# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 4 Trailing Edge Serrations
### Aeroacoustic Performance of Trailing Edge Serrations
#### 4.1.2 Instability Tonal Noise  
[PERSON] et al. [109] investigated the effectiveness of the serrated geometries with flat plate inserts to reduce the laminar boundary layer instability noise at low to moderate Reynolds numbers (\(3.33\times 10^{5}\)-\(4.66\times 10^{5}\)). Experimental results show that pressure side instability was the main cause of the instability noise emission for the free transition NACA 0018 airfoil and trailing edge serrations could reduce up to 40 dB acoustic emission. [PERSON] et al. [101; 103; 129] presented an acoustical study of the use of directly cutting serrated trailing edges to control instability tonal noise under different low to medium Reynolds numbers and angles of attack, with the aim to establish the causal link between the spectral properties of the radiated tones and the separation bubbles on the pressure side of the NACA 0012 airfoil. The results show that trailing edge serrations did not alter the streamwise pressure gradient, but triggered a bypass transition of the laminar boundary layer into turbulence and suppressed the length of the separation bubble near the serrated trailing edges, resulting in reduced amplitude and tonality of the radiated noise. Several studies showed that the effectiveness of trailing edge serrations to reduce instability tonal noise was sensitive to several factors: (1) Instability tonal noise attenuation improved with increasing flow speeds or Reynolds numbers [101; 103] when the serrations were directly cut or decreased with increasing flow speeds when thin serrated flat plate inserts were added [109]. (2) Tonal noise reduction was more effective at a larger angle of attack [101; 103], since the onset of the boundary layer separation bubble at the NACA 0012 airfoil's pressure side had moved downstream with increasing angle of attack, which led to a smaller bubble length or total amplification factor of the T-S wave. (3) When the ratio between the wavelength and amplitude of serrations \(\lambda/h\) was the same, the one with larger amplitude produced less instability noise [101; 103; 109] since more separation regions were affected under this situation. On the other hand, when the amplitude of the serrations was the same, the one  
Figure 13: Captured raw active liquid crystal images (redrawn from [134], reddish black color and green color denote low temperature and high temperature, respectively) for: (**a**) the baseline trailing edge ( serrated trailing edge with triangular add-on); and (**b**) the serrated trailing edge. with wider wavelength or larger oblique angle would further suppress the broadband hump and the discrete tones (see also [96; 123]). This conclusion is contrary to the theoretical study of [PERSON] [114] who pointed out that serration oblique angle of less than 45\({}^{\circ}\) was necessary for the reduction of broadband self-noise for the sawtooth trailing edges. The authors speculated that the reason may be due to the fundamentally different generation mechanisms [101]: The generation mechanism of the instability tonal noise is related to the aeroacoustic feedback loop at the pressure side, which involves the diffraction of the two-dimensional T-S waves at the vicinity of the sharp trailing edge. When the serrations have a larger oblique angle, the incoming T-S waves have an overall shorter \"edge\" or separation bubble to be amplified sufficiently in the boundary layer and further be scattered into discrete noise, which resulted in a larger tonal noise reduction. To explore the instability tonal noise attenuation mechanism of the trailing edge serrations, researchers [106; 109; 129] employed several advanced flow field measurement techniques, including surface mounted hot-film arrays, hot-wire anemometry and PIV. The results of the surface mounted hot-film sensor arrays[106; 129] showed that the onset of the boundary layer separation was located at around 83% chord length for the baseline NACA 0012 airfoil at \(Re_{c}=4\times 10^{5}\) and geometrical angle of \(\alpha=15^{\circ}\). [PERSON] et al.