# Research on Characteristics of Flow Noise and Flow-Induced Noise
## 4 Noise Calculation of Cylindrical Shell Model
### Comparison of Flow Noise and Flow-Induced Noise  
The spatial distribution of the flow-induced noise can be roughly understood through the above acoustic directivity. Next, we analyzed and compared the flow-induced noise at different detection points along the X, Y, and Z axes with the flow noise.  
We arranged 12 detection points along the X, Y, and Z axes, with a gradual transition from dense to sparse spacing.  
Firstly, we selected detection points at both the near-field and far-field along the three axes. We conducted a comparative analysis of the spectral characteristics of flow noise and flow-induced noise, as well as the differences between the near-field and the far-field.  
As shown in Figure 11, the variations in flow noise and flow-induced noise were consistent in different directions. Along the x axis, when the frequency was below 200 Hz, the flow noise and flow-induced noise were nearly equal. For frequencies above 200 Hz, both near and far-field conditions exhibited higher flow-induced noise compared with flow noise. Moreover, as the distance increased, both values gradually decreased. Beyond 400 Hz, both values tended to stabilize. It is noteworthy that at a frequency of 240 Hz, the flow-induced noise reached its maximum value, approximately 130 dB, while the corresponding flow noise levels were around 60 dB, resulting in a difference of approximately 70 dB. The noise variation trend along the y axis was similar to that along the x axis. Along the z axis, the overall noise levels were slightly lower than those along the x and y axes, but the first peak of flow-induced noise was larger. Because of the noise generated by the vibration of the shell structure, peaks in the flow-induced noise were observed at frequencies of 240 Hz, 520 Hz, and 620 Hz. These frequencies were associated with the inherent frequencies of the wet modes, indicating that resonance had a significant impact on flow-induced noise.  
As shown in Figure 12, at the 12 detection points, the flow noise was consistently lower than the flow-induced noise. Along the x axis, with the increase in distance from the shell surface, the value of flow noise decreased from 117 dB to 65 dB, while the flow-induced noise decreased from 153 dB to 107 dB. The difference between the two remained around 40 dB. When the distance from the shell surface was less than 0.25 m, the reduction in flow noise was more significant than that in the flow-induced noise, and stabilized as the distance increased. For each of the detection points along the y axis, the value of flow-induced noise decreased from 150 dB to 102 dB, while the flow noise decreased from 125 dB to 57 dB, as the distance from the shell surface increased. The trend along the z axis was consistent with that of the x and y axes, with a difference of about 48 dB between the flow noise and flow-induced noise. As the distance between the detection points and the shell surface increased, flow noise decreased from 103 dB to 37 dB, while the flow-induced noise decreased from 138 dB to 80 dB, with a greater variation in flow noise. It is noteworthy that along the z axis, at the same distance, both flow noise and flow-induced noise were lower compared with the X and Y directions. This is attributed to significant variations in the flow field around the bow and stern of the shell, which reduced noise propagation.  
Figure 10: Flow-induced noise acoustic directivity diagram: (**a**) vertical section; (**b**) horizontal section.  
Figure 11: Noise comparison diagram: (**a**) detection point X1; (**b**) detection point X11; (**c**) detection point Y1; (**d**) detection point Y11; (**e**) detection point Z1; (**f**) detection point Z11.