# Investigation of Cavitation Noise in Cavitating Flows around an NACA0015 Hydrofoil
## 5 Conclusions  
In this study, the cavitation evolution around a hydrofoil and its acoustic characteristic of flow induced noise were numerically investigated. The filter-based turbulence model and Zwart cavitation model were employed to accurately calculate the flow field and predict the cavitation phenomenon. Besides this, an acoustic analogy method based on the FW-H equation was applied to analyze the flow-induced noise. To research the regularities and features of hydrofoil in different cavitation conditions, seven cavitation numbers were selected and investigated.  
1. In the comparison of non-cavitation, sheet cavitation, and cloud cavitation, it can be observed that as cavitation number decreases, cavitation cycle length gets shorter and the magnitude of acoustic power spectral density increases dramatically. The cause can be contributed to their difference in cavitation morphology. For \(\sigma=3.3\), higher acoustic pressure can be observed, meanwhile, the magnitude of pressure for \(\sigma=3.3\) is clearly larger compared with that of other cavitation numbers. Thus, it is speculated that pressure fluctuation is possibly the cause of high acoustic pressure for the condition of non-cavitation;
2. For the condition of sheet cavitation, the distance from the location of peak value in the designated time is 0.2-0.4 \(C\) away from the cavitation growth and collapsing point. Only one peak value of acoustic power spectral density, which was due to the extending and retracting of the leading-edge cavitation, could be observed;
3. For the condition of cloud cavitation, the results show that the distance between peak values of acoustic pressure and cavitation is 0.2-0.4 \(C\). Besides this, acoustic pressure drops abruptly in the point behind the tail. For point upper surface of the hydrofoil, two peak values of acoustic power spectral density are the result of superposition from the leading-edge cavitation and trailing vortex. As a comparison, for points behind the hydrofoil, only one peak value can be observed, for the reason that the impact of leading-edge cavitation has dissipated over such a long distance from the collapsing point. Differences in PSD for different cavitation numbers were also observed. When \(\sigma=0.55\) or \(\sigma=0.65\), the peak value of acoustic power spectral density reveals that the growth and collapsing of trailing vortex occurs once a cycle, but that of front bubbles happen twice in one cycle. When \(\sigma=0.75\), the growth and shedding of leading-edge cavitation occur three times a cycle. Additionally, the first acoustic power spectral density peak is relatively higher, which is caused by the phenomenon that the collapsing of the trailing vortex and front bubbles occurs at the same time.  
Conceptualization, A.Y. and D.Z.; Methodology, Q.T. and H.C.; Validation, A.Y., and D.Z.; Formal analysis, A.Y.; Investigation, X.W. and Z.Z.; Writing-original draft preparation, X.W. and Z.Z; Writing-review and editing, A.Y.; Supervision, A.Y.; Funding acquisition, A.Y. This research was funded by the National Natural Science Foundation of China (Project No. 51806058) and the Fundamental Research Funds for the Central Universities (Project No. 2019B15014).  
The authors declare no conflict of interest.