# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 3 Leading Edge Serrations
### Flow Mechanisms Involved with the Noise Reduction by Using Leading Edge Serrations  
The mechanisms responsible for the noise reduction obtained in the past generally include the generation of streamwise vortices and the reduction of spanwise correlation:  
1. **Vortex generators**: Hot-wire measurements conducted by [PERSON] and [PERSON] [45] showed that leading edge sawtooth serrations severely dampened the mean flow and the near wake turbulence intensities at the position of 75% span and two chord lengths downstream of a two-bladed propeller. Combined with the studies of flow visualization (kerosene-burning smoke generator and stroboscopic lights), the authors suggested that the leading edge serrations along the blades acted as \"vortex generators\", which served to introduce small scale instabilities into the main flow and altered the turbulence structure by quickly breaking up large eddies shed from the blade tips or airfoils, resulting further in reduction of vortex noise. Another effect of \"vortex generators\" was that it could induce the formation of counter-rotating stream-wise vortex pairs at each root [47; 54], which may trigger the laminar boundary layer \"bypass transition\", reduce Tollmien-Schlichting (T-S) instability waves, and then destroy the acoustic feedback loop, which in turn further decrease or totally suppress instability tonal noise.  
Figure 8: Lift and drag coefficients for the baseline (solid lines) and serrated (triangles) whale flipper model, reprinted from [60] with permission from AIP Publishing: (**a**) lift coefficient; and (**b**) drag coefficient.  
Figure 7: 3D scale model of an idealized humpback whale flipper with and without sinusoidal leading edges, reprinted from [60] with permission from AIP Publishing.  
When turbulence impinging to the leading edge, the local sweep angle of the sinusoidal serrations (particularly in the mid-slopes regions) reduced the pressure fluctuations [56; 57; 69], which decreased the effectiveness of the sound source and thus the turbulence-airfoil interaction noise.
2. **Spanwise correlation loss**: [PERSON] et al. [61] compared the pressure distribution between both the unmodified and modified (\(\lambda 30H8\) configuration) airfoils at \(\alpha=5^{\circ}\) by using a low-speed wind tunnel, and speculated that another possible explanation of tonal noise reduction was the spanwise variations in separation location: separation bubble extended over the entire span of the unmodified airfoil, while it was localized to or separated earlier behind the roots of the airfoil with serrations (the flow remained largely attached on the peaks; see also [47; 54; 75; 89; 95]). This characteristic might affect the coherence of the vortex generation, reduce the sensitivity of the boundary layer to external acoustic excitation and minimize the potential for the development of trailing edge tonal noise feedback loop (T-S waves). This characteristic might also dramatically reduce the size of the separated flow region and thus play a critical role in the corresponding improvements of the aerodynamic performance [75; 81; 82; 89; 95], i.e., higher \(C_{L}\) in the post-stall region.  
Numerical simulations by [PERSON] et al. [62; 64] using RANS/LES and a CAA code solving the nonlinear Euler equations in conjunction to the FW-H acoustic analogy showed that pressure distribution was almost identical away from the leading edge region, while it was noticeably different at the region of leading-edge serrations (also experimentally observed in [48]): The pressure peak and the pressure fluctuation behind the roots of the leading edge serrations were slightly amplified compared to the unmodified configuration, while they were reduced by more than a half at both the peaks and mid-slopes (see also [53]). Therefore, the pressure signals from different locations had significant phase shifts [67] and reduced spanwise correlation [69; 70], which might reduce the efficiency of the interaction noise radiation.