The noise problem of wind turbines is prominent
With the goal of "30 carbon peak, 60 carbon neutrality", wind energy as an important part of renewable energy, in recent years, the vigorous development of wind farms will be closer and closer to residential areas. However, the wind turbine will make a unique whizzing sound during rotation, and the periodic change of noise will make people very uneasy. Noise at particularly lower frequencies, because the atmosphere does not absorb attenuation noise well, especially at night, the sound refracts downward so that people can hear wind turbine noise at a distance. Wind turbine noise has been reported to have a serious negative impact on psychology and can reduce sleep quality, leading to frequent reports of wind disturbances in recent years.
Wind turbine (image from the Internet)
A megawatt-scale wind turbine locates the sound source and finds that its main noise source is generated by the rotation of the blade of the wind turbine, and is mainly concentrated near the side of the leaf tip (the darker the color, the louder the noise). The aerodynamic noise generated by the rotation of the blade is mainly the turbulence noise and the blade self-noise, both of which are caused by the interaction between the turbulent flow and the surface of the wind power blade. In the case of a MW-class wind turbine, the noise generated by the rotation of its blades can be as high as about 105 decibels, which is comparable to the noise emitted by the lawn mower. When it is transmitted to a distance of 300m downstream, it can meet the living standards stipulated by the state (55 decibels during the day and 45 decibels at night).
Wind turbine noise source localization (Image source: Reference 1)
Wind turbine noise size (image from the network)
Noise reduction research based on biomimicry
Biomimicry studies the structure, function, and working principles of biological systems and attempts to create new engineering techniques based on these concepts found in nature, and to invent devices to help solve practical engineering problems. Many cases of biomimicry have already been applied in our lives: people invent radar through the principle of sonic positioning of bats, people have invented oars inspired by fish fins, drag-reducing swimsuits have been invented through shark skin surfaces, and so on.
In view of the noise problems in the actual operation of wind turbines, some noise reduction methods based on biomimics have emerged in recent years. Many species of owls are known to have a unique ability to fly silently, which is attributed to their unique feather adaptability. Inspired by the owl, the researchers attempted to reduce the aerodynamic noise of wind turbines, aircraft and other structures by exploring the noise reduction characteristics of wind turbines, aircraft and other structures from different perspectives, and then using the knowledge gained to develop innovative noise reduction schemes that reduce the aerodynamic noise of wind turbines, aircraft and other structures.
The secret of the owl's silent flight
Owl feather characteristics (Image source: Reference 2)
To explore the secrets of the owl's silent flight, you must first start with the structure of the owl's wings. As early as 1934, British scientist Graham discovered that the three main characteristics of the owl's silent flight, namely the comb-shaped leading edge, the jagged tail edge and the soft villi on the surface, it is these special structures that allow the owl to achieve quiet flight. Enlarging the owl's feathers reveals that the edges of the feathers have fine tooth-like structures, and the length of its teeth changes along the extension position.
After years of research, it has been found that these special leading and trailing edge tooth structures will cause interference between the radiated sound waves to disappear, resulting in a decrease in noise. In addition, these anterior caudal dendritic structures also induce additional vortex structures that change the original vortex structure flowing through the wings, and can also reduce the intensity of the noise source.
The serrated caudal margin changes the original caudal flow field (Image source: Reference 3)
Bionic noise reduction applications
Inspired by the owl's quiet flight, the researchers reduced its aerodynamic noise by fitting the toothed leading edge and tail edge on the fluid part. The toothed leading edge reduces the leading edge noise due to the turbulence inflow and the leading edge, while the toothed tail edge reduces the tailing noise due to the interaction of the boundary layer with the tail edge. In recent years, the serrated tail edge has also been gradually applied to wind power blades, and it has been found that the installation of serrations on the tail edge of the blade can obtain a noise reduction effect of 3.2 dB.
In addition, this bionic noise reduction technology is also gradually applied to rotating machinery such as aircraft, drones, and wind power. With the maturity of bionic noise reduction technology, it is believed that it will be promoted and applied on household appliances (such as air conditioners, exhaust fans, etc.).
Application of bionic noise reduction technology (Image source: References 4 and 5)
bibliography:
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2. Weger M, Wagner H. Morphological variations of leading-edge serrations in owls (Strigiformes)[J]. PLoS One, 2016, 11(3): e0149236.
3. Gruber M, Joseph P, Chong T. On the mechanisms of serrated airfoil trailing edge noise reduction[C]//17th AIAA/CEAS aeroacoustics conference (32nd AIAA aeroacoustics conference). 2011: 2781.
4. Oerlemans S, Fisher M, Maeder T, et al. Reduction of wind turbine noise using optimized airfoils and trailing-edge serrations[J]. AIAA journal, 2009, 47(6): 1470-1481.
5. Fish F E, Weber P W, Murray M M, et al. The tubercles on humpback whales' flippers: application of bio-inspired technology[J]. 2011.
Source: Institute of Engineering Thermophysics, Chinese Academy of Sciences