# 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
### Coupled Fluid Flow Phenomena
#### 7.3.1 Interaction of VIVs and Galloping
A conventional galloping-based FEH typically employs a prismatic bluff body while a standard VIV energy harvester often utilizes a cylindrical bluff body. To combine both effects, [PERSON] et al. [196] utilized a cross-shaped beam along a cylindrical bluff body and two cuboid bluff bodies shown in Figure 13c. Similarly, in [197], a novel bluff body with rounded and folded corners superimposed galloping and VIV effects. According to the experimental findings, the harvester's voltage output reached 14.6 V when it had a half cuboid and half cylindrical bluff body. The introduction of a dumbbell-shaped bluff body presented in [198] integrated higher amplitudes during galloping and a lower cut-in speed for the VIV depicted in Figure 13d. [PERSON] et al. [199] evaluated the performance of the wind-induced vibration cylinder with fin-shaped rods (FSRs) for energy harvesting. Their findings demonstrate that the output power of cylinder-based aeroelastic energy harvesters can be greatly increased by the proper installation of FSRs on a bluff body. However, all these investigations consider changing the bluff body geometry. [PERSON] et al. [45] developed physical models for circular cylinders with passive turbulence control (PTC), square, trapezoid and triangular prisms and observed VIVs combining with galloping. [PERSON] et al. [46] inserted a downstream diamond-shaped baffle to change the aeroelastic instability of the cylindrical bluff body. By including the piezoelectric transducer inside the cylindrical shell (bluff body), the system also gained an advantage from the indirect interaction between the PEH and the wind. Recently, [PERSON] et al. [200] reported another way of coupling the VIV and galloping effect, i.e., by introducing protrusions on the surface of the bluff body and finding an optimum aspect ratio for the bluff body. The cut-in velocity varied for each scenario. Experiments revealed that at a wind speed of 5.1 m/s, the maximum output power from the analyzed scenarios was 0.992 mW for a 3:2 aspect ratio of the bluff body and protrusion length of 10 mm. All the above studies incorporate only one cylinder to generate wind energy; however, [PERSON] et al. [147] utilized the synergistic effect of the transverse and interference galloping phenomenon. They installed an interference in the shape of mirror symmetry with respect to the bluff body. The developed harvester's output performance was compared to that of a traditional transverse galloping-based PEH. The coupled synergistic impact resulted in an electrical power almost 20 times greater than that of the traditional galloping FEH. [PERSON] et al. [201] proposed a 2-DOF coupled VIV and wake galloping FEH by utilizing two parallel and elastically connected cylinders. The cylinder facing the wind flow, i.e., the windward cylinder (bluff body 2) or the leeward cylinder (bluff body 1), matched the first or second natural frequency resonance during operation, and the system underwent VIVs. However, at relatively high wind speeds, VIVs convert to wake galloping and scavenge wind energy in a wide range of wind velocities with two lock-in regions.