# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 3 Leading Edge Serrations
### Aeroacoustic Performance of Leading Edge Serrations
#### 3.1.3 More Innovative Geometries  
Different from the above-mentioned leading edge serrations, where the chord changed triangularly or sinusoidally in the spanwise direction (see Figure 6a), [PERSON] et al. [72] proposed a novel leading edge model where the geometric angle of attack varied sinusoidally along the span  
Figure 5: Typical comparison of acoustic spectra, reprinted from [63] with permission from AIP Publishing: (**a**) variation in amplitude (the prediction by using Amiet theory is shown in this subfigure to validate the measurement method and to establish the baseline acoustic spectra); and (**b**) variation in wavelength (dashed lines and solid lines for each speed denote \(\lambda=0.033c_{0}\) and \(\lambda=0.133c_{0}\), respectively).  
(see Figure 6b). Both predictions by using [PERSON]'s nonlinear lifting-line theory and wind tunnel experiments have shown that the newly proposed design had similar aerodynamic characteristics to those with conventional sinusoidal serrations. However, no acoustic measurements were presented in this work.  
In 2016, several innovative leading-edge geometries were developed by [PERSON] et al. [65], who further modified the geometry profiles of conventional sawtooth or sinusoidal leading edge serrations, with the hope to provide substantially greater noise reductions. These innovative geometries can be classified into two categories according to their noise reduction mechanisms: control of noise radiation and control of noise source. The first category includes the double-wavelength serrations, which were made up of the summation of two single-frequency sinusoidal wavelengths of roughly the same amplitude, and the chopped-peak serrations by clipping the peak. The second category includes the leading edge slitted-root serrations where a narrow slit was added at the root position of the serrations and the leading edge slitted-V-root serrations. Acoustic measurement results confirm the effectiveness of these innovative geometries: (1) Double-wavelength serrations obtained up to 4 dB additional noise reductions around the peak of sound power level, compared to the single wavelength serrations, due to the interference between adjacent serration roots. Moreover, it was observed that, under the same wavelengths, low-frequency noise reduction of the double-wavelength serrations reduced while high-frequency noise reduction increased, as the streamwise distance between two adjacent roots increased. (2) Chopped-peak serrations increased the low-frequency noise reductions, which can be put down to the increased source strength at the clipped peak of the serrations and the enhanced degree of destructive interference between the chopped-peak and the root. (3) As shown by [PERSON] et al. [51] and [PERSON] et al. [73], the dominant noise sources of the serrated sinusoidal leading edges in the low-frequency range are concentrated near the root of the serrations. Thus, the introduction of the leading edge slitted-root serrations reduced the pressure difference and vortex strength at these locations, which in turn increased the noise attenuations at low frequencies in the far field. Slitted-V-root serrations further eliminated the negative noise reductions at high-frequency ranges of the slitted-root serrations, which was caused by a Helmholtz resonance formed in the cavity across the narrow slits.