# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 5 Flutter Energy Harvesting
### Flutter PE-FBEHs  
In 2008, a morphing airfoil was employed by [PERSON] et al. [104] to capture aeroelastic energy from airflow. Since then, a wide range of energy harvesters have been developed and investigated. For instance, [PERSON] et al. [105] proposed the idea of using aeroelastic flutter to harvest wind energy. Numerical simulation and experimentation was performed, and the impact of flutter velocity on piezoelectric energy harvesting was examined. [PERSON] et al. [106] developed a semi-empirical unstable aerodynamic model to simulate and analyze the stall behavior of the structure. In [107], a T-shaped PE-FBEH operated on the principle of aeroelastic flutter, which occurs at low fluid speeds. It was found that a harvester of size 1006 cm\({}^{3}\) could provide 4.0 mW of power at a wind speed of 4 m/s. The maximum speed to allow safe operation was not identified (it was tested up to 15 m/s), so the device maximum output was not concluded. Moreover, there was no clear evidence of the onset flutter speed, hence the frequency range was not defined. To enhance the performance of a flutter-based energy harvester, it was integrated with an airfoil as shown in [108]. The device was proven to function better under the combined loading, wind and base excitations in terms of its transduction abilities, and power density even while operating below its optimum speed. Experimentally, a 150% increase in output power per unit acceleration was observed when the wind speed was increased from 0 to 2 m/s, which was below the flutter speed of 2.3 m/s. However, the reported model does not account for adaptability in the real environment, and also, a prediction of the harvester response in terms of stability potentials was not performed.  
In [109], a stepper motor connected to the composite beam was controlled to set the static angle at the base of the beam. The experiments were performed at several angles. At a flow speed of 9 m/s and 7.2\({}^{\circ}\) angle, a maximum output of 0.272 mW was achieved. Such types of airflow energy harvesting from flutter are widely used in providing power to autonomous sensors in applications like building management systems. [PERSON] et al. [110] investigated flag-flutter-based energy harvesting via piezoelectric transduction using numerical analysis and experimentation. Furthermore, the flutter instability of a cantilevered flag with piezoelectric (PZT) and Aluminum (Al) patches was explored. For a 66.6 k\(\Omega\) resistance, the highest power output produced was 1.12 mW during the experiments. [PERSON] and [PERSON] studied the dynamics of a planar structure's flutter oscillation in the presence of an additional wake field with the goal of enhancing the efficiency of a flutter-based energy harvester [111]. The output of 1.46 V and 6.81 uW were obtained at a critical flutter velocity of 6.6 m/s. The study in [112] also sought to improve the understanding of bluff bodies; optimization parameters and piezoelectric flags, as shown in Figure 8, were used. Various rigid cylindrical shapes were placed upstream in the airflow. The results showed that adjusting the stiffness of the flag resulted in a significant increase in power generation. Notably, the highest power output of 746 uW and 19.31 V (RMS) was achieved when the 120\({}^{\circ}\) bluff body was coupled with a stiff, short flag.  
Figure 8: Piezoelectric flag tested with various bluff bodies reproduced with permission from [112] Elsevier, 2023.