Evolution of ventilated cavitation and supersonic jet coupling in underwater launch: Experimental and numerical insights

Underwater launch technology uses high-pressure gas to propel vehicles from launch tubes, with the nozzle activating, and accompanied by complex fluid dynamics phenomena such as supersonic jets, turbulence, and cavitation. Despite significant advancements, challenges remain in understanding the interactions between the tail cavity and jet coupling, especially regarding flow field evolution and the hydrodynamic effects on the vehicle surface. This study combines experimental and numerical simulations using the volume of fraction model to analyze the coupling dynamics of the tail cavity and jet during vertical motion. Morphological changes in the tail cavity under different parameters were observed, identifying distinct pulsation mechanisms and flow patterns in two primary evolution modes: the integrity cavity mode (IC) and the foam-conical cavity mode (FC-CC). The IC mode is characterized by smooth internal flow, venting via twin-vortex tubes, and reentrant jet closure, whereas the FC-CC mode exhibits turbulent flow with cyclic expansion, compression, and shedding. Parametric studies revealed that nozzle stagnation pressure ratios determine mode selection, while crossflow delays reentrant jets and amplifies their intensity. In the IC mode, the vehicle's bottom pressure frequencies are primarily concentrated in the 0-300 Hz range. In contrast, in the FC-CC mode, frequencies persist primarily in the higher range of 0-1200 Hz, indicating heightened hydrodynamic instability. This work establishes a foundation for enhancing posture control and investigating nonlinear instabilities in launch technologies.