# The Noise-Reduction Characteristics of Microstructure of Dragonfly Wing Leading Vein
## 5 Conclusions  
During the development of flapping-wing aircraft, our team found that the surface microstructure of dragonfly wings is related to their low-noise flight capability. In this paper, the geometric characteristics of dragonfly wings' leading edge are extracted, optimized and modeled. Then, according to the practical application requirements and the principle of scale law, the bionic leading-edge microstructural model is enlarged. After simulation analysis and experimental verification, the following conclusions are obtained:  
(1) The noise-reduction effect of the leading-edge microstructure of the wing is remarkable with the increase of the overall size. When amplified 4 times, it has the best noise-reduction effect, with an overall sound-pressure-level reduction of 12.5 dB. After this, the noise-reduction effect decreases rapidly and even increases the noise level. (2) The height of the serration microstructure affects the noise-reduction effect and level of the bionic wing. The noise-reduction effect is remarkable with the increase of serrated height. When the height is amplified 1.8 times, the noise-reduction effect is the best. At this time, the serration height of the microstructure is 230.58 \(\upmu\)m, with an overall sound-pressure-level reduction of 11.5 decibels; when we continue to increase the altitude, the effect of noise reduction rapidly attenuates. (3) The change of serration microstructural width can obviously change the pressure fluctuation distribution at the leading edge and extend the strong pressure fluctuation to the upper surface of the wing, but it has no significant effect on the overall sound pressure level of serration microstructural, that is, the width of serration microstructure has no significant effect on noise reduction. (4) Observing the 1/3 octave sound pressure level curve of the bionic wing, it can be concluded that the sound pressure level in the leading edge region has a decisive influence on the overall sound pressure level of noise, and the bionic serration microstructure has an obvious noise-reduction effect in the leading-edge region. In conclusion, the microstructure of dragonfly wings' leading edge has an effect of reducing aerodynamic noise. Moreover, the microstructure can significantly change the eddy current state, the vortex size and its distribution on the wing surface, and then affect the pressure fluctuation on the leading edge, trailing edge and the upper surface of wings, thus affecting the overall sound pressure level of the noise generated by the wing, thereby reducing the noise of the bionic wing. In the follow-up work, the degree of simulation and manufacturing accuracy of microstructure will be improved, and the noise-reduction effect of dragonfly wing leading-edge microstructure will be further studied to promote practical application. Figure 11: Overall sound pressure level of the basic fan blade and bionic fan blade at different rotational speeds. **Author Contributions:** Conceptualization S.J. and Y.H. ; conducting a research and investigation process Y.H. ; draft preparing Q.L. ([PERSON]); design of methodology and revision support H.W. ; software development Y.L. ; management and coordination responsibility for the research activity planning and execution X.Z. ; providing the experiment ideas and funding acquisition Q.L. ([PERSON]). All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by National Nature Science Foundation of China (NSFC), grant number U1601203, U19A20104; Jilin Province Science and Technology Development Program, grant number 20180101321 JC, 20190302099 GX; Jilin Province industrial Technology of Research and Development, grant number 2019C037-3; and Science and Technology project of Jilin Provincial Department of Education, grant number JJKH20200955 KJ. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.