# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 5 Flutter Energy Harvesting
### Bio-Inspired Flutter FEHs  
The application of FIVs is extended to natural environments. Nature provides several sources of FIVs, e.g., fluttering grass leaves or dancing tree leaves are interesting vibrating structures. Several leaf flutter theories [127] and bio-inspired structures [128; 129] utilized in wind energy harvesting in the literature are reported. [PERSON] et al. [128] investigated a unique cross-flow stalk configuration for energy harvesting, which differs from standard fluttering devices positioned parallel to the flow direction. The cross-flow stalk arrangement with flexible piezoelectric stalks positioned perpendicular to the flow amplifies vibrations by an order of magnitude, considerably increasing energy extraction. The device was made of low-cost organic piezomaterials for the stalks and polymer films for the leaves and used aerodynamic forces to maximize oscillations, making it suited for cost-effective applications. In addition to leaf-inspired designs, nature's diverse mechanisms for motion extend beyond flora to the intricate wing motions of birds and insects, which offer equally valuable insights for energy harvesting applications. Figure 10a illustrates a TEEH that was inspired by the design and movement of hummingbird wings, intended for efficient and small-scale flutter-based energy harvesting, as reported in [130]. The fluttering mechanism within the wing structure creates movement that leads to contact-separation and free-standing electrification between different triboelectric materials. The device is lightweight (10 g), making it one of the lightest TENGs ever created. This bio-inspired design allows the device to operate effectively under low-frequency wind conditions, even in harsh environments. Inspired by the fluttering mechanics of dipteran insects (such as flies), a bionic multi-domain energy harvester was developed [131]. The wing structures allow for agile flight through rapid flapping motions. The harvester utilizes a flexible cantilever beam that simulates the wings of dipteran insects. This beam can bend and vibrate when subjected to wind or other forces, mimicking the fluttering motion of insect wings. However, it did not account for the output in response to wind energy; vibration energy was used to drive the harvester. The research in [132] aimed to produce energy from the motion of a branch of a palm tree. The branch was positioned in a wind tunnel and forced to vibrate at an airflow speed of 7 m/s and at an optimum load of 1 M\(\Omega\); a maximum power output of 0.157 mW was generated, which corresponded to a power density of 393 mW/m\({}^{2}\). The vertical stalk-leaf arrangement was used by [PERSON] et al. [133] to create a macro fiber composite (MFC)-based flutter energy harvester, as shown in Figure 10b. [PERSON] et al. [134] introduced the inclined angle into the stalk-leaf system. The authors recently developed a bio-inspired architecture of a PEH that has a low cut-in wind speed and constant output frequency [135]. The process of leaf flutter was comprehended by including aeroelastic modeling and laboratory testing of palm leaves. A fake leaf FEH was tested in a wind tunnel at 2 m/s wind speed and at a stable frequency of 3.56 Hz, and an output power density of 1.238 \(\upmu\)W/cm\({}^{3}\) was obtained by the harvester. Table 4 summarizes and compares several flutter-based energy harvesters, indicating a higher output generation of EM and TE energy harvesters compared with PEHs. Fluttering is a divergent phenomenon, and for larger deflections during fluttering, utilizing EMEHs is a better option. However, the research in [112] focuses on the optimal configurations of the PEH. Optimizing the stiffness of the flag considering the role of structural dynamics in terms of bluff bodies notably increases the power generation. In comparison, the EMEH in [115] considers essential factors such as wind speed, magnet positioning, angle of attack and harvester orientation of the wind flow. All these factors contribute to enhanced output power.