Mode decomposition of pressure effects on coherent structures in self-excited oscillating cavitation waterjets

Self-excited oscillating cavitation waterjets are widely utilized in marine engineering applications such as deep-sea mining, well drilling, and natural gas hydrate extraction. The operating pressure critically influences their frequency response and oscillation characteristics, thereby impacting operational efficiency. This study investigates the effect of pressure on the vortex and cavitation cloud structures. Large eddy simulation was employed to simulate the cavitation flow within waterjets. The generalized S-transform method was utilized to analyze the difference in pressure oscillation frequency between the interior and exterior of the oscillator. Dynamic mode decomposition further elucidated the coupling characteristics of vortex structures in the flow field. The energy transfer coefficient was used to quantitatively characterize the energy exchange between coherent vortex structures and cavitation clouds. The main findings are that large-scale bubbles enhance vortex shedding and promote the collapse of coherent structures. Near the Helmholtz nozzle outlet, vortex structures exhibit relative compactness, and the waterjets demonstrate strong coherence. Prior to modulation, the time–frequency spectrum reveals a dominant peak at 105 Hz. Post-modulation, transient pressure fluctuations are predominantly concentrated in the low-frequency range, with a dominant frequency of 200 Hz. Cavitation bubble expansion is identified as the primary factor driving the stretching of coherent structures. Furthermore, the shedding of vortex rings at the waterjet's fundamental frequency is synchronized with its entire cycle of cavitation bubble expansion and collapse.