# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 4 Trailing Edge Serrations
### Aerodynamic Properties of Trailing Edge Serrations  
Comprehensive studies (see Table 6) showed that the aerodynamic performance of trailing edge serrations relied on the applied model, flow speed, angle of attack, and geometrical characteristics of the serrations.  
Measurements of the pressure distribution along the chord and wake velocity downstream of the asymmetric NACA 65-(12)10 airfoil [102] showed that both lift and drag coefficients were not significantly influenced by the introduction of trailing edge serrations (see also [99; 124] for wind turbine models). However, the measurements on a linear cascade made of 7 NACA 65-(12)10 blades showed that the loading was not affected in the presence of the serrations but the drag coefficient was increased by 14% [105]. Moreover, directly aerodynamic lift and drag measurements [83; 131; 132; 136] by using a force balance showed that NACA 65-(12)10 airfoil attached with different sawtooth serrations reduced up to 15% lift coefficient over the angles of attack ranging from \(-5^{\circ}\) to about 10\({}^{\circ}\), compared to the baseline airfoil with the same surface area, while increased the lift coefficient in the pre-stall region. Sawtooth serrations increased the drag coefficient at angles of attack larger than 10\({}^{\circ}\) and more drag was produced by the serrations with increasing wavelengths.  
The aerodynamic lift and drag measurements on the symmetric NACA 0012 airfoil [83; 131; 132; 136] showed noticeably different behavior to that of the asymmetric NACA 65-(12)10 airfoil: The effect of sawtooth serrations on lift coefficient was less significant, especially at lower speeds and smaller angles of attack (from 0\({}^{\circ}\) to 9\({}^{\circ}\)). Serrations decreased the maximum lift coefficient over the critical angle region (from 12\({}^{\circ}\) to 15\({}^{\circ}\)) but improved the lift performance at deep stall region (from 17\({}^{\circ}\) to 20\({}^{\circ}\)). Moreover, NACA 0012 airfoil with serrations generally produced lower drag than that of the baseline airfoil with the same surface area.  
[PERSON] and [PERSON] [141] investigated the effect of trailing edge 60\({}^{\circ}\) sawtooth serrations ahead of a single slotted flap on the aerodynamic forces of a multi-element airfoil configuration. Measurements on both a generic flat plate model and a 2D high lift model indicated that trailing edge serrations could enhance slotted flap performance across a range of flap lap/gap settings and flap deflections. Flap lift coefficient increased up to 17% and this effect diminished with increasing flap deflection angles. The effect on the flap drag coefficient was found negligible.  
Different from all these cases where static serrated airfoils were used, [PERSON] et al. [142] investigated the effect of a dynamic serrated airfoil (i.e., the airfoil with serrated trailing edges is subjected to highly oscillating angles of attack, which is typical behavior for a wind turbine under field operating conditions). The results show that the lift values of the serrated case were within reasonable agreement with the original airfoil at low angles of attack, while the dynamic stall angle changed from 18.5\({}^{\circ}\) to 19.5\({}^{\circ}\), which led to higher lift values close to the dynamic stall angle.