Coupling mechanism of structure–cavitation impact in reflux self-excited oscillating nozzles

Abstract

Aiming at improving the application of reflux self-excited oscillating nozzles (RSONs) in underwater cleaning, this study focuses on the influence of nozzle structure parameters on jet morphology evolution and cavitation impact characteristics at 10 MPa pressure. We establish a multi-scale analysis methodology integrating study of internal and external flow fields through high-speed camera visualisation experiments and large eddy simulation. A coverage growth rate index is proposed to normalise the evaluation of jet coverage area across nozzles with varying dimensions. Dynamic mode decomposition (DMD) reveals distinct modal energy distributions in jet velocity fields and cavitation fields. The results demonstrate that at the characteristic ratio K_t ranging from 0.4 to 0.5, symmetric vortex structures in the main chamber induce periodic jet sweeping motions. Adjacent jet bundles exhibit opposing velocity vectors, maximising the flow field coverage growth rate. However, when K_t exceeds 0.6, vortex de-stabilisation occurs, suppressing jet sweeping behaviour. Cavitation bubble clusters predominantly grow and collapse near the outlet. DMD analysis demonstrates that high-energy modes exhibit anti-symmetric vortex pairs, whereas low-energy modes represent small-scale turbulent structures with broad-band frequencies. This study elucidates the cavitation–vortex interaction mechanism in RSONs and provides a theoretical foundation for optimisation of nozzle designs in underwater cleaning.