# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 4 Trailing Edge Serrations
### Aeroacoustic Performance of Trailing Edge Serrations
#### 4.1.1 Broadband Self-Noise
These experimental studies revealed that the assumptions and simplifying approximations made by [PERSON] to derive the serration noise reduction model might be inaccurate [117; 119; 120; 123]. [PERSON] et al. [99] and [PERSON] et al. [124] did validation measurements on the reduction of trailing edge noise from scaled wind turbine models in different anechoic wind tunnels. The results show that acoustically optimized serrated rotors could achieve 6-7 dB turbulent boundary layer trailing edge noise reduction over a variety of flow conditions, with little to no changes to aerodynamic performance. Based on these encouraging studies, [PERSON] et al. [100; 125] and [PERSON] et al. [124] further performed acoustic measurements on full-scale wind turbines. Several parameters were considered in these works, such as wind speed, observer position and the effect of the blade roughness. The results showed that: (1) Although lower than that obtained on the model wind turbines, the full-scale wind turbines with serrated blade still obtained significant trailing edge noise reduction at frequencies below 1 kHz and high wind speeds: average overall sound power level reduction of 3.2 dB for the upwind measurements on the clean rotor, and 1.2 dB and 1.6 dB reductions for the downwind measurements on the clean and tripped rotor, respectively. (2) The serrated blade was substantially quieter than the baseline one during the downward movement, while much noisier during the upward movement. [PERSON] et al. [104] and [PERSON] et al. [96; 117; 123; 126] experimentally investigated the influence of different parameters on the noise reduction performance of flat plate inserted serrations. In the experiments of [PERSON] et al. [104], over 30 serrated trailing edges with different sawtooth geometries were tested on a NACA 65-(12)10 airfoil. Several interesting and valuable observations were found: (1) At lower frequencies (300 Hz to 400 Hz), sawtooth serrations reduced less than 1 dB noise due to the dominance of jet noise. Sound power level decreased significantly in the mid-frequency range (more than 7 dB) while increased for higher frequencies (a maximum of about 3 dB). The critical frequency \(f\) above which sawtooth serrations began to increase noise followed a Strouhal number dependency of about \(St_{\delta}=f\delta/u_{\infty}\sim 1\), where \(\delta\) is the boundary layer thickness. (2) With increasing inflow speed or Reynolds number, boundary layer thickness \(\delta\) decreased and thus the critical frequency \(f\) moved towards higher frequency, i.e., the frequency region where sawtooth serrations has noise reduction capability increased (also observed by [PERSON] et al. [105] for a linear cascade of seven loaded airfoils and by [PERSON] et al. [107] for a cambered SD 2030 airfoil). However, the maximum noise reduction levels decreased when the inflow speed or Reynolds number was increased [96; 104; 105; 127]. (3) Spectral shape and dependency on the angle of attack appeared to be small, compared to other parameters. (4) When the wavelength of the serrations \(\lambda\) decreased, noise reduction increased at low to mid frequencies while decreased at higher frequencies. Thus, there existed an optimal value for the serration wavelength. (5) A critical value of serration amplitude is \(h/\delta>0.5\), below which sawtooth serrations are inefficient at attenuating noise radiation since the eddies in the boundary layer are too large to be influenced by a small amplitude \(h\), while above which significant noise reductions occur. Noise radiation efficiency decreased (see also [112]) and noise reduction frequency bands became wide with increasing \(h/\delta\), but at the same time noise increase at higher frequencies increased. On the other hand, in the experiments of [PERSON] et al. [96; 117; 123; 126], two different sawtooth geometries with the same amplitude were tested on a flat plate at low-to-moderate Reynolds numbers. The results show similar but slightly different observations to the works of [PERSON] et al.