Ventilated supercavitation around a moving body in a still fluid: observation and drag measurement

Abstract

This experimental study examines ventilated supercavity formation in a free-surface bounded environment where a body is in motion and the fluid is at rest. For a given torpedo-shaped body and water depth (H), depending on the cavitator diameter (d(c)) and the submergence depth (h(c)), four different cases are investigated according to the blockage ratio (B = d(c)/d(h), where d(h) is the hydraulic diameter) and the dimensionless submergence depth (h* = h(s)/H). Cases 1-4 are, respectively, no cavitator in fully submerged (B = 0, h* = 0.5), small blockage in fully submerged (B=1.5 %, h* = 0.5), small blockage in shallowly submerged (B = 1.5 %, h* = 0.17) and large blockage in fully submerged (B = 3 %, h* = 0.5) cases. In case 1, no supercavitation is observed and only a bubbly flow (B) and a foamy cavity (FC) are observed. In non-zero blockage cases 2-4, various non-bubbly and non-foamy steady states are observed according to the cavitator-diameter-based Froude number (Fr), air-entrainment coefficient (C-q) and the cavitation number (sigma(c)). The ranges of Fr, C q and a c are Fr = 2.6-18.2, C q = 0-6, a, = 0-1 for cases 2 and 3, and Fr = 1.8-12.9, C-q = 0-1.5,sigma(c) = 0-1 for case 4. In cases 2 and 3, a twin-vortex supercavity (TV), a reentrant-jet supercavity (RJ), a half-supercavity with foamy cavity downstream (HSF), B and FC are observed. Supercavities in case 3 are not top-bottom symmetric. In case 4, a half-supercavity with a ring-type vortex shedding downstream (HSV), double-layer supercavities (RJ inside and TV outside (RJTV), TV inside and TV outside (TVTV), RJ inside and RJ outside (RJRJ)), B, FC and TV are observed. The cavitation numbers (sigma(c)) are approximately 0.9 for the B, FC and HSF, 0.25 for the HSV, and 0.1 for the TV, RJ, RJTV, TVTV and RJRJ supercavities. In cases 2-4, for a given Fr, there exists a minimum cavitation number in the formation of a supercavity while the minimum cavitation number decreases as the Fr increases. In cases 2 and 3, it is observed that a high Fr favours an RJ and a low Fr favours a TV. For the RJ supercavities in cases 2 and 3, the cavity width is always larger than the cavity height. In addition, the cavity length, height and width all increase (decrease) as the decreases (increases). The cavity length in case 3 is smaller than that in case 2. In both cases 2 and 3, the cavity length depends little on the Fr. In case 2, the cavity height and width increase as the Fr increases. In case 3, the cavity height and width show a weak dependence on the Fr. Compared to case 2, for the same Fr, C-q and sigma(c), case 4 admits a double-layer supercavity instead of a single-layer supercavity. Connected with this behavioural observation, the body-frontal-area-based drag coefficient for a moving torpedo-shaped body with a supercavity is measured to be approximately 0.11 while that for a cavitator-free moving body without a supercavity is approximately 0.4.