# Structural Design and Parameter Optimization of Bionic Exhaust Tailpipe of Tractors
## 3 Results and Discussion
### Physical Mechanism of Noise Reduction in Bionic Exhaust Tailpipe
As shown in Figures 18a and 19a, the sound pressure level amplitude of the prototype exhaust tailpipe at the inlet was concentrated at 146 dB, and the sound pressure level amplitude at the outlet was concentrated at 144 dB. The sound pressure level amplitude of the whole tailpipe was mainly between 141 dB and 144 dB. The sound pressure level amplitude of the bionic exhaust tailpipe concentrates at 146 dB at the inlet and 143 dB at the outlet. The sound pressure level amplitude of the whole tailpipe was mainly between 139 dB and 143 dB. The bionic texture plays a role in reducing the sound pressure level amplitude inside the tailpipe. From the velocity amplitude nephogram in Figures 18b and 19b, the velocity amplitude at the inlet of the prototype tailpipe was mainly between 3.22 m/s and 3.57 m/s, and the outlet was mainly between 1.81 m/s and 2.51 m/s. The velocity amplitude of the entire tailpipe was mainly between 0.752 m/s and 1.46 m/s. However, the velocity amplitude of the entire pipeline of the bionic exhaust tailpipe was concentrated at 0.37 m/s. This shows that the bionic triangle texture reduced the velocity amplitude, reducing the sound pressure level. From the velocity vector diagrams in Figures 18c and 19c, the velocity at the inlet of the prototype tailpipe was 0.644 m/s, and the direction was the same as the exhaust direction. The velocity at the outlet was between 1.61 m/s and 1.93 m/s, and the direction was opposite to the exhaust. The minimum velocity was in the middle and lower parts of the prototype exhaust tailpipe, and the speed was 2.14 \(\times\) 10\({}^{-3}\) m/s. The velocity of the bionic tailpipe inlet was mainly between 0.426 m/s and 0.852 m/s. The direction was the same as the exhaust direction, and the speed at the outlet was between 2.13 m/s and 4.26 m/s, while the direction was opposite to the exhaust direction. The minimum velocity was in the middle and lower parts of the tailpipe, and the speed is 4.8 \(\times\) 10\({}^{-5}\) m/s. From the diagrams of the entire tailpipe speed vector, the internal velocity of the prototype tailpipe was between 2.14 \(\times\) 10\({}^{-3}\) m/s and 0.964 m/s, and the internal speed of the bionic tailpipe was between 4.8 \(\times\) 10\({}^{-5}\) m/s and 0.426 m/s. The maximum reverse velocity at the outlet of the prototype tailpipe was 3.21 m/s. However, the reverse velocity of the bionic tailpipe reached 4.26 m/s, which had a greater hedging effect on sound wave propagation along the pipeline, reduced the sound velocity in the pipeline, and reduced the internal sound pressure of the pipe. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Velocities (m/s)** & **Prototype** & **Bionic Tailpipes** & **Drag Reduction** \\ \hline
40 & 4.219 & 3.901 & 7.54\% \\
50 & 6.313 & 5.650 & 10.50\% \\
60 & 8.859 & 7.641 & 13.75\% \\
70 & 11.752 & 9.905 & 15.72\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The surface friction coefficient of the tailpipe wall at different inlet velocities. Figure 17: X-vorticity nephogram of vertical flow section in **(a)** prototype and **(b)** bionic tailpipe.