Experimental Study of Break-off Frequency in Cavitation Disappearance Phenomenon on NACA16-012 Hydrofoil

Abstract

Under the specific angle of attack on NACA16-012 hydrofoil, generated cavitation suddenly disappears even in the low cavitation number condition in which cavitation normally develops. This cavitation disappearance phenomenon was confirmed to be unique to NACA16-012 hydrofoil in our previous research. It was experimentally confirmed that cavitation disappearance phenomenon occurs in a transient cavitation condition where the sheet cavity is periodically broken off and the cloud cavity is released. In the previous study, it was predicted that there is the frequency band in which sheet/cloud cavitation cannot exist and cavitation disappearance phenomenon occurs. In this study, the break-off frequency of NACA16-012 hydrofoil was experimentally investigated and relationship between the break-off frequency and cavitation disappearance phenomenon was considered. The pressure fluctuation was measured with the pressure transducer downstream reagion and the dominant frequency was estimated by the frequency analysis. As the results, cavitation disappearance phenomenon was confirmed at angles of attack of 6° and 7°. There was the region increasing the break-off frequency according to decreasing of cavitation number at angles of attack 6° to 12°. It means that the break-off frequency of NACA16-012 hydrofoil has unusual characteristics. When the break-off frequency was expressed by the Strouhal number, it was found that the Strouhal number took each same value immediately just before and after cavitation disappearance phenomenon in decompression experiment. It was suggested that the break-off frequency was related to cavitation disappearance phenomenon.