# The Noise-Reduction Characteristics of Microstructure of Dragonfly Wing Leading Vein
## 1 Introduction  
The dragonfly has been attracting much interest due to its excellent characteristics, such as low-noise high mobility flight [1]. As the most important organ to support silent flight, the dragonfly wing is becoming a research hotspot in many fields. It has been found that the coupled effects of wing shape, configuration, structure, material and other factors make dragonflies have multiple functions of vibration reduction, noise reduction, self-cleaning and fatigue resistance [1]. However, the study in anti-vibration and noise-reduction characteristics of dragonfly wings mainly focused on the pterostigma of dragonfly wings, and the low-noise flight characteristics of dragonfly wings are rarely reported in detail. The flying noise of dragonflies is very gentle, which is not only related to the wing pterostigma but also to the veins of dragonfly wings and the microstructure on their wings. According to hydrodynamic analysis, when the solid surface is in an unstable and nonuniform incident flow, aerodynamic noise will be generated. When a dragonfly glides, the wing and airflow have a similar interaction relationship. Therefore, this noise generation mechanism can be analogous to the noise formation of a fixed-wing in an aircraft. Generally, the noise can be divided into six categories, and each source produces the scattering noise of the wing independently. The six types of noise sources are inflow turbulence, turbulent boundary layer trailing edge interaction, separating flow, laminarboundary layer vortex shedding, trailing edge bluntness vortex shedding and tip vortex formation [2]. Wind tunnel tests confirmed that the trailing edge noise dominated when the incident airflow was laminar and that the leading edge noise became the main source of noise and concealed the trailing edge noise when the incident airflow was severe turbulence [3]. In the process of flying, dragonflies will inevitably be affected by natural wind; that is, the incoming airflow is turbulent, but the turbulence intensity is not high. It can be concluded that the noise source is composed of leading-edge noise and its self-noise. The flying noise of dragonflies is very small. This paper infers that it is related to the microstructure of the leading edge of dragonfly wings, which can effectively reduce the leading-edge noise and the self-noise of the wings and then effectively reduce the overall noise of the wings. Noise reduction characteristics of the surface microstructure of flying biological wings have been studied extensively. The quiet flight characteristics of the long-eared owl are closely related to the special morphological structure of its wings. The circular-arc toothed structure of the leading edge of the wing has a function similar to an eddy current generator. It delays the separation of the airflow boundary layer on the wing surface, makes the airflow adhere to the upper surface of the entire leading-edge boundary layer of the wing, and makes the air eddy flow passing through the wing edge change from larger air eddy to smaller eddy flow, thus reducing the aerodynamic eddy noise generated by flight [4]. Generally, the incident turbulence amplifies the pressure fluctuation near the leading edge of the wing, and the pressure fluctuation scattering on the wing will become broadband noise [5]. Some studies have shown that the front serration structure can significantly reduce the pressure fluctuation in the front and middle regions of the serration structure, thereby reducing the broadband noise level [6]. The experimental results show that the noise-reduction effect of the serrated leading edge is not ideal in the low-frequency range, but it has an obvious noise-reduction effect in the middle frequency range (500 Hz to 8 kHz). Moreover, the height of the serrated leading edge is an important parameter affecting the noise-reduction effect of the flat wing [7].