Experimental and Numerical Research on the Formation Conditions and Entrainment Characteristics of Supercavity by Rear Jet Reflux

Aiming at better controlling the ventilated supercavity flow for drag reduction, the experimental and numerical researches of supercavity by rear gas reflux are proposed in this paper. Several experiments with different test bodies have been carried out to study the formation and collapse conditions of jet-reflux supercavity. An opencirculation water tunnel for ultra-high-speed jet experiment and air jet system is employed to form jet-reflux supercavity around the bodies installed in the forward strut. The experiment results show that the supercavity can be maintained by the reflux of tail jet flow when an initial supercavity covering the jet outlet is formed. However, the supercavity will be destroyed when the jet intensity is further enhanced. Under the same jet coefficient, the scale of jet-reflux supercavity extends as the increase of the body length, while the critical jet coefficient for the collapse of the supercavity decreases as the increase of the body length. The multiphase flow model coupling the VOF model and the level-set method is applied to capture the air-water interface. Then, the flow field characteristics of the jet-reflux supercavity are analyzed and compared with the ventilated supercavity. The streamline inside the cavity presents considerable three-dimensional asymmetry inflating flow characteristics. The variation of the gas reflux coefficient along the axial direction is obtained, which indicates that a handful of reflux gas are required to sustain the head cavity. Therefore, the jet-reflux supercavity can be formed within a certain range of the tail jet intensity. Although jet intensities are not equal to each other, the scale of cavity head is roughly maintained under the same reflux coefficient. When the supercavity gets closed to the nozzle outlet, the maximum scale of cavity is decreased, which leads to a weaker reflux at the outlet. The cavity interface will be impinged by the high-speed gas and mixture liquid, which obviously causes deformation and final collapse. In order to improve the stability of the jet-reflux supercavity, it is necessary to use the gas re-directed structure to control the reflux.