# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 3 Karman Vortex Street-Based Energy Harvesting
### Karman Vortex Street-Based PE-FBEHs  
[PERSON] et al. [41] utilized piezoelectric harvesters to investigate various attributes of energy harvesting from unstable and turbulent flows. A beam with piezoelectric material was placed in the Karman vortex street, with the static pressure on the beam altered dynamically, causing the beam to oscillate. A lift force is generated by the vortex formation. The voltage induced by piezoelectric beams that were positioned in the wakes of a turbulent flow of circular cylinders at high Reynolds numbers was examined. [PERSON] et al. [39] developed PE-FBEHs for harvesting energy from the VIVs established in the water flow due to the presence of a bluff body. The experimental results showed that an open circuit peak-to-peak output voltage and power of 0.12 V and 0.7 nW are produced, respectively. An optimization design method or the use of a piezoelectric material with higher piezoelectric constants can improve the device's low output power. The study in [42] presents a PE-FBEH comprising a Lead Zirconate Titanate (PZT) beam with a cylinder. At a resonant vibration velocity of 0.35 m/s and optimum resistance range (100-150 k\(\Omega\)), output power of 84.49 uW was obtained which corresponds to an energy density of 60.35 mW/m\({}^{2}\). The authors concluded that optimizing the mass and diameter of the cylinder can improve the efficiency of the energy harvester. [PERSON] et al. [43] presented a PE-FBEH where a flexure beam was oscillated by von Karman vortices that were dislodged from the upstream bluff body, as depicted in Figure 4a. The ideal positioning of the beam behind the bluff body  
Figure 3: Typical trend of VIVs: (**a**) synchronization region; (**b**) Karman vortex street. has been determined from the simulation findings. During experimental characterization, the electrical output of the harvester was measured for various flow velocities and beam orientations. The system could be improved by extending the operational spectrum and attaining a wider range of optimum velocities. In addition, the bluff body presented here is quite a generic obstacle; the optimization of the bluff body shape can be another consideration within the scope of this work. Researchers developed unimorph and bimorph piezoelectric cantilever beams [44; 45] with different shapes of bluff bodies. [PERSON] et al. [46] implemented changes in the bluff body geometry. The interaction between a downstream diamond-shaped baffle and a cylindrical shell presents a novel way to modify the bluff body's aeroelastic instability. The maximum output voltage increased from 19.8 V to 200 V (by over 900%), offering a viable way to improve the efficiency of conventional VIV-based energy harvesters. Therefore, altering the geometric properties of structures and introducing attachments on the surface of oscillators can increase the output of FBEHs. Figure 4: Karman vortex street-based PE-FBEHs. (**a**) Schematic of energy harvesting from von Karman vortices in airflow [43]; (**b**) Direction-adaptive EH with a guide wing reproduced with permission from [47], Elsevier, 2019; (**c**) VIV energy harvester reproduced with permission from [48], Elsevier, 2018; (**d**) Schematic of VIV micro windmill with three blades reproduced with permission from [49], Elsevier, 2022. The majority of energy harvesting systems prioritize piezoelectric optimization over design performance in real environments with low-density wind energy [49]. To extract wind energy, a micro windmill with three blades (Figure 4d), a circular polyvinylidene diffluoride (PVDF) piezoelectric harvester and an air blast blower is set in a wind tunnel. The vortex-induced high-frequency resonance subsequently raises the excitation force amplitudes and enhances the piezoelectric energy harvesting efficiency. However, there is a need to design the energy harvester with strong coupling in the wind field in order to improve the performance at resonance.