## 4 The Study about the Distribution Characteristics of Flow-Induced Noise
### Noise Changing Characteristics Analysis under Various Operating Conditions  
The centrifugal pumps always need to operate under various conditions to meet corresponding working demands and the flow-induced noise changes accordingly. The analysis of the noise changing characteristics is always included in the other studies mentioned above. Figures 15-17 depict the experimental results of internal flow noise changing curves at inlet and outlet pipes with different rotational speeds [107], flow rates [20] and medium temperatures [108; 109], respectively. It is found that the noise at inlet and outlet pipes shows a similar changing trend and the noise at the outlet pipe is higher than that at the inlet pipe. With the growth of rotational speed, the noise at the inlet and outlet pipes increases gradually. Under different flow rates, the noise at inlet and outlet pipes reaches a minimum at a rated operating condition, that is, near the best efficiency condition and increases at  
Figure 14: TSPL directivity characteristics of external field obtained by experiment [98].  
Figure 13: TSPL directivity characteristics of external sound field obtained by simulation. (a) Arrangement of monitoring points; (b) TSPL directivity characteristics induced by impeller dipole source; (c) TSPL directivity characteristics induced by solute dipole source [96].  
off-design conditions. Besides, it is found from Figure 18 that the medium temperature could also affect the noise level and the noise at inlet and outlet pipes reaches a maximum at a certain temperature.  
The changing trend of external radiation noise and internal flow noise is similar under different rotational speeds, while that of different flow rates is different. Figure 18 depicts the experimental results of external radiation noise changing curves with different flow rates [99], where P1-P13 are the monitoring points shown in Figure 10a. It is observed that the external radiation noise follows an increasing trend with the growth of flow rate, which is different from the trend of internal flow noise by comparing Figures 16 and 18. However, the mechanism that leads to the difference of changing trend still needs to be explored further.  
Figure 16: The changing curves of internal flow noise at inlet and outlet pipes with different flow rates [20].  
Figure 15: The changing curves of internal flow noise at inlet and outlet pipes with different rotational speeds [107].  
In summary, based on the above analysis in Sections 2-4, it could be concluded that remarkable results have been gained from analyzing the noise generation and distribution characteristics from using different study methodologies. On this foundation, the flow-induced noise propagation mechanism still needs to be further explored, to form a systematic analysis model of the noise generation, distribution and propagation mechanism. The analysis of the noise propagation mechanism could not only extend the noise evaluation criterion but also lay a foundation for flow-induced noise reduction from the view of the noise propagation route.  
Besides, the generation of noise and vibration is associated with the unsteady flow characteristics. It is indispensable to take the flow field effect into account on the basis of the acoustic-vibration-coupling method, to further improve the numerical simulation method.  
Additionally, in the process of experimental measurement, the measurement object and mode are relatively monotonous currently and only the noise at representative monitoring positions is measured. Considering the limitation of the current experimental measurement, it is necessary to develop new  
Figure 17: The changing curves of internal flow noise at inlet and outlet pipes with different medium temperatures [108].  
Figure 18: The changing curves of external radiation noise with different flow rates [98].  
measurement technology to realize the real-time measurement of the whole sound field of centrifugal pumps, namely the visualization of the sound field.