# Structural Design and Parameter Optimization of Bionic Exhaust Tailpipe of Tractors
## 3 Results and Discussion
### Aeroacoustics and Transmission Loss Analysis
This indicates that the form of secondary vortices near the texture can suppress the generation of turbulence, which was the main reason for reducing aerodynamic noise. As the height reached a certain level, the airflow drag inside the tailpipe increased correspondingly with increasing texture height, which weakened the noise reduction effect. As shown in Figures 8c and 9c, the influence of the top angle \(\theta\) had little impact on the sound pressure level of bionic tailpipes, which indicated that the top angle \(\theta\) was not the main factor in reducing aerodynamic noise. The vibration noise reduction performance of the bionic exhaust tailpipes was studied by calculating the transmission loss. The transmission loss curves of the bionic exhaust tailpipes with different top angles, heights, and numbers of circumferential columns were analyzed to study the regular changes in the size parameters of the bionic triangle texture and the transmission loss. The transmission loss of the bionic exhaust tailpipe was compared with that of the prototype exhaust tailpipe. A comparative analysis of transmission loss curves between the bionic exhaust tailpipes with different triangular texture parameters and the prototype exhaust tailpipe is shown in Figure 10. As seen from the transmission loss curve of the prototype exhaust tailpipe, between 10 Hz and 50 Hz, the transmission loss is relatively high: between 5.288 dB and 19.745 dB. From 70 Hz to 110 Hz, the transmission loss is between 0.752 dB and 2.897 dB. The noise reduction effect decreases significantly when the frequency is between 130 Hz and 410 Hz, and the transmission loss is between 0.035 dB and 0.418 dB. Above 430 Hz, the transmission loss becomes negative, and the noise reduction effect is the worst. As a whole, the transmission loss of the prototype exhaust tailpipe decreases with increasing frequency. Since diesel engine exhaust noise is mainly low frequency and medium frequency [49], the target frequency of this paper is between 10 Hz and 490 Hz. The transmission loss of the prototype exhaust tailpipe within the target frequency range is relatively low, and the noise reduction effect is not very good and needs to be improved. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Velocity (m/s)**} & \multicolumn{3}{c}{**Total Sound Pressure Level (dB)**} \\ \cline{2-4}  & **Prototype** & **Bionic Tailpipes** & **Difference** \\ \hline
40 & 111.256 & 109.406 & 1.850 \\
50 & 114.233 & 111.673 & 2.560 \\
60 & 116.724 & 115.422 & 1.302 \\
70 & 119.461 & 117.303 & 2.158 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Total sound pressure level between 10 Hz and 2500 Hz. Figure 7: Sound pressure level frequency response curve of prototype and bionic exhaust tailpipe at different airflow velocities: (**a**) at 40 m/s, (**b**) at 50 m/s, (**c**) at 60 m/s, (**d**) at 70 m/s. Figure 8: Effect of dimension parameters on sound pressure level of bionic tailpipes: (**a**) circumferential column number n, (**b**) texture height h, (**c**) top angle \(\theta\). Figure 9: Effect of dimension parameters on total sound pressure level of bionic tailpipes: (**a**) circumferential column number n, (**b**) texture height h, (**c**) top angle \(\theta\). mission loss is increased by 0.172 dB for a texture height of 0.75 mm. Figure 10(a\({}_{3}\)) shows that \(\rm TL_{1 mm}>TL_{0.75 mm}>TL_{1.25 mm}>TL_{1.5 mm}>TL_{0.5 mm}\) when the top angle \(\theta\) is 60\({}^{\circ}\). The maximum transmission loss is increased by 6.12 dB, and the minimum transmission loss is increased by 0.348 dB for a texture height of 1 mm. Figure 10(a\({}_{4}\)) shows that \(\rm TL_{0.75 mm}>TL_{1 mm}>TL_{1.5 mm}>TL_{0.5 mm}>TL_{1.25 mm}\) when the top angle \(\theta\) is 75\({}^{\circ}\), the maximum transmission loss is increased by 2.951 dB, and the minimum transmission loss is decreased by 1.5 dB for the texture height of 0.75 mm.