# Fluid Flow-Based Vibration Energy Harvesters: A Critical Review of State-of-the-Art Technologies
## 5 Flutter Energy Harvesting  
Flutter is a type of self-excited motion, which occurs owing to coupling between the aerodynamics and the structure. The flutter motion is related to the bending and torsional vibration of the structure, which usually occurs at high wind-speeds. When the environmental wind speed exceeds a critical value, the structure starts to vibrate with a large amplitude [101]. Just like galloping, the flutter phenomenon results from negative damping. Unlike galloping, it typically occurs in structures with a sheet-like shape (such as flat plates, membranes, wing-shaped structures, etc.) and flutters frequently with a chaotic aerodynamic force [102]. Due to system nonlinearities like geometric nonlinearity caused by substantial deformation, structural gaps and nonlinear aerodynamic behavior due to high angle of attack, the increasing amplitude is constrained practically. As a result, the system attains limit cycle oscillation (LCO). Due to the self-excitation and large amplitudes of vibration induced by flutter, the flutter-based energy harvester offers a great deal of scope for advancement in terms of energy harvesting [103]. Owing to its efficiency, compact size and flexibility, piezoelectric flutter FEHs dominate the literature, in comparison with electromagnetic flutter FEHs in most research and practical applications.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \begin{tabular}{c} **Shape of the Bluff** \\ **Body** \\ \end{tabular} & \begin{tabular}{c} **Cut-in Velocity** \\ **(m/s)** \\ \end{tabular} & \begin{tabular}{c} **Optimum** \\ **Resistance** \\ **(k\(\Omega\))** \\ \end{tabular} & \begin{tabular}{c} **Velocity at** \\ **Max Power** \\ **(m/s)** \\ \end{tabular} & \begin{tabular}{c} **Voltage** \\ **(V)** \\ \end{tabular} & \
\begin{tabular}{c} **Output Power** \\ **(mW)** \\ \end{tabular} & **References** \\ \hline \multicolumn{7}{c}{**PE-FEHs**} \\ \hline Y-shape & 1.32 & 2006 & 2.098 & 1.19 & [PERSON] et al. [76] \\ \hline Curved plate & 1.8 & 820 & 5.5 & 10.7 & 0.0356 & [PERSON] et al. [78] \\ \hline Splitter plate & 3 & 0.593 & 7 & 14.8 & [PERSON] et al. [80] \\ \hline Square & 1.9 & 2730 & 4 & 0.018 & [PERSON] et al. [81] \\ \hline Square with V-shaped & \multirow{2}{*}{3.04} & \multirow{2}{*}{9} & \multirow{2}{*}{10} & \multirow{2}{*}{0.93} & \multirow{2}{*}{[PERSON] et al. [82]} \\ grooves & & & & & & \\ \hline Square with V-shaped & \multirow{2}{*}{1.75} & \multirow{2}{*}{180} & \multirow{2}{*}{9} & \multirow{2}{*}{15.24} & [PERSON] et al. [83] \\ grooves & & & & & & \\ \hline Cut-corner prism & 3.8 & 100 & 6.24 & 47.5 & [PERSON] et al. [84] \\ \hline Funnel shape & 7 & 300 & 24 & 4.3 & [PERSON] et al. [85] \\ \hline
\begin{tabular}{c} Hyper-structure \\ corrugated \\ \end{tabular} & \multirow{2}{*}{3} & \multirow{2}{*}{6} & \multirow{2}{*}{0.89} & \multirow{2}{*}{31.3} & [PERSON] et al. [86] \\ \hline Double bluff body & 0.96 & 2000 & 15 & 90.35 & 2.57 & [PERSON] et al. [87] \\ \hline Double-airfoil & 1.5 & 411.2 & 8 & 163.39 & 26.67 & [PERSON] et al. [88] \\ \hline Elliptical & 0.3 & 4000 & 0.55 & 38.4 & 4.2 & [PERSON] et al. [90] \\ \hline \multicolumn{7}{c}{**EM-FEHs**} \\ \hline D-shape & - & 0.3 & 3.25 & 0.105 & 0.037 & [PERSON] et al. [92] \\ \hline Y-shape & 1.5 & 0.3 & 4 & 1.4 & 2.5 & [PERSON] et al. [93] \\ \hline Square & 3 & - & 14 & 0.103 & 0.79 & [PERSON] et al. [94] \\ \hline Y-shape & 2.75 & 0.044 & 10 & 0.118 & 0.31 & [PERSON] et al. [95] \\ \hline Hollow square tube & 1.6 & 1.2 & 4 & 3.2 & 7.8 & [PERSON] et al. [96] \\ \hline \multicolumn{7}{c}{**TE-FEHs**} \\ \hline Y-shape & 1.4 & 50,000 & 6 & 200 & 0.01 & [PERSON] et al. [97] \\ \hline Trapezoid & 2.9 & 44,000 & 7.8 & - & 1.3 & [PERSON] et al. [98] \\ \hline Fishtail-shaped & 0.24 & - & 0.89 & 313 & - & [PERSON] et al. [99] \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._