On the unsteady behavior of a freely closed ventilated supercavity at low Froude numbers

An experimental study was conducted to examine the unsteady flow characteristics of a freely closed ventilated supercavity induced by a disk-shaped cavitator. Within the explored parameter range, four distinct cavity closure modes were identified: Foam Cavity (FC), Twin Vortex (TV), hybrid Foam Cavity and Re-entrant Jet (FCRJ), and hybrid Twin Vortex and Re-entrant Jet (TVRJ). Among these, the Twin Vortex mode was most commonly observed at low Froude numbers, characterized by periodic shedding of small bubbles and the presence of a pair of vortex tubes connected to the cavity tail, exhibiting regular breakup events and fluctuations in both diameter and axial position. High-precision dynamic pressure sensors was employed to investigate spectral features of internal cavity pressure and surrounding ambient pressure across various closure modes and flow conditions. Results revealed that, in the low-frequency range 0∼50Hz, incoming flow disturbances predominantly influenced cavity pulsations, whereas cavity oscillations themselves also significantly affected pressure fluctuations in the external flow field. In the intermediate-frequency range 50∼300Hz, distinct energy peaks emerged, attributable to intrinsic unsteady flow structures within the supercavity. Moreover, at high-frequency range 300∼3000Hz, evenly spaced spectral peaks were observed, corresponding to the excitation of standing wave modes within the slender supercavity body.