# 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
### Nonlinear FBEHs
#### 7.1.1 Nonlinear VIV-Based PE-FBEHs  
Due to its distinctive characteristics of self-excited oscillations, maximum power is produced within the synchronization zone or (linear) resonance when vortex shedding frequency is near to one of the natural frequencies of the harvester's structure. VIV-based energy harvesting has gained significant attention; however, the narrow lock-in region of VIV energy harvesting is one of its major shortcomings. As the effectiveness of VIVs energy harvesters is optimum when vortex frequency approaches the natural frequency of the harvester, any variation in wind speed or vortex frequency causes a sharp reduction in the power. Contrarily, nonlinear harvesters use nonlinear characteristics to increase the frequency bandwidth, making them more robust and responsive to random and wide range vibrations [150]. Several such FEH configurations are depicted in Figure 12. Researchers are interested in the vibration response of VIVs energy harvesters reinforced by nonlinear springs. The primary characteristic of a nonlinear restoring force is that the concept of natural frequency does not exist. The system can excite a wider range of structural modes, introducing broadband frequency response. The limited frequency bandwidth of VIVs devices has been addressed from the perspectives of monostability, bistability and magnetic interaction. The nonlinear distributed parameter model was developed by [PERSON] et al. [151]. They considered the first four modes of FEHs. A [PERSON] wake oscillator was used as the basis for the concept of the lift force generated by VIVs. The study in [152] investigated improving wind energy conversion by utilizing a buckled beam vibration. Experiments have shown that when the buckling beam becomes unstable due to wind, it causes substantial vibration in the PVDF harvester, which results in significantly higher power output compared with traditional cantilever beam type harvesters. To create an effective and broadband harvesting system, [PERSON] et al. [153] included attractive magnetic force (Figure 12a) to develop a nonlinear VIVs energy harvester, whereas [PERSON] et al. [154] incorporated nonlinear repulsive forces in their system. Two small magnets with repulsive forces were introduced, attached on the lower support and the bottom of a circular cylinder, respectively, and were subjected to a uniform wind speed. The dynamics of the energy harvester were affected by altering the relative positions of the magnets' center-line horizontal (\(\Delta\)x) and vertical (\(\Delta\)y) distances. Experiments revealed a significant increase in the dominating frequency of the energy harvester results in a wider synchronization region by 138% of the original configuration. The harvested power was gradually increased to the highest value of 150 \(\mu\)W (29% increase) at 3.2 m/s. [PERSON] et al. [155] explored a cylindrical monostable EH based on vortex excitation. A maximum power of 0.21 mW was achieved at a flow velocity of 1.6 m/s. A self-aligning airfoil cantilever design adds nonlinear energy harvesting capabilities that can boost conversion efficiency. Although nonlinearities introduced in VIVs can greatly improve the output performance, there is a need for environmental adaptability and robustness from wind erosion. This issue was addressed by [PERSON] et al. [156] by developing a non-contact PEH. Theoretical and experimental investigations were conducted on a piezoelectric transducer implanted in a cylindrical shell. The results showed that as the wind speed rose, the vibration amplitude and output were increased with an increasing transducer mass and decreasing shell mass. The research conducted in [157] introduced a nonlinearity in the system to design, develop and test a nonlinear airfoil-shaped PE-FBEH for VIVs as depicted in Figure 12b. The harvester converted FIVs from water into electrical energy. A passive self-adjustable base compensated for the changing flow direction that could reduce the conversion efficiency of energy harvesters.