# Structural Design and Parameter Optimization of Bionic Exhaust Tailpipe of Tractors
## 3 Results and Discussion
### Experience Results and Analysis  
In this section, the results of the insertion loss tests are analyzed. The analysis focused on the noise reduction effect of the application of bionic textures in the inner surface of tailpipes. As shown in Figure 20(a\({}_{1}\)) and Figure 21(a\({}_{1}\)), the top angle of the bionic texture was 45\({}^{\circ}\), and the texture height was 0.75 mm. When the frequency is 100-2000 Hz, the maximum insertion loss could reach 4.351 dB at 1000 Hz. In Figure 20(b\({}_{1}\)) and Figure 21(b\({}_{1}\)), the top angle of the bionic texture was 45\({}^{\circ}\) and the texture height is 1 mm. Except for the negative value at 50 Hz, the average insertion loss at other frequency bands was 3.142 dB. In Figure 20(c\({}_{1}\)) and Figure 21(c\({}_{1}\)), the top angle was 45\({}^{\circ}\) and the texture height  
Figure 19: Optimized bionic exhaust tailpipe: (**a**) sound pressure level amplitude nephogram, (**b**) velocity amplitude nephogram, and (**c**) velocity vector diagram.  
Figure 18: Vorticity amplitude nephogram (**a**) prototype (**b**) bionic tailpipe. Prototype: (**a**) sound pressure level amplitude nephogram, (**b**) velocity amplitude nephogram, and (**c**) velocity vector diagram.  
was 1.25 mm. The insertion loss was positive throughout the test frequency range, with an average insertion loss of 1.338 dB. In Figure 20(a2) and Figure 21(a2), the top angle is 60\({}^{\circ}\), and the texture height is 0.75 mm. The maximum insertion loss was 2.372 dB at 630 Hz. In Figure 20(b2) and Figure 21(b2), the top angle was 60\({}^{\circ}\) and the texture height is 1 mm. The insertion loss was greater than 1 dB at most frequencies from 63 Hz to 1600 Hz. The insertion loss was 4.374 dB at 1000 Hz. In Figure 20(c2) and Figure 21(c2), the top angle is 60\({}^{\circ}\) and the texture height was 1.25 mm. Between 63 Hz to 2000 Hz, the maximum insertion loss was 3.887 dB at 2000 Hz. In Figure 20(a3) and Figure 21(a3), the top angle was 75\({}^{\circ}\), the texture height was 0.75 mm, and the insertion loss was about 2 dB at most frequencies. The maximum insertion loss at 2000 Hz was 5.182 dB. In Figure 20(b3) and Figure 21(b3), the top angle is 75\({}^{\circ}\) and the texture height was 1 mm. Between 100 Hz and 630 Hz, insertion loss was about 2 dB at some of these frequencies and 3.274 dB at 630 Hz. In Figure 20(c3) and Figure 21(c3), the top angle was 75\({}^{\circ}\), the texture height was 1.25 mm, and the average insertion loss of other frequency bands was 1.918 dB except for the negative value of 50 Hz.  
Figures 20 and 21 show the curves of the sound pressure level and insertion loss measured in the experiment. The sound pressure level of the bionic exhaust tailpipes decreased to different degrees in most frequency bands compared with the prototype exhaust tailpipe. On the whole, the noise reduction effect was better when the top angle was approximately 60\({}^{\circ}\), which was consistent with the simulation results. In the range of 45\({}^{\circ}\) and 60\({}^{\circ}\), the insertion loss tends to increase first and then decrease with increasing texture height at the same top angle, and the effect of noise reduction was better when the texture height was 1 mm, which was also consistent with the simulation results. When the top angle reached 75\({}^{\circ}\), the effect of different texture heights on the insertion loss was less than that of other angles, and the change trend was not obvi  
Figure 20: Sound pressure level curves at different size parameters: (**a1–a3**) texture height h 0.75 mm and top angle \(\theta\) 45\({}^{\circ}\)–75\({}^{\circ}\), (**b1–b3**) texture height h 1 mm and top angle \(\theta\) 45\({}^{\circ}\)–75\({}^{\circ}\), (**c1–c3**) texture height h 1.25 mm and top angle \(\theta\) 45\({}^{\circ}\)–75\({}^{\circ}\).  
different from the simulation results. The disparity was mainly due to the limitation of experimental conditions, and the influence of airflow velocity was not considered in the experiment.