# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 4 Trailing Edge Serrations
### Aeroacoustic Performance of Trailing Edge Serrations
#### 4.1.1 Broadband Self-Noise
[104], which were attributed to the significant differences between the geometry of the applied models (NACA 6512-10 airfoil vs. flat plate): (1) Both broadband noise levels and high-frequency peak due to vortex shedding from the blunt trailing edge of the reference flat plate decreased with decreasing flow speed or Reynolds number. Trailing edge serrations slightly reduced broadband noise levels (up to 3 dB) at low frequencies, produced equivalent noise levels or a minor noise increase in the mid-frequency range, while reduced up to 13 dB vortex shedding noise near the high-frequency peak. Noise radiation performance of the serrated trailing edges also appeared to follow three constant Strouhal numbers based on boundary layer thickness. However, the values were different from the study of [PERSON] et al. [104]. (2) The wide serrations produced larger noise reductions, since the region of low-frequency noise attenuation was much wider and the radiated noise in the mid-frequency region was approximately equal to the reference plate. To suppress high-frequency noise increase normally observed with trailing edge serrations, [PERSON] et al. [128] introduced several innovative serrated-type trailing edge treatments. They showed that cutting slits into the sawtooth serrations provided an additional overall sound power reduction of up to 2 dB in the mid-frequency range (1-7 kHz) and about 4 dB in the higher frequency range (7-20 kHz), since slitted sawtooth distributed the pressure difference between the suction side and pressure side of the serrated trailing edges by introducing flow permeability and thus reduced the intensity of the cross flow in each sawtooth root [104]. Moreover, They found that noise reduction gradually improved with increasing depth and reducing the separation distance of the slits. [PERSON] et al. [128] also showed that random trailing edge geometry provided up to 3 dB broadband noise reduction with no significant noise increase at high frequencies since a random pattern reduced the scattering effect at the trailing edge by maximizing the length of the wetted edge and reducing the correlation lengths of trailing edge turbulence [104]. Different from all the above cases where the serrations were flat plate inserts adding into the trailing edge of the main body, [PERSON] et al. [97; 115; 129] investigated the noise performance of serrations directly cutting into the main body, which offers better structural strength and integrity. The measurements showed that the cutting serrations not only obtained similar broadband self-noise reductions (2-8 dB) over a larger frequency range but also could suppress the noise increase at high frequencies caused by the flat plate inserts. However, a by-product of the directly cutting serrations was extraneous low-frequency narrowband tonal noise due to the additional vortex roll-up and shedding involving horseshoe vortices [118; 130; 131; 132] (see Figure 11). The vortex shedding noise was produced by the bluntness of varying degrees introduced across each sawtooth side (maximum at the root while minimum at the tip) or the periodic oscillatory flow (upwash and downwash) within each sawtooth gap, and thus resulted in a large penalty for the reduction of the overall sound pressure level. Moreover, they found that the low-frequency tonal noise increased with higher flow speed and longer serration amplitude (see also [130]), while decreased when the wavelength of the serrations \(\lambda\) increases. To inhibit the growth of the vortical structures at the near wake and thus suppress these extraneous noise components originating from the gap of each sawtooth, the authors further explored a hybrid trailing edge configuration which covered the serrations with a flow-resistant woven-wire mesh screen [97; 115] (see Figure 12a) or filled the gaps between adjacent teeth with porous metal, synthetic foams, or thin brush bundles [110].