# Pressure Fluctuation and Flow-Induced Noise of the Fin and Rudder in a Water Tunnel
## 4 Flow Field and Pressure Fluctuation
### Comparison of Pressure Fluctuation Results  
In this section, an analysis of the deviation between the experimental data and simulated results regarding pressure fluctuation at the same location is conducted. In the simulation, pressure measuring points were established, and their positions are consistent with those of the pressure sensors mentioned in Section 2.2. The accuracy of the sensors was verified, and the circuit connections were checked prior to the experiments. However, during the experiments, a few pressure sensors located at monitoring points #2, #12, and #14 experienced circuit connectivity issues, resulting in an inability to obtain pressure readings at these measurement points. Most of the pressure sensors worked normally. A comparison of the pressure fluctuation spectra between the simulation results and experimental data at each monitoring point for a flow velocity of 3 m/s is shown in Figure 16. The top-right corner of the graph represents the magnitude of the OSPL for pressure fluctuations. The pressure fluctuation monitoring points are located on the fin in Figure 16a-d. The OSPL values of these monitoring points obtained through the simulation are relatively close. In the experiment, monitoring point #3 exhibited the highest value at 114.88 dB. In contrast, in the simulation, monitoring point #1 showed the highest value at 123.20 dB. In Figure 16e-l, the monitoring points for pressure fluctuations are positioned on the rudder. Among these, monitoring points #7, #9, and #11 show similar OSPL values for pressure fluctuations between the experiment and the simulation. The highest OSPL values among these points are observed at monitoring point #9, reaching 130.53 dB in the experiment and 140.80 dB in the simulation. Using Equation (10), the rate of decrease in the pressure fluctuation spectrum at each measuring point in the experiment can be calculated, and the average pressure fluctuation at 3 m/s can be obtained, as shown in Table 7. The mean rate of decrease in the pressure fluctuation for all monitoring points at a flow velocity of 3 m/s was determined to be 10.08 dB/Oct. Figure 15: Vorticity distribution for different flow velocities (Q = 12 s\({}^{-1}\)): (a) 3 m/s; (b) 5 m/s; (c) 8 m/s. Figure 16: Cont. A comparison of the pressure fluctuation spectra between the simulation results and experimental data at each monitoring point for a flow velocity of 5 m/s is shown in Figure 17. In Figure 17a-d, the monitoring points for pressure fluctuations are located on the fin. Among these, monitoring points #3 and #5 show a close resemblance in terms of the OSPL of the pressure fluctuation between the experiment and simulation. In the experiment, monitoring point #5 showed the highest pressure fluctuation value at 128.08 dB. In the simulation, monitoring point #1 exhibited the highest value at 134.71 dB. In Figure 17e-l, the monitoring points for pressure fluctuation are positioned on the rudder. Among these, monitoring points #7 and #9 show similar pressure fluctuation levels between the experiment and simulation. In the experiment, monitoring point #11 showed the highest pressure fluctuation value at 148.96 dB, while in the simulation, monitoring point #6 showed the highest value at 163.41 dB. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring** & **Decease Rate** & **Monitoring** & **Decease Rate** & **Mean Value** \\
**Point** & **(dB/Oct)** & **Point** & **(dB/Oct)** & **(dB/Oct)** \\ \hline \hline
#1 & 9.21 & \#8 & 8.50 & \\ \hline
#3 & 7.97 & \#9 & 10.37 & \\ \hline
#4 & 7.88 & \#10 & 11.16 & \\ \hline
#5 & 9.04 & \#11 & 12.40 & \\ \hline
#6 & 12.31 & \#13 & 8.68 & \\ \hline
#7 & 11.52 & \#15 & 11.96 & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Rate of decrease in pressure fluctuation spectrum at different monitoring points in the experiment at a velocity of 3 m/s. Figure 16: Comparison of simulated and experimental pressure fluctuations in the 100–5000 Hz range at 3 m/s.