# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 6 Wake Galloping Energy Harvesters  
Wake galloping refers to the diverging oscillations of the downstream bluff body triggered by the wake that originates from the upstream bluff body. The galloping phenomenon is the term used to describe the vibration that occurs when a single cylinder with a flexible base is placed in the wind. However, when an obstacle with a fixed foundation is positioned upstream of the energy harvester, the vortices shed from the upstream obstacle reattach to the downstream body (i.e., the energy harvester). As denoted in Figure 11a [136], the oscillations in wake galloping are created by the wind flow interaction with the structure's wake. The region that consists of disturbed air behind the structure is known as the wake. The wake may become unstable when the wind flow is strong enough. Large-scale vortices may develop in the wake because of this instability. The structure oscillates due to the interaction with these vortices. Wake galloping has its features in that of VIVs, however, there is a slight difference in the position of the oscillating body in relation to the vortex shedding. In fact, for wake galloping, the oscillating body is positioned downstream of an obstacle. As a result, the body oscillates as it interacts with the vortices that the obstacle is shedding. Lock-in or synchronization takes place when the vortex shedding frequency approaches the oscillator's natural frequency, as shown in Figure 11b. Large amplitude motions are produced in this region when flow relates to the oscillator's natural modes of vibration. When the vibration frequency is a multiple or submultiple of the shedding frequency, synchronization can also happen, though to a lower extent. On the other hand, in VIVs, the body oscillates due to the alternate lift forces brought on by the vortices that are shed from its trailing edge. [PERSON] et al. [137] numerically investigated two bluff bodies working in tandem. Based on previous studies, for example, [PERSON] et al. [138] and [PERSON] et al. [139], the authors utilized interference cylinders for energy harvesting from VIVs, and the wake galloping phenomenon emerged as an efficient method to harvest energy in a wide range of wind speeds. A PEH was developed by [PERSON] et al. [140] by positioning an elastic beam behind a bluff body at a specific distance. Due to the wake created by the bluff body, the structure consequently vibrated, generating a maximum power of 4 \(\mu\)W. [PERSON] et al. [141] experimentally found that the EM-FBEH can produce 370 mW of power under a wind speed of 4.5 m/s at a natural frequency of 4.8 Hz. [PERSON] et al. [142] evaluated a number of crucial parameters to optimize the setup of PEHs using the analysis in COMSOL. A flexible beam was installed behind bluff bodies of various shapes in their computational model. According to the results, the cylindrical bluff body produced the highest average power of 0.35 mW. The study in [143] demonstrated the vibration behavior of the tip body (downstream cylinder) affixed to a cylindrical shaped bluff body through a cantilever beam.