# Structural Design and Parameter Optimization of Bionic Exhaust Tailpipe of Tractors
## 3 Results and Discussion  
### Aeroacoustics and Transmission Loss Analysis  
The sound pressure level frequency response curves of the prototype and bionic exhaust tailpipes at different inlet velocities are shown in Figure 7. The size parameters of the bionic exhaust tailpipe were selected from Table 1. The top angle \(\theta\) was 60\({}^{\circ}\), the texture height h was 1 mm, and the number of circumferential columns n was 24. The inlet airflow velocities of the tailpipe were 40, 50, 60, and 70 m/s, and the corresponding Reynolds numbers were 1.20 \(\times\) 10\({}^{5}\), 1.51 \(\times\) 10\({}^{5}\), 1.81 \(\times\) 10\({}^{5}\), and 2.21 \(\times\) 10\({}^{5}\). The results suggested that the sound pressure level gradually increased at the exit monitoring point with increasing air velocity. As shown in Table 2, the total sound pressure level of the prototype tailpipe increased from 111.256 dB to 119.461 dB, increasing by 8.205 dB. The total sound pressure level of the bionic tailpipe increased from 109.406 dB to 117.303 dB, increasing by 7.897 dB. The total sound pressure level of the bionic exhaust tailpipe decreased by 1.302 dB to 2.560 dB compared with that of the prototype tailpipe. The noise reduction effect was most obvious when the air velocity was 50 m/s and the sound pressure level was reduced by 2.560 dB. [PERSON] et al. studied the noise reduction of a wavy multi-copter rotor; the attenuation of total sound pressure level of the wavy rotor with respect to the baseline rotor was about 1.4-2 dB [45]. Aerodynamic and acoustic investigations of multi-copter rotors with trailing edge serrations have been performed. The results suggested that the serrated rotor had a total sound pressure level attenuation of 0.9-1.6 dB [46; 47]. The noise reduction effect through leading-edge serrations was studied on two-dimensional airfoils, and alleviated the total sound pressure level of 1.5 dB [48]. Therefore, the role played by the bionic exhaust tailpipe in this paper achieved a significant effect in aeroacoustics. At different inlet airflow velocities, the total sound pressure level of the bionic tailpipe was decreased compared with that of the prototype tailpipe. The results showed that the bionic tailpipe reduced the aerodynamic noise. The main reason was that the bionic texture could suppress the turbulent flow near the wall of the tailpipe and reduce aerodynamic drag and aerodynamic noise. The noise reduction performance of 70 m/s is worse than 60 m/s, mainly due to the increase of airflow velocity, which leads to the increase of airflow regeneration noise. The sound wave at high frequency propagates in the form of non-plane wave, which leads to more obvious airflow regeneration noise. Therefore, the performance of the bionic exhaust tailpipe is better at 60 m/s than 70 m/s, especially in the 2000-2500 Hz range. To research the influence of size parameters on the aerodynamic noise reduction performance of bionic tailpipes, different size parameters of bionic tailpipes were simulated, as shown in Figures 8 and 9. As shown in Figures 8a and 9a, the noise reduction effect gradually increased, and the total sound pressure level of bionic tailpipes gradually reduced as the circumferential column number n increased from 18 to 36. The total sound pressure level of bionic tailpipes was reduced by 1.901 dB-2.790 dB compared with the prototype tailpipe. There was more low-speed airflow near the wall of the bionic tailpipe with an increase in the number of circumferential columns n, which produced a better drag reduction effect and reduced the generation of aerodynamic noise. In addition, as shown in Figures 8b and 9b, the total sound pressure level of the bionic tailpipes decreased by 0.223 dB-1.911 dB compared with the prototype tailpipe with increasing texture height h. The total sound pressure level of bionic tailpipes first decreased and then increased.