# The Noise-Reduction Characteristics of Microstructure of Dragonfly Wing Leading Vein
## 2 Bionic Modeling of Dragonfly Wing Microstructure
### Effect of Serrated Microstructure Height on Noise Reduction  
The parameters of the profile of wing 3 leading-edge microstructure were selected as reference objects due to owning moderate noise-reduction value. By changing the height of leading-edge microstructure serration on the bionic wing 3 and calculating the overall sound pressure level generated by the bionic wing, the influence of the height of microstructure serration on the noise-reduction effect was obtained. The height of the leading edge serration microstructure was enlarged from 0.5 times to 4 times based on wing 3, numbered as wing a to wing f. The \(\Delta\)OASPL calculation results are shown in Table 5. It could be drawn that when the magnification of serration microstructure height was between 1.5 and 2, the noise-reduction effect was obvious, and when the magnification increased, the noise-reduction effect decreased rapidly, and even noise improved. According to the above analysis results, the height of the leading-edge microstructure serration would be divided more carefully. The noise-reduction effect of the serration microstructure with a magnification of 1.6, 1.7, 1.8 and 1.9 was analyzed, numbered \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Wing Type** & **Wing 1** & **Wing 2** & **Wing 3** & **Wing 4** & **Wing 5** & **Wing 6** \\ \hline \(\Delta\)**OASPL (dB)** & 7.2 & 7.0 & 6.8 & 12.5 & 0.9 & 3.7 \\ \hline \end{tabular}
\end{table} 
Table 4: Overall sound-pressure-level reduction for different bionic wings. \begin{table}
\begin{tabular}{c c c c} \hline
**Wing Type** & **OASPL (dB)** & **Wing Type** & **OASPL (dB)** \\ \hline Basic wing & 44.7 & Wing 6 & 41.0 \\ Wing 1 & 37.5 & Wing 7 & 52.6 \\ Wing 2 & 37.7 & Wing 8 & 51.9 \\ Wing 3 & 37.9 & Wing 9 & 51.6 \\ Wing 4 & 32.2 & Wing 10 & 51.5 \\ Wing 5 & 43.8 & & \\ \hline \end{tabular}
\end{table} 
Table 3: Overall sound pressure level of different bionic wings. as wing g, wing h, wing i and wing j, respectively. The overall sound-pressure-level reduction of each bionic wing is shown in Table 6, and the OASPL of the bionic wing was plotted as a curve, as shown in Figure 4. It could be seen that under the given conditions in this paper, the change of serration height of the leading-edge microstructure greatly affected the OASPL value and noise-reduction effect of the bionic wing. The leading-edge microstructure, whose serration height ranged from 128.1 to 384.3 \(\upmu\)m, had a certain noise-reduction effect, and the optimum height was 230.58 \(\upmu\)m with 11.5 dB \(\Delta\)OASPL value. In order to further explore the mechanism of noise reduction by the bionic leading-edge microstructure, the wings with different noise effects were selected for comparative analysis. That is to say, the basic wing, wing a, wing i, wing c and wing e were selected as the research objects, and the 1/3 octave sound pressure level analysis was carried out. The results are shown in Figure 5. At the same time, three bionic wings (wing c, wing e, wing i) and basic wings were selected to analyze the pressure fluctuation on the upper surface of the bionic wing. The pressure nephogram is shown in Figure 6.  
As seen in Figure 5, compared to the basic airfoil, the sound pressure level of the bionic wing a and wing i was always lower than that of the basic airfoil in the full frequency range, which shows an obvious noise-reduction effect. These bionic wings had different acoustic characteristics in the different frequency range, which was the reason that the \(1/3\) octave sound pressure level curves of different bionic wings in Figure 5 have intersects. By comparing the SPL curves of the basic wing and bionic wing c, it was found that the sound pressure level of the bionic wing c was lower than that of the basic wing in the mid-frequency range (200 Hz to 6 kHz). It could be found that sound pressure level value was closely related to the acoustics frequency range and the bionic serrated microstructure on the leading edge in this paper had a certain noise-reduction effect in the mid-frequency range.