# 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
Figure 17: _Cont._  
Using Equation (10), the rate of decrease at each measuring point in the experiment can be calculated, and the average pressure fluctuation at 5 m/s can be obtained, as shown in Table 8. The mean rate of decrease in the pressure fluctuation for all monitoring points at a flow velocity of 5 m/s was determined to be 10.30 dB/Oct. Figure 18 provides a comparison of the pressure fluctuation spectra between the simulation results and the experimental data at each monitoring point for a flow velocity of 8 m/s. In Figure 18a-d, the monitoring points for pressure fluctuation are located on the fin. In the experiment, monitoring point #5 had the highest pressure fluctuation value at 135.01 dB. In the simulation, monitoring point #1 exhibited the highest value at 143.90 dB. Regarding Figure 18e-l, the monitoring points for pressure fluctuations are positioned on the rudder. In the experiment, monitoring point #11 showed the highest pressure  
Figure 17: Comparison of simulated and experimental pressure fluctuations in the 100–5000 Hz range at 5 m/s. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring Point** & **Decrease Rate (dB/Oct)** & **Monitoring Point** & **Decrease Rate (dB/Oct)** & **Mean Value (dB/Oct)** \\ \hline \#1 & 8.15 & \#8 & 8.50 & \\ \hline \#3 & 7.00 & \#9 & 12.05 & \\ \hline \#4 & 6.73 & \#10 & 12.23 & \\ \hline \#5 & 7.80 & \#11 & 14.17 & \\ \hline \#6 & 14.53 & \#13 & 8.33 & \\ \hline \#7 & 12.40 & \#15 & 11.69 & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Rate of decrease in pressure fluctuation at different monitoring points in the experiment at a velocity of 5 m/s. fluctuation value at 162.09 dB, while in the simulation, monitoring point #6 showed the highest value at 174.18 dB. As can be seen from the data in Figures 16-18, the trends regarding variation between the numerical results and the experimental data are similar; that is, the pressure fluctuation decreases with an increase in frequency. The difference is that the experimental values of most measurement points are smaller than the numerical results, and the decreasing trend in the experimental values with an increase in frequency is greater than that in the simulation values. The observed differences may be due to experimental error and discrepancies between the actual testing conditions and the assumptions inherent in the simulation model. For example, due to the size of the sensor, the pressure sensor measures the pressure change of a plane instead of a point; further, the inlet of the flow in the simulation is set to be uniform, which is inconsistent with the experimental environment. The pressure fluctuations at different flow velocities show that the simulation results can reflect the amplitude of the pressure and the change trend with respect to frequency. In addition, it can be seen that the pressure fluctuations at monitoring points #1 to #5 are smaller compared to those at monitoring points #6 to #15 at the same flow velocity. As monitoring points #1 to #5 are located on the fin, while monitoring points #6 to #15 are located on the rudder, it can be inferred that the intensity of the pressure fluctuations on the fin is lower than that on the rudder. The OSPL values of pressure fluctuations at the same monitoring points on both sides of the fin and rudder are approximately consistent. Using Equation (10), the rate of decrease at each measuring point in the experiment can be calculated, and the average pressure fluctuation at 8 m/s can be obtained, as shown in Table 9. The mean rate of decrease in the pressure fluctuation for all monitoring points at a flow velocity of 8 m/s was determined to be 8.84 dB/Oct. By taking the average of the decrease rate obtained at three different velocities, the overall decrease rate of pressure fluctuations in this experiment was determined to be 9.7 dB/Oct. Figure 18: Comparison of simulated and experimental pressure fluctuations in the 100–5000 Hz range at 8 m/s.