## 6 Experimental Validation of the Model
### Results of the Experimental Measurement  
The change of the numbers of the opening valves in the gravity water tunnel at the outlets can achieve the velocities of the flow. The level of sound radiation power of the hydrodynamic noise from the model is shown in Figure 19. Some conclusions can be drawn:  
(1) The high peaks of sound radiation power are mainly concentrated in the low-frequency range, which shows that the flow-induced noise induced by the horseshoe vortex is confirmed in the experiment.  
(2) The level of sound radiation power in the low-frequency range is greater than that in the high-frequency range, and with the increase of the frequency, the change of sound radiation power is relatively mild, which shows that the flow-induced noise of the model excited by the boundary layer separation is measured in the experiment.  
(3) At the frequency of 550 Hz, the peak shows that the model is excited by the tail vortex shedding measured in the experiment.  
Therefore, the results in the experimental measurement validate the analysis in the numerical calculation.  
Figure 18: Diagram of the measurement. (**a**) Chart of the experimental test. The model is rigidly placed at the bottom of the working section, and the hydrodynamic noise from the model is excited by the water flow due to the gravity. A vertical array of several hydrophones is placed in the water tank outside the working section, which collects the hydrodynamic noise from the model. (**b**) Practical photo of the model.  
However, some differences between the measurement and the calculation can be observed. The total level of sound radiation power from the model in the experiment is 119.47 dB, while that in the calculation is 113.52 dB. Some reasons may be ascribed to the difference made.  
(1) Because the level of the flow noise is much lower than that of the flow-induced noise, the effect of the flow noise on the measurement can be ignored. However, the system of the gravity water tunnel is very similar to a long sound guide. Two tanks are at both ends of the gravity water tunnel, the formation of the design can make the flow-induced noise reflect many times, which enhances the level of the flow-induced noise. Even if we have calibrated the gravity water tunnel by a standard sound source, the effect of the reflection of the flow-induced noise cannot be accurately quantified since the process of the generation of the flow-induced noise is too complicated.  
(2) In the numerical calculation, the wall surface of the flow field is set up as the boundary of the solid wall. However, the wall surface cannot meet the solid wall boundary in the experiment, especially in the water.  
(3) The model in the experiment is made up of copper, and weld at the boundaries, which is different from the model in the simulation, and the latter is established completely and has no weld points.  
(4) The background noise may interfere with the level of the hydrodynamic noise. Since the cavitation usually occurs, the velocity of the flow cannot be made too high, and the signal-to-noise ratio is relatively low.  
Besides, the hydrodynamic noise is a random process. All these factors make the experimental results unable to match accurately with that of the numerical calculation.