The effects of cavitation number on multiphase flow characteristics and trans-media motion behaviors of a bionic manta-ray vehicle

To investigate the trans-media motion dynamics of a bionic manta-ray, this study integrates the improved delayed detached eddy simulation model, energy equation, volume of fluid method, Schnerr–Sauer cavitation model, and overlapping grid technique. In addition, validations of the numerical approach are conducted, and the results are satisfactory. Load and motion characteristics during water–air transition under varying cavitation numbers are systematically examined. The cavitation number dominates cavity morphology and stability: low values facilitate large-scale, well-adhered cavities continuous with the tail surface, while high values suppress cavitation, inducing cavity shedding, fragmentation, and discrete low-water-volume-fraction zones. At cavitation number 0.694, the effective cavity volume fraction drops below 5% of that at cavitation number, with the downstream low-volume-fraction area shrinking over 80%. Vortex evolution is tightly coupled with cavity dynamics: high cavitation numbers disrupt shear-driven vortex generation, reducing high-magnitude vorticity volume by about 60% and weakening vortex coherence by about 70% relative to cavitation number 0.394. Moreover, cavity dynamics govern load characteristics and trans-media motion stability. High cavitation numbers trigger abrupt pressure changes, causing sharp force jumps and moment oscillations. This work provides a theoretical basis for optimizing bionic trans-media vehicles.