# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 3 Leading Edge Serrations
### Aeroacoustic Performance of Leading Edge Serrations
#### 3.1.2 Sinusoidal Serrations
A systematic parametric study on 12 sinusoidal leading edges with different wavelengths and amplitudes by [PERSON] et al. [52] revealed that: (a) Significant laminar instability tonal noise reduction can be achieved by the sinusoidal leading edge serrations with a smaller wavelength and larger amplitude (also observed in [47; 54; 61]). Smaller wavelength tends to generate more streamwise vortices per unit span, while larger amplitude would produce strong streamwise vortices. Both are more effective in restraining laminar flow separation and destroying the instability of theincoming boundary layer. (b) Turbulence-leading edge interaction noise attenuation improves slowly with reducing serration wavelength (see also in numerical simulations [67; 68]). However, small wavelength serrations might increase noise at frequencies greater than 10 kHz, which would translate into the corresponding overall sound pressure level, thus the smallest wavelength is not necessarily the most optimum choice. Moreover, the reduction of turbulence-leading edge interaction noise is increased (nearly linearly [63; 68; 69]) by increasing the serration amplitude. On the other hand, as shown by the numerical simulation results of [PERSON] et al. [70], this trend would reach a maximum beyond which no further increase could be achieved. Moreover, the optimal serration varies depending on the parameters of the inflow eddy, and it is clear that no one optimal serration exists that would reduce the noise for all the eddy parameters [71]. 2. **Inflow speed or Reynolds number:** For both the flat plate [50; 63] and the NACA airfoils [48; 50; 54], the level of noise reduction (both boundary layer instability noise and turbulence-airfoil interaction noise) decreases as the mean flow velocities or Reynolds numbers increase (see Figure 5b), although the variations in the noise reductions are not significant. 3. **Angle of attack**: [PERSON] et al. [52] showed that the best turbulence-leading edge interaction noise reduction actually occurs at the angle of attack \(\alpha=0^{\circ}\) for the modified airfoils with sinusoidal leading edge serrations, and noise reduction becomes slightly better at \(\alpha>0^{\circ}\) but less effective at \(\alpha<0^{\circ}\). Moreover, both experimental [48] and numerical [71] studies showed that the maximum noise reduction becomes smaller with increasing angle of attack. 4. **Directivity**: OASPL directivity patterns of the symmetrical NACA 0012 airfoil with sinusoidal leading edges at different azimuth angles all reduced 2-5.5 dB rod-airfoil interaction noise [53], while the observations on the non-symmetrical NACA 65-(12)10 lifting airfoil at 60 m/s [48] showed that the noise reduction in the rear arc (gain of 5 dB) is higher than that in the front arc (gain of 3 dB). 5. **Frequency range**: Significant noise reductions are confined to the mid to high range of frequencies (typically, 500 Hz to 8 kHz) but with negligible reductions at low frequencies (below about 500 Hz) for all the examined cases [50; 63; 69; 63].