin{tabular}{c} **Optimum Resistance** \\ **(k\(\Omega\))** \\ \end{tabular} &
\begin{tabular}{c} **Velocity at Max Power** \\ **(m/s)** \\ \end{tabular} & **Output Power (mW)** & **References** \\ \hline \multirow{2}{*}{VIVs} & 400 (PE) & \multirow{2}{*}{0.6} & \multirow{2}{*}{16.55} & [PERSON] et al. [184] \\  & 2.2 (EM) & & & \\ \hline \multirow{3}{*}{Rotational} & 10,000 (TEEH) & \multirow{3}{*}{6} & 1.67 (TEEH) & \multirow{3}{*}{[PERSON] et al. [185]} \\  & 330 (PE) & & 1.38 (PE) & \\  & 180 (EM) & & 286.6 (EM) & \\ \hline \multirow{3}{*}{VIVs} & 1 (PE) & \multirow{3}{*}{1.8} & 0.7375 (PE) & \multirow{3}{*}{[PERSON] et al. [186]} \\  & 0.01 (EM) & & 0.095 (EM) & \\ \hline \multirow{3}{*}{Galloping} & 2000 (PE) & \multirow{3}{*}{14} & 0.1 (PE) & \multirow{3}{*}{[PERSON] et al. [187]} \\  & 100,000 (TEEH) & & 0.064 (TEEH A) & \\ \hline \multirow{3}{*}{Flutter and vibration} & 160 (PE) & \multirow{3}{*}{14.5} & 4.4 (PE) & \multirow{3}{*}{[PERSON] et al. [188]} \\  & 0.04 (EM) & & 3.68 (EM) & \\ \hline \multirow{3}{*}{Flutter} & 150 (PE) & \multirow{3}{*}{6.7} & 1.35 (PE) & \multirow{3}{*}{[PERSON] et al. [190]} \\  & 0.419 (EM) & & 36.63 (EM) & \\ \hline \multirow{3}{*}{Magnetically coupled} & 750 (PE) & 11 & 1.79 (PE) & \multirow{3}{*}{[PERSON] et al. [191]} \\ \cline{2-2} \cline{5-5} Galloping & 0.05 (EM) & 7.15 & 3.92 (EM) & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Hybrid flow energy harvesters. VIVs and galloping. A circular-based PEH with three various small-size attachments (circular, triangular and square), as shown in Figure 13a, was tested. They concluded that, when compared with the other two varieties, the triangle rod attachment at an angle of 60\({}^{\circ}\) outperformed. [PERSON] et al. [194] presented an experimental technique to show the separation or superposition of a VIV and galloping by changing the aspect ratio of a rectangular cylinder. In order to improve performance over a wider range of wind velocities, a T-section fin FEH was subjected to the vortex shedding-galloping coupled mechanism [195]. [PERSON] et al. [77] discussed how to combine VIVs and galloping to increase the effectiveness of extracting wind energy through the synergetic interaction of two typical geometries: circular and rectangular cylinders. Figure 13b explains the dynamic interaction between VIVs and galloping for energy harvesting. As wind speed increases from point 1 to point 2, VIV lock-in dominates, followed by a decrease in oscillation amplitude, reaching equilibrium at point 6. At point 5, galloping causes a sudden amplitude jump to point 4, and then gradually decreases to point 3 before dropping to equilibrium. Upon further speed reduction, the VIV reappears before the system returns to rest, showing a subcritical bifurcation and hysteresis effects.