# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 1 Introduction  
In recent years, the problem of aerodynamic noise and its control have become a matter of serious concern and thus attracted a number of researchers' interest. With the rapid development of the aviation industry and the ever-increasing popularity of air travel, the number of large commercial aircraft, as well as expanded and newly built airports, has increased dramatically. The problem of aircraft noise control has attracted more and more attention, and more stringent standards have been set by the International Civil Aviation Organization (ICAO) on aircraft movements during take-off and landing approach phases. Therefore, NASA, Boeing and so on regard the reduction of noise emission of the aircraft itself aerodynamically as the basic requirement for modern aircraft design and are devoted to the development of silent aircraft [1; 2; 3; 4; 5]. As an environmentally acceptable and technologically mature renewable energy, wind energy has been gradually utilized by human beings. However, with the rapid growth of the total installed capacity of the global wind power, large-scale wind farms and wind turbines with increased rotor diameter will inevitably get closer to the densely populated residential areas. As a result, the accompanying noise pollution problems lead to morecomplaints from the surrounding residents [6; 7]. At present, the speed of trains has reached more than 200 km per hour. High-speed trains at 350 km per hour or even faster speed are also being planned. Therefore, to restrain the increase of wayside noise along railway lines is a key issue that cannot be avoided since the acoustic power of the radiated aerodynamic noise satisfies a sixth-order velocity dependence [8]. In the past few decades, a number of methods and devices have been investigated with the aim to reduce the aerodynamic noise, which can be classified into active and passive techniques [9]. Active control techniques need some energy input or relevant auxiliary equipment to manipulate the flow around the objects, such as wall-normal suction or blowing [10; 11; 12; 13] and plasma actuator [14]. On the other hand, passive techniques can control noise emission by making small geometry changes or adding other materials to the surface of the objects, such as vortex generator [15]. However, the advancement of current low noise technologies has gradually entered the bottleneck period and appears to be insufficient to achieve the target of \"A Vision for 2020\" to reduce the perceived noise to 50% of the noise level in 2000 by 2020 [16], not to mention an even greater challenging objective of \"Flightpath 2050\" to reduce the Effective Perceived Noise (EPN) levels by 65% in 2050 [17]. Therefore, there is an urgent need to introduce innovative ideas and the engineering of new technologies. Among them, bio-inspired aerodynamic noise control is one of the most promising techniques. Bionics investigates the structure, function and working principles of biological systems, and tries to transplant these concepts found in nature to create new engineering technologies, and to invent superior instruments, devices, and machines to help solving complex human problems [18]. In the past few decades, bionics has achieved a lot of research results. For example, the bullet-shaped front of the high-speed trains, inspired by the streamlined long-tailed beak of the kingfisher, can reduce the energy consumption of trains and solve the \"sonic explosion\" problem that occurs when trains pass through narrow tunnels [19]. Swimsuits made in the style of sharkskin can reduce the resistance of the water when athletes are swimming. The invention of the aircraft was inspired by bird flight and bird wings more than 100 years ago. Recent observations and studies on most genera of owls found that they can fly quietly close to their prey, and have the well-known ability of silent flight, which may be an excellent biological clue for finding solutions for quieter aircraft and other aerodynamic structures.