# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 3 Leading Edge Serrations
### Aeroacoustic Performance of Leading Edge Serrations
#### 3.1.1 Sawtooth Serrations  
In the early 1970s, a series of extensive studies of the application of sawtooth serrations to the leading edge of stationary and rotating airfoils was suggested by [PERSON] [56; 57] when Director of the NASA Ames Research Center, who was aware that the noise radiated from the airfoils can be reduced by using the silent flight adaptations of the owl wing for reference. In these studies, all serrations were sawtooth with various configurations and attached on the pressure side near the leading edge of airfoils or rotor blades. The acoustic measurements conducted by [PERSON] and [PERSON] [44] at chord-based Reynolds numbers ranging from \(Re_{c}=8.33\times 10^{4}\) to \(Re_{c}=3.33\times 10^{5}\) showed that the loud, distinct tone noise of both an airfoil and a propeller operating at low angles of attack in a smooth inflow can be definitely reduced or eliminated by using various types of sawtooth serrations attached in various positions on the pressure surface near the leading edge. The authors found that the tones were generated by laminar boundary-layer wake vortex shedding near the trailing edge of the airfoil and propeller at the examined Reynolds number range. The leading edge serrations removed virtually all the tones by generating chordwise trailing vortices which changed the boundary layer from laminar to turbulent on both pressure and suction surfaces and altered the wake vortex shedding from periodic or near periodic to broadband [58]. [PERSON] and [PERSON] [45] experimentally examined the possibility of using leading edge sawtooth serrations to reduce rotor noise in an anechoic chamber. The results indicate that the leading edge serrations could effectively reduce both rotational noise and vortex noise components under some specific configurations and running conditions. [PERSON] [46] investigated the acoustic effects of sawtooth serrations mounted near the leading edges of two different size rotors, operating in hover conditions under various rotor speeds, blade angles, as well as the shape and position of the serrations. He observed that the overall sound pressure level reductions varied from 4 to 8 dB for the small-scale rotor (1.52 m diameter, NACA 0012 airfoil, chord-based Reynolds number at the tip was \(1.83\times 10^{5}\)-\(5.5\times 10^{5}\)) with 3.6% chord serrations, while it was only up to 4 dB for the large-scale rotor (2.59m diameter, NACA 0015 airfoil, chord-based Reynolds number at the tip was \(9.94\times 10^{5}\)-\(3.18\times 10^{6}\)). The effectiveness of the sawtooth serrations to suppress tonal noise was sensitive to several factors, such as the geometry and location of the serrations, inflow speed and angle of attack, as well as the profile shape of the models. [PERSON] and [PERSON] [44] found that the greatest noise reduction of the NACA 0012 shaped airfoil was achieved by the deepest serration (about 1.1% chord), while the greatest reduction in noise of the two-bladed propeller was obtained by the shallow serration (about 0.75% chord). The noise measurement results on the large-scale rotor [46] revealed that the smaller serrations (0.6-1.2% chord) and serrations with spacing between prongs outperformed than the larger serration (3% chord) and the serration without spacing. However, [PERSON] and [PERSON] [45] found that the effectiveness of the serrations was strongly related to blade pitch angle and rotor speed, instead of sawtooth configurations (tooth depth and tooth tip spacing): 5 dB attenuation was realized at 6\({}^{\circ}\) pitch angle and 3000 Revolutions Per Minute (RPM), whereas a maximum noise reduction of 8 dB was observed at 10\({}^{\circ}\) pitch angle and 2000 RPM. Moreover, the positions of the serrations had a significant effect on noise reduction: the best reduction was obtained when the sawtooth serrations were located near the stagnation point (move downstream with increasing angle of attack) [46], and when they were sticking out from the surface as opposed to when they were attached flush against the surface [44].