# Structural Design and Parameter Optimization of Bionic Exhaust Tailpipe of Tractors
## 3 Results and Discussion
### Physical Mechanism of Noise Reduction in Bionic Exhaust Tailpipe  
To study the noise reduction mechanism of the bionic tailpipe better, the pressure nephogram, velocity amplitude nephogram, surface friction coefficient nephogram, and vorticity distribution nephogram were analyzed, as shown in Figures 13-17. As seen from Table 2, when the inlet velocity was 50 m/s, the noise reduction effect was better than those of other speed conditions. Therefore, the flow field under this inlet velocity condition was compared with the prototype tailpipe to study the noise reduction mechanism. Figure 13 shows a pressure nephogram of the prototype and bionic tailpipe. The wall pressure of the bionic tailpipe was less than that of the prototype tailpipe. The results show that the bionic texture reduced the wall air pressure, increased the stability of airflow near the wall, and weakened the generation of turbulence. This had a positive effect on the flow stability near the whole pipe wall, which reduces the aerodynamic noise of the exhaust tailpipe. As seen from Figure 14, the comparison nephogram of the vertical flow section velocity amplitude between the prototype and bionic tailpipe and the cross-section was at 300 mm in the \(z\)-axis direction. The velocity amplitude near the texture of the bionic tailpipe was smaller than that at the same position as the prototype tailpipe. The results showed that the bionic texture can reduce the velocity amplitude near the wall and weaken the turbulence near the wall. As shown in Figure 15, the friction coefficient of the surface near the inlet of the pipeline was significantly reduced through numerical simulation. Therefore, the bionic texture reduces the resistance between the airflow and the wall of the bionic tailpipe. As shown in Table 4, the drag reduction effect under different conditions was analyzed, and the drag reduction rate of the smooth surface model and nonsmooth surface model was obtained under different conditions. The drag reduction rate improved with increasing inlet velocity. A comparison of the vorticity distribution nephogram between the prototype and bionic tailpipe is shown in Figure 16. The research suggests that the vorticity amplitude in most areas of the bionic tailpipe wall is reduced compared with the prototype tailpipe. The bionic texture can make the airflow near the tailpipe wall more stable, suppress turbulence bursts, and reduce aerodynamic noise. An X-vorticity nephogram of the vertical flow section is shown in Figure 17 (z = 300 mm). There are obvious secondary vortices at the tip of the bionic texture. The secondary vortex is the fundamental reason for drag reduction. The rotation direction of the secondary vortex is opposite to that of the large vortex, which weakens the large eddy and weakens the turbulent flow. The two opposite vortices that appear near the bionic texture undermine each other, which makes the airflow around the  
Figure 13: Comparison of pressure nephogram: (**a**) prototype and (**b**) bionic tailpipe. wall of the bionic tailpipe more stable and reduces the generation of aerodynamic noise. At the same time, the interaction between the secondary vortex pair and the reverse vortex pair weakens the reverse vortex pair, suppresses the turbulence burst, reduces the energy dissipation, and produces drag reduction and noise reduction. Figure 16: Vorticity amplitude nephogram (**a**) prototype and (**b**) bionic tailpipe. Figure 14: Vertical flow direction section velocity amplitude nephogram: (**a**) prototype and (**b**) bionic tailpipe. Figure 15: Surface friction coefficient nephogram: (**a**) prototype and (**b**) bionic tailpipe. Figures 18 and 19 show the sound pressure level amplitude nephogram (a), velocity amplitude nephogram (b), and velocity vector diagram (c) of the prototype and optimized bionic exhaust tailpipe of the tractor at 490 Hz, respectively, to better study the vibration noise reduction mechanism of the bionic tailpipe.