Effect of mounting strut and cavitator shape on the ventilation demand for ventilated supercavitation

Abstract

The present work systematically investigates the effect of cavitator mounting strut and cavitator shape on the ventilation demand to form and sustain a ventilated supercavity under different flow conditions. Three cavitators of different shapes (i.e. 2D cavitators including triangle and disk shape, and 3D cone-shaped cavitator) with the same frontal area are employed for the experiments. The cavitator is connected to a ventilation pipe extended upstream from a mounting strut, referred to as forward-facing model (FFM). The minimal ventilation coefficients to generate (CQf) and to collapse (CQc) a supercavity are measured over a wide range of Froude number (Fr) for each cavitator. Images of overall cavity shapes and topology near closure region as well as the cavity pressure under different experimental conditions are captured simultaneously. The results are compared across different cavitator shapes and the disk cavitator situated on the downstream side of the mount strut, referred to as backward-facing model (BFM). Similar CQf-Fr curves were observed in BFM and FFM-configured disk cavitators. The cone-shaped cavitator requires the least amount of ventilation to generate a supercavity among all different shapes across the range of Fr in our experiments. The CQcof disk FFM is lower than that of its BFM counterpart at small Fr and exceeds the BFM CQcwith further increase of Fr. The cone cavitator has the smallest CQc among all the cavitators across the range of Fr in our experiments. To elucidate the trends of CQc upon changing Fr and cavitator shape, the geometry of supercavity under CQc including its overall shape, the cavity maximum diameter (Dmax) and half length (L1/2) are also investigated. Both Dmax and L1/2 show an increasing then plateauing trend upon increasing Fr across different FFM cavitators despite the smaller values for the 3D cone cavitator. Subsequently, such cavity geometric information and cavity pressure measurements in conjunction with high speed imaging of re-entrant jet are employed to estimate the re-entrant jet momentum under different Fr for disk and cone cavitators. The estimated re-entrant jet momentum shows reasonable match with the ventilation air momentum under CQc in lower Fr for both cavitator cases, with the disk cavitator case yielding significantly stronger re-entrant jet, providing support to the re-entrant jet mechanism governing on the cavity collapse. Our study sheds some light on the cavitator design and ventilation strategy for a supercavitating vehicle in practice.