## 9 The Experimental Validation
### The Description of the Experimental Measurement  
The gravity water tunnel was composed of a water tank, a contraction section, a rectifying section, a working section, a diffusion section, and the pipes. The models were installed in the working section. To reduce the vibration from the other sections, an iron sand box and a Helmholtz muffler were installed at both ends of the working section. Outside the working section, a reverberation tank made up of steel and plastic is shown in Figure 24. The bottom of the reverberation tank was vibration-damped to eliminate ground vibration. Figure 25 shows the picture of the original model and the model with mechanical VGs. The parameters were as follows: the mechanical VGs were triangular; the mechanical VGs were at an angle of 30\({}^{\circ}\) to the flow direction; the mechanical VGs were 0.1 \(H\) high and 0.1c long, where \(H\) was the height of the sail hull and c was the chord length; the mechanical VGs were setup at a distance of 0.1c from the leading edge of the sail hull. The radiated sound power from the two models was recorded by the reverberation method. Owing to the measured limitation frequency of the reverberation tank, we put a fluctuation pressure sensor on the top surface of the sail hull to measure the turbulent fluctuation pressure instead of the inside placement of a hydrophone, because the hydrophone's sensitivity was lower than the sensor's sensitivity. Figure 26 shows a vertical array of five hydrophones in the reverberation area to measure the spatial average sound pressure in the reverberation tank. The radiated sound power of the hydrodynamic noise was obtained according to Equation (29). The measured limitation frequency using the reverberation method in the reverberation tank was estimated as 500 Hz. Below this frequency, we could compare the sensitivities from the fluctuation pressure sensor and the hydrophone to estimate the sound pressure level from the data of the fluctuation pressure sensor. Therefore, we could achieve the total level of radiated sound power. The velocity of water flow was 4.62 m/s and 8.68 m/s. Figure 27 shows the turbulent fluctuation pressure collected by the fluctuation pressure sensor. Figure 24: The gravity water tunnel and the reverberation tank. (**a**) The whole water tunnel; (**b**) The reverberation water tank  
Figure 25: The photo of the two models. (**a**) The original model; (**b**) the model with triangular VGs. In Figure 27, we observed that the turbulent fluctuation pressure was suppressed in the low-frequency range by the triangular VGs. However, a high peak was observed at a frequency of 50 Hz. Through a detailed analysis, we found that the pressure level at this frequency remained unchanged in the two experimental measurements. First, in the measurement of the original model, and second, the measurement of the model with the mechanical VGs. Then, we could conclude that this peak was generated by the electricity power supply. Since the fluctuation pressure sensor must be amplified by a conditioner, the conditioner could only work under the alternate current (AC) supply. In our country, the frequency of the AC supply is 50 Hz. Therefore, the high peak of 50 Hz came from the AC supply, and not from the flow-induced noise. In the analysis, we neglected this peak. Compared with Figure 11, we observe that the trend of the pressure changing with the frequency is very similar. If the frequency was less than 1200 Hz, the mechanical VGs could suppress the hydrodynamic noise. Meanwhile, if the frequency was higher than 1200 Hz, the mechanical VGs could enhance the hydrodynamic noise. Influenced by the background noise, the peaks in Figure 27 are greater than the peaks in Figure 11.  
Figure 28 shows the radiated sound power collected in the reverberation tank.