Unsteady cavitation causes noise, damage, and performance decline in the marine engineering and fluid machinery systems. Therefore, finding a method to control the cavitation and its destructive effects is important for the industrial applications. In this work, we proposed a passive method to control the unsteady behavior of transient cavitation at the medium Reynolds number. For this aim, we performed an experimental study using a high-speed camera to analyze the effects of hemispherical vortex generators (VGs) on the cavitation dynamics around a hydrofoil surface. In addition, the pressure pulsations induced by the collapse of the cavity structures in the wake region of the hydrofoil were captured with a pressure transducer mounted on the wall downstream of the hydrofoil. The results showed that the instability behaviors of the cavity structures on the hydrofoil were mitigated using the proposed cavitation passive control method. In addition, the pressure pulsations in the wake region of the hydrofoil were reduced significantly. It can be concluded that the suppression of cavitation instabilities can improve the operating life and reliability of the marine and hydraulic systems.
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.
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