# 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
#### 7.1.3 Nonlinear Flutter PE-FBEHs  
The performance of flutter-based energy harvesters can be enhanced in a variety of ways, for example, by the addition of nonlinearities. The harvester's bandwidth can be widened, allowing it to produce energy for a broad range of wind speeds due to the presence of multi-resonances. Since nonlinearities can produce a subcritical Hopf bifurcation, the cut-in wind speed of the harvester can be reduced. Moreover, the harvester's capacity to capture energy can be amplified. This is mainly due to the chaotic vibrations caused by additional nonlinearities in flutter energy harvesters, which can improve the energy transmission between the structure and the piezoelectric component. The impacts of the structural nonlinearities of the PE-FBEH were examined theoretically and experimentally by [PERSON] et al. [169]. It was demonstrated that the cubic hardening stiffness contributed to sustained oscillations with appropriate amplitude over a broad spectrum of flow velocities. The combined nonlinearities were investigated analytically by taking advantage of freeplay nonlinearity [170,171]. [PERSON] et al. [172] examined the application of a passive self-aligning mechanism to account for shifting wind directions in an inverted flag introducing nonlinear behavior. At a wind speed of 9 m/s, 5.0 mW/cm\({}^{3}\) power density was obtained. Even in low-wind speed regimes (3.5 m/s), the harvester was ideal for ambient wind energy collection and was capable of producing a power density of 0.4 mW/cm\({}^{3}\). In addition, a self-aligning system was incorporated to account for fluctuating wind directions that did not cause any hindrance in powering the sensors. With the harvester's enhanced power, the data output of the operated sensor was increased by 20 times. These results provide new possibilities for autonomous devices under variable conditions and low wind-speed environments. [PERSON] et al. [173] numerically investigated a double-plunge-degree-of-freedom PE-FBEH via the dynamic stall model. Their findings reveal that the cut-in wind speed was lower and the output power was larger than those of the single-plunge-degree-of-freedom PE-FBEH. A dynamic multistable harvester for flutter energy was proposed by [PERSON] et al. [174] and achieved snap-through and high-power output (at 1.5-7.5 m/s wind speeds). Magnetic nonlinearity was incorporated into the PE-FBEH design by [PERSON] et al. [175, 176]. A flutter EH with a flap was proposed by [PERSON] et al. [177]. The flap was easier to incorporate nonlinearity than the airfoil structures. The developed EH had a maximum power generation of 5.5 mW. [PERSON] et al. [178] carried out a comparative analysis of unsteady aerodynamic loads and dynamic stall models. The results predicted the unsteady aerodynamic model was more accurate in terms of output performance. To simulate the plunging motion of a typical airfoil, [PERSON] et al. [179] utilized two flexural springs symmetrically attached to the airfoil shaft and the holder. The two flexural springs allowed for periodic bending and deformation perpendicular to the airflow. By incorporating the cubic structural stiffness coefficient, the maximum output voltage was 29.08 V and the maximum output power was 3.382 mW, which demonstrates the improved energy harvesting performance. Recently, [PERSON] et al. [180] investigated the feasibility of using a bistable dual cantilever for flutter energy collecting. Two identical pole magnets were linked to the free ends of the cantilever to create a bistable effect on beams. The bistable state of the beams showed 2.5 times increase in amplitude of oscillations compared with monostable beams. The energy harvester design shown in Figure 12f is a tristable flutter energy harvester reported by [PERSON] et al. [181]. It is capable of large amplitude interwell plunging displacements and high-performance energy harvesting.