## 3 Results And Discussion
### Calculation Results of Flow Noise
#### 3.2.1 Calculation Results for Pulsating Pressure Excition  
In the case where the pulsating pressure on wall is used as the excitation, the vehicle wall pressure pulsation is extracted from the flow field data and used as a boundary condition for further calculation. The extracted wall pressure pulsation at some frequencies is shown in Figure 5, and the unit was set to dB. Figure 5 shows that the pulsating pressure distribution on the wall of underwater vehicle is inextricably related to the distribution of vortex region of flow field, and there is a high pulsating pressure distribution in the vortex-dense area of both the leading and trailing edges of underwater vehicle, more obvious at the trailing edge. It is easy to observe that the pulsating pressure on the vehicle wall is higher in the low-frequency part and lower in the high-frequency part. For the same frequency, the underwater vehicle has a large pulsating pressure area in the wake region, which is caused by the wake vortex area. Below 500 Hz, as the frequency increases, the large pulsating pressure region in the tail gradually decreases, and by around 500 Hz, it decreases to a very small portion and does not change much until higher frequencies are applied. Large vortices have a greater impact on the low-frequency noise situation, while small vortices affect the high-frequency noise results. A comparison of the vortex structure diagram of transient flow field can lead to the conclusion that large vortices play a major role in generating the pulsating pressure on the vehicle wall. Figure 5: Pulsating pressure on the vehicle wall. Figure 6a shows the calculation results of sound pressure at monitoring points 1, 2, and 3. It can be observed that the spectral characteristics of sound pressure levels at these monitoring points are very similar, although monitoring point 2 is located above the axis, while monitoring points 1 and 3 are symmetrically distributed on both sides of the axis. This indicates that the acoustic radiation distribution at the front of the underwater vehicle is approximately the same in all the directions. This can be explained by the absence of substantial vortex and turbulence properties in the front, which would not produce a large source of stream noise. In addition, the amplitude and trend of the frequency domain distribution curves of sound pressure at symmetric monitoring points (e.g., receivers 6 and 7 and receivers 8-11) are almost the same, although there is a certain directionality at the corresponding frequency values. Several representative monitoring points were selected to compare the changes in sound pressure levels along the flow direction, as shown in Figure 6. In the case where the pulsating pressure is the excitation, the sound pressure level substantially varies along the flow direction, with the highest value of about 135 dB in the low-frequency part and also around 90 dB in the high-frequency part at several monitoring points (receivers 1-5) at the front of underwater vehicle. By the middle of the underwater vehicle (receivers 6 and 7), the sound pressure level improved considerably, probably by about 6 dB, and in the whole tail section of underwater vehicle and the subsequent part of the area (receivers 8-14), the sound pressure level increased integrally by about 8 dB. The highest value of low-frequency part in this region is about 150 dB, whereas the high-frequency part is also about 110 dB. The sound pressure distribution diagram of the middle section of underwater vehicle can show the sound field radiation more clearly, as shown in Figure 7. The sound pressure level shows a general trend of gradually decreasing with increasing frequency, but a small increase in sound pressure level might have occurred at some frequencies. The highest values of sound pressure levels appear around the blades in the wake of torpedo, which is due to the complex shape of blades and the tendency to obtain stronger disturbances in the flow.