# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 4 Trailing Edge Serrations
### Aeroacoustic Performance of Trailing Edge Serrations
#### 4.1.1 Broadband Self-Noise
The results showed significant suppression of the amplitude and frequency of the narrowband tonal noise, as well as the broadband self-noise (as compared in Figure 12b,c). However, the introduced surface roughness of the mesh screen to the sawtooth surfaces slightly increased noise level at high frequencies (around 8 kHz), and thus resulted in similar mean noise performance metric distribution to the non-meshed trailing edge serrations. On the other hand, the integration of porous material into the serrated trailing edges allowed the pressure and suction sides to \"communicate\", and thus reducing the acoustic dipole strength and completely suppressing vortex shedding tonal noise at the trailing edge while maintaining the benefits of the serrations. The optimal range of airflow resistivity for the porous material at the sawtooth gaps was found to be around 10 kN.s.m\({}^{-4}\)[133]. When the sawtooth gaps were partially filled with a thin layer of brushes, vortex shedding tonal noise was suppressed and slightly wider broadband noise reduction was achieved at mid to higher Reynolds numbers. Moreover, the overall noise performance improved with decreasing brush density. Noise reduction mechanism when using serrated trailing edges is very complicated, as pointed out by [PERSON] et al. [107], since the variation in turbulence velocity fluctuating strength is different for different positions and directions. However, several aerodynamic mechanisms are candidates to explain the noise reduction in the low to mid frequency ranges and the noise increase at higher frequencies:  
1. The mean pressure difference between the suction and pressure sides at the trailing edge drove the wake to start mixing together at the roots of the serrations and finally created cross flow [104]. The cross-flow increased the distance between the model surface and the suction side boundary layer (from 7.1 mm to 8 mm [102]), and thus led to a less efficient scattering source [105]. 2. Flow visualization by an advanced optically active liquid crystal technique [108; 134] showed that stronger turbulence existed on the predominantly sawtooth's oblique side edges and peaks since lower surface temperatures and higher convective heat transfers existed in these regions (see Figure 13). [PERSON] and [PERSON] [108; 134] conjectured that there were convective pressure-driven spanwise vortical structures near the sawtooth side edges and amalgamation of the vortical structures on both sides near the sawtooth peaks. The interaction between these vortical structures and the local turbulent boundary layer could be an effective mechanism to redistribute the momentum transfer, turbulent shear stress, and energy spectrum, resulting in reduced convection velocity of the turbulent eddies and weakened scattering of the turbulence interaction noise. Figure 11: Contours of streamwise vorticity (redrawn from [134]) for: (**a**) the baseline trailing edge; and (**b**) the serrated trailing edge. Figure 12: Photographs of the serrated trailing edges attached with woven-wire mesh screen (**a**); and color maps of the \(\Delta\)SPL between serrated and baseline trailing edges (**b**) or \(\Delta\)SPL between serrated and poro-serrated trailing edges (**c**), where positive dB value represents noise reduction. Reprinted from [97] with permission from Elsevier. 3. The flow was attached on the suction and pressure sides of the serrated edges [104] and the suction side turbulent boundary layer was suppressed [105], which in turn reduced the broadband noise amplitudes and tonal noise components. 4. The particular sawtooth geometry reduced the spanwise coherence (related to the spanwise correlation length), which further reduced the noise generation efficiency. In the streamwise direction, the turbulent eddies propagated at a similar speed for both the baseline and serrated trailing edges.