Study on the coupling mechanism of nozzle structure-cavitation impact under submerged environment

Water jet technology has broad potential in marine engineering due to its high efficiency, energy efficiency, and environmental friendliness. This study aims to enhance the cavitation fragmentation performance of jets by integrating underwater erosion experiments with large eddy simulations to investigate the influence of petal-shaped nozzle structural parameters on cavitation impact characteristics. A multiscale analysis method integrating cavitation impact and damage mechanisms, along with a metric for cavitation intensity, has been established to quantify the cavitation impact performance of jets. Experimental and computational fluid dynamics results show that the organ-pipe nozzle (O.N₄₀) with a diffusion angle of 40° has the maximum cavitation strength at target distance L/D_e = 6. The optimum target distance L/D_e = 8 occurs for a petal-shaped nozzle (P.N₂₇) with a diffusion angle of 27°. Under these conditions, the cavitation erosion intensity is 2.29 times greater than that of O.N₄₀. Microporous regions caused by bubble collapse were identified via scanning electron microscopy (SEM). Localized high-pressure phenomena emerge during cavitation cloud collapse, accompanied by outwardly propagating pressure waves. P.N₆₀ cavitation bubbles exhibit a greater development rate and spatial range with higher bubble density, generating stronger cavitation impacts at the target surface. Dynamic mode decomposition (DMD) was applied to cavitation data in the shock flow field. The steady cavitation flow field was captured in the high-energy modes. The spatio-temporal evolution of microbubbles was accurately characterized by higher-order modes. This study elucidated the interaction mechanisms between nozzle structure and cavitation impact and provided theoretical foundations for optimizing nozzle designs in submerged rock-breaking applications.