Evolution of vortex-cavitation coherent structures in self-excited cavitation waterjets under sound waves excitation.

Self-excited cavitation waterjets have been widely employed in surface treatment, material cutting, and equipment cleaning owing to their low cost and high energy conversion efficiency. This study investigates the evolution of vortex-cavitation coherent structures in self-excited cavitation waterjets under ultrasonic excitation using Large Eddy Simulation (LES) and Proper Orthogonal Decomposition (POD). External vibrational excitation was applied via a user-defined vibrating boundary within the Helmholtz nozzle chamber. The results showed that synchronization between vortex shedding and cavitation evolution was enhanced at an resonant frequency excitation of f_e = 2.029 kHz. In contrast, long-distance cavitation bubble transport was enhanced, while dominant microbubble collapse fragmented vortex structures under ultrasonic excitation at f_e = 25 kHz. POD analysis revealed that resonant excitation concentrated 80 % of the total energy in the first 200 modes, highlighting the dominance of vortex-induced cavitation. High-frequency excitation dispersed energy more broadly, with only 50 % of the total energy captured by the first 200 modes. Although the first vorticity mode remained large-scale, the second to fourth modes revealed disordered small-scale vortices due to intensified shear. These results elucidate the dynamic interplay between vortices and cavitation under ultrasonic excitation and provide a theoretical foundation for the active optimization of cavitation waterjet performance.