# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 3 Leading Edge Serrations
### Aerodynamic Properties of Leading Edge Serrations
#### 3.2.1 Sawtooth Serrations  
[PERSON] and [PERSON] (2019) found that the fluctuating lift (the dipole source of the acoustic tone), drag, and pitch moment levels were greatly reduced by attaching the sawtooth serrations, but no significant improvements or adverse effects in steady or average aerodynamic performance were observed. Serrations placed near the stagnation point substantially increased the slope of the lift curve by 12% for the symmetrical NACA 0015 airfoil and 22% for the camber NACA 2412 airfoil ([PERSON], 2019; [PERSON], 2019), respectively. Both drag and stall angle were not measurably increased by the serrations for either airfoil. [PERSON] and [PERSON] (2019) showed that both vortex noise and rotational noise were attenuated by using leading edge serrations onto a two-foot diameter rotor, but it was accompanied by obvious losses in rotor efficiency (thrust coefficient reduced and torque coefficient increased), thus the authors believed that the practicality of the leading edge sawtooth serrations as a noise attenuator might be questionable ([PERSON], 2019).  
[PERSON] (2019) at NASA Ames systematically studied the effect of sawtooth leading edge serrations on aerodynamic performance. He found that serration size was an important parameter for determining the aerodynamic effects on a 2D wind tunnel model, while serration lean (spanwise cant angle) had very little effect: smaller sawtooth serrations (0.13% to 0.67% chord length), when properly attached tangentially to the leading edge surface of the NACA 661-012 airfoil's pressure side, increased maximum lift by approximately 12% and angle of attack for maximum lift (i.e., stall angle) but slightly reduced the slope of the lift-curve, by generating streamwise counter-rotating vortices from each root that re-energized the boundary layer, decreased the airfoil wake thickness and delayed flow separation on the suction side at various angles of attack. On the other hand, larger serrations reduced the maximum lift since they obstructed the flow over the suction side. Maximum lift and lift-curve slope can be further improved when there is some spacing or gaps between the roots rather than immediately adjacent, however, the spacing is not as big as possible but has a suitable intermediate value. The drag was not affected by the introducing serrations at low angles of attack but was reduced at high angles of attack when the stall angle was approached. [PERSON] also found that serration location was the most critical parameter to performance: serrations attached as close as possible to the stagnation point region (1.25% chord) near the stall angle of attack caused the greatest increase in maximum lift (up to 21%), while the performance was degraded when the serrations were attached forward to the frontline of the airfoil. [PERSON] (1987) performed wind tunnel experiments with laminar wing models which were equipped with or without different serrated leading edges. The author found that the aerodynamic characteristics of sawtooth serrations had a strong Reynolds number effect: In the lower Reynolds number of \(Re_{c}=2.1\times 10^{4}\), the sawtooth leading edges showed higher lift coefficient than the unmodified airfoil beyond \(\alpha=10^{\circ}\), and serrations with lower wavelength had higher lift coefficient. While in the higher Reynolds number of \(Re_{c}=2.1\times 10^{5}\), no significant difference was seen among the different leading edge configurations.