# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
The kinetic energy from the ocean waves and gushing winds can be harvested with mesoscale rotary turbines or with FIV energy harvesters. Recognizing the substantial research advancements in the field of FIV energy harvesters, rapid progress is driven towards sustainable energy solutions for powering WSNs. The ambient fluid flow [16; 17; 18] is in abundance in day-to-day life and industrial environment. Flow-based energy harvesters (FBEHs) are developed to take advantage of flow-induced structural vibrations generated by the natural fluid flow when encountering obstacles in the flow field. The fluid structure interaction (FSI) produces aerodynamic effects, variations in air pressure and velocity over time which cause oscillatory patterns in the flow. The FIVs for energy harvesting can be split into distinct vibration mechanisms such as vortex-induced vibrations (VIVs) [19], flutter [20], galloping [21] and wake galloping [22]. These mechanisms refer to aeroelastic instabilities. These instabilities are caused by the interaction of the harvester's structure with the wind flow. However, in some cases  
Figure 1: Potential energy sources for FIV-based harvesting for powering WSNs in IoT applications. turbulence-induced vibrations [23; 24], which are caused by pressure fluctuations in the airflow, are utilized. Based on aeroelastic instabilities, the power generation process incorporates the aero-electro-mechanical coupling comprising the integration of fluid dynamics, structural dynamics and electricity. The nonlinear aerodynamic force sets up mechanical vibrations through the fluid-structure coupling, which are subsequently converted into electrical energy by the harvester's transduction mechanism [25]. In this paper, a comprehensive review is conducted on FBEHs developed to power WSNs in IoT-based monitoring systems. Fundamental concepts and aerodynamic instabilities (VIVs, flutter, galloping and wake galloping) are discussed. A detailed comparison of energy harvesting devices offers insights into optimizing wind energy harvesting technologies. It extensively covers different energy transduction mechanisms such as piezoelectric (PE), electromagnetic (EM) and triboelectric (TE), and moreover highlights their performance and applicability. Likewise, hybrid energy harvesters are also discussed that incorporate more than one energy transduction method, along with harvesters based on coupled fluid phenomena, highlighting how these innovative designs enhance energy conversion and overall performance. In addition, nonlinearities introduced in FIV energy harvesters are thoroughly examined, covering each flow-induced mechanism and transduction technique employed. Additionally, common wind-driven rotary energy harvesters are reviewed, emphasizing their usefulness and practicality for powering monitoring sensors. This paper not only highlights the advantages and practical applications but also indicates the limitations of flow energy harvesters regarding their transduction mechanisms and ways to improve performance. Finally, strategies to enhance power output and key challenges related to FBEHs are debated.