# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 5 Fringe-Type Trailing Edge Extensions  
The basic idea behind the introduction of fringe-type (brush-type or slit) trailing edge extensions is inspired by the trailing edge fringes at the primary feathers of some owl species (see Table 7). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline
**Year** & **Author(s)** & **Serration Type** & **Bionic Objects(s)** & \(\mathbf{R_{e}}\) (**10\%**) & **AoA (\({}^{\circ}\)**)** & **_H/\(\mathbf{\log}\) (\%)_ & **_A/\(\mathbf{\log}\) (\%)_ & **_A/\(\mathbf{\mu}\)H_** \\ \hline
2005 & Knepper and Garry [141] & Flat plate inserts & Flat plate & 3.58 & 0 & 0.67, 1.33 & 0.77, 1.54 & 1.15 \\  & & 2D high lift system & 16.3 & \(-2\) to 20 & 0.67, 1.33 & 0.77, 1.54 & 1.15 \\ \hline
2010 & [PERSON] et al. [102] & Flat plate inserts & NACA 6512-10 airfoil & 2.15 to 8.62 & \(-5\), 0, 5, 10, 15 & 12.5, 18.75 & 0.94, 1.88, 3.13, 5.63 & 0.05, 0.08, 0.10, 0.15, 0.25, 0.30 \\ \hline
2014 & [PERSON] et al. [142] & Flat plate inserts & SD 7037 airfoil & 4 & 0 to 22 & 13.33 & 4.44 & 0.33 \\ \hline  & & & NACA 0012 airfoil & 3.5 & 0 to 20 & 19.35 & 1.94, 5.81, 14.52 & 0.1, 0.3, 0.75 \\
2015 & [PERSON] et al. [136] & Flat plate inserts & NACA 65-(12)10 airfoil & 3.5 & \(-5\) to 20 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Selected investigations of aerodynamic effect of trailing edge serrations. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Year** & **Author(s)** & **Serration Type** & **Bionic Objects(s)** & \(\mathbf{R_{e}}\) (**10\%**) & **AoA (\({}^{\circ}\)**)** & **_H/\(\mathbf{\log}\) (\%)_ & **_A/\(\mathbf{\log}\) (\%)_ & **_A/\(\mathbf{\mu}\)H_** \\ \hline
2005 & Knepper and Garry [141] & Flat plate inserts & Flat plate & 3.58 & 0 & 0.67, 1.33 & 0.77, 1.54 & 1.15 \\  & & 2D high lift system & 16.3 & \(-2\) to 20 & 0.67, 1.33 & 0.77, 1.54 & 1.15 \\ \hline
2010 & [PERSON] et al. [102] & Flat plate inserts & NACA 6512-10 airfoil & 2.15 to 8.62 & \(-5\), 0, 5, 10, 15 & 12.5, 18.75 & 0.94, 1.88, 3.13, 5.63 & 0.05, 0.08, 0.10, 0.15, 0.25, 0.30 \\ \hline
2014 & [PERSON] et al. [142] & Flat plate inserts & SD 7037 airfoil & 4 & 0 to 22 & 13.33 & 4.44 & 0.33 \\ \hline  & & & NACA 0012 airfoil & 3.5 & 0 to 20 & 19.35 & 1.94, 5.81, 14.52 & 0.1, 0.3, 0.75 \\
2015 & [PERSON] et al. [136] & Flat plate inserts & NACA 65-(12)10 airfoil & 3.5 & \(-5\) to 20 & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Selected investigations of acoustic effect of fringe-type trailing edge extensions (”...” denotes “not clear”). The noise reduction capability of various closely spaced fringe-type trailing edge extensions was tested in DLR's open jet aeroacoustic wind tunnel in Braunschweig (AWB) at comparatively high chord-based Reynolds numbers of \(2.1\times 10^{6}\) to \(7.9\times 10^{6}\) (for the flat plate [143; 147]) or \(1.1\times 10^{6}\) to \(1.6\times 10^{6}\) (for the 2D NACA 0012-like airfoil [144; 148]) (see Figure 14). Noise measurement results by using a directional elliptical acoustic mirror showed that the examined fringe-type trailing edge devices led to a significant reduction of both broadband turbulent boundary layer trailing edge noise (in excess of 10 dB within the sensitive frequency range of human perception) and narrow-band vortex shedding noise (up to about 14 dB) due to finite trailing edge bluntness, for all the tested Reynolds numbers and plate lengths. Acoustic measurements on a cambered NACA 65(12)-10 airfoil at chord-based Reynolds numbers of \(1.73\times 10^{5}\) to \(3.47\times 10^{5}\) showed that a relatively low maximum noise reduction of 3 dB was achieved around 600-2000 Hz by a single row of flexible polypropylene fibers, due to the high curvature of the incident flow and a strong secondary emission of the leading edge acoustic source [145].