## 7 Conclusions  
This paper reviews more than 130 articles to summarize the current study status on the flow-induced noise of centrifugal pumps. The review contents include the analysis of noise generation mechanisms and propagation routes, the study methodologies comparison, the distribution characteristics analysis of flow-induced noise and the analysis of noise optimization design. Then the outlook for future study is presented based on the literature review. The main conclusions are shown as follows:  
1. The flow-induced noise of centrifugal pumps is related to various complex flow phenomena. During the normal operating condition, the noise induced by rotor-stator interaction is the main component of overall noise. The rotor-stator interaction could cause the frequency response characteristics that are dominated by characteristic frequencies in a low frequency range. The cavitation could inhibit the rotor-stator interaction to some extent. With the development of cavitation, the frequency response characteristics of flow-induced noise that are dominated by characteristic frequencies weaken and the noise level in a high frequency range increases gradually. In addition, when the flow-induced noise generates inside centrifugal pumps, the noise could not only propagate to the inlet and outlet pipes but also propagate to the external environment, namely the internal flow noise and external radiation noise, respectively. 2. Theoretical study, numerical simulation and experimental measurement are the main methodologies to analyze the flow-induced noise of centrifugal pumps. Theoretical study analyzes the subject through the governing equation level. Because of the obscurity of the theoretical study, the theoretical conclusions are always combined with numerical simulation. Numerical simulation is currently the most popular and versatile method to analyze the flow-induced noise by combining the Lighthill acoustic analogy theory with CFD and taking the acoustic-vibration-coupling effect into consideration, but the numerical simulation results need to be verified by experiment. For experimental measurement, the flow-induced noise could be directly measured but the measurement object and mode are relatively monotonous and the measurement results are affected by mechanical vibration. 3. The analysis of the distribution characteristics of flow-induced noise mainly includes the noise source identification and comparison, the frequency response analysis, the directivity characteristics of sound field and the noise changing characteristics under various operating conditions. The main noise source region is located near the volute tongue and the volute dipole source has more contribution to internal flow noise compared with the impeller dipole source. Besides the dipole source, the vibration of pump structures induced by unsteady flow fluctuations is also a kind of noise source and it makes the biggest contribution to external radiation noise. Under the influence of a noise source, specific frequency response characteristics of flow-induced noise are formed and the internal flow noise shows a trend of fluctuating downward with the increase of frequency, while the maximum of external radiation noise appears between 1000 and 2000 Hz. Based on the frequency response analysis results, directivity characteristics could be obtained. The internal flow noise and external radiation noise show the similar dipole directivity characteristics and the noise level near the tongue is higher than that far away from the tongue. The internal flow noise and external radiation noise increase with the growth of rotational speed. With the growth of flow rate, the internal flow noise reaches a minimum at rated operating conditions generally and increases at off-design conditions, while the external radiation noise increases gradually. Besides the effects of rotational speed and flow rate on noise, the medium temperature could also affect the noise level. 4.