Numerical investigation of low-noise airfoils inspired by the down coat of owls

Aeroacoustic characteristics of owl-inspired blade designs in a mixed flow fan: effects of leading- and trailing-edge serrations

There is an increasing need in industry for noise reduction in fans. Inspired by owls' silent flight, we propose four owl-inspired blade designs for a mixed-flow fan to examine whether leading-edge (LE) and/or trailing-edge (TE) serrations can resolve the tradeoff between sound suppression and aerodynamic performance. We investigate the blades' aeroacoustic characteristics through various experimental methods and large-eddy simulation (LES)-based numerical analyses. Experimental results suggest that 'slotted', simply-fabricated LE serrations can achieve a lowering of the noise level while sustaining the aerodynamic performance of the fan, whereas TE serrations fail. In addition, the inclination angle can improve LE serration performance in aeroacoustic and aerodynamic performance with a reduction in the specific noise level by around 1.4 dB. LES results and noise spectral analysis indicate that the LE serrations can suppress flow separation, reducing the broadband noise at low-to-middle frequencies (40-4k Hz). This passive-flow-control mechanism, likely due to local higher incidence angles associated with LE serrations, is capable of alleviating the intensive pressure gradient while suppressing wall-pressure fluctuations over the LE region, hence weakening the Kelvin-Helmholtz instability. The tonal noise also shows a marked reduction at the highest peak frequency associated with fan-vane interaction. Moreover, we find that the high-frequency noise by-product radiates mainly from the LE serrations andsurroundings, due to the small eddies broken up when the vortical flows pass through the LE serrations. Our results demonstrate that the biomimetic design of the LE serrations can facilitate the break-up of LE vortices passively and effectively without negatively impacting aerodynamic performance, which can be utilized as an effective device to improve the aeroacoustic performance of fan blades.

Optimal design of aeroacoustic airfoils with owl-inspired trailing-edge serrations

The trailing-edge serration that imitates the silent owl wing is used as a flow control method to suppress the aerodynamic noise generated from the rotating wind turbine blade. Recent studies have found that the addition of serrations could degrade the overall aerodynamic performance of the airfoil. To this end, an optimal design method for airfoils with the trailing-edge serration is developed. Combined with the modeling methods of aerodynamics for serrations, the fundamental parameters of serrations are integrated into the optimal design of wind turbine airfoils. Specifically, based on the existing multidisciplinary optimization method for airfoils, the aerodynamic prediction and evaluation module for the serrated airfoil was introduced to develop an aerodynamic-structural optimal design platform. In this way, a novel serrated airfoil equipped with high aerodynamic performance can be designed. Compared with the reference airfoil, the maximum lift-to-drag ratio and lift coefficient of the optimal serrated airfoil at the design point have been increased by 1.9% and 32.5%, while the aerodynamic noise could also be reduced. Finally, experiments were conducted in an anechoic chamber to verify the noise-reduction level of the optimal serrated airfoil, which sufficiently demonstrate the capability to improve the comprehensive performance of the airfoil using such a developed optimal scheme.

Effect of trailing-edge serrations on noise reduction in a coupled bionic aerofoil inspired by barn owls

Noise reduction is an important development direction for aircrafts and wind turbines. Owl wings have three unique morphological characteristics (leading-edge serrations, trailing-edge serrations and velvet-like surfaces) that effectively suppress aerodynamic noise in low Reynolds numbers. Among them, trailing-edge serrations are widely considered the most effective noise-reduction method. Although different serrations have been studied, the quantitative relationship and influence mechanism between the serration shape, wavelength and amplitude are poorly understood. The acoustic characteristics of asymmetrical aerofoils with different trailing-edge serrations have not been fully studied. This work investigates the flow characteristics and acoustic scattering mechanisms of novel owl-based aerofoils with different trailing-edge serrations. A sensitivity analysis is utilized to quantitatively investigate the influence and interaction mechanisms of the shape, wavelength and amplitude in trailing-edge noise reduction. Numerical simulations of the transient flow over the aerofoil are performed via the large eddy simulation method, and the acoustic far-field is obtained by solving the Ffowcs Williams and Hawkings equation. The results indicate that the sawtooth and sinusoidal serrations provide the most significant noise reduction effects; the maximum noise reduction is 8.74 dB. The wavelength and amplitude play similar roles, but the amplitude has relatively greater influence. For the sawtooth and sinusoidal serrations, the large-scale vortex structures are broken into many small-scale spiral vortex structures due to the presence of the sharp serration tip. The serrations can effectively reduce the coherence of the turbulent fluctuations due to spanwise variations in the edge and may be the main reason for noise suppression. The original owl-based aerofoil generates more low-frequency noise and less high-frequency noise than aerofoils with trailing-edge serrations. The peak noise frequencies of all aerofoils are approximately 400 Hz; hence, low-frequency noise is a dominant influence in noise generation. Furthermore, the acoustic sources generated by transient pressure fluctuations are mainly located on the serration root.