Nozzle-lip effects on co-flow self-excited cavitating waterjets for cavitation peening

Cavitation peening offers a promising non-contact surface treatment alternative to conventional shot peening, but its application is often limited by the requirement of submerged operation. Co-flow cavitating waterjets overcome this constraint by using an annular liquid curtain to sustain cavitation in air. However, the role of nozzle geometry remains poorly understood. Here, we examine how the nozzle-lip geometry governs this process by comparing two distinct configurations—a horn lip and a cylindrical lip—in a co-flow self-excited cavitating waterjet equipped with an organ-pipe resonator. Through erosion tests on pure aluminum, high-speed imaging, proper orthogonal decomposition, acoustic and pressure spectral analysis, and complementary large-eddy simulations, we establish a mechanistic link between lip-induced near-field hydrodynamics and the resulting erosion intensity. The horn lip produces a markedly stronger cavitation response: it increases the mean mass loss by approximately 31% and expands the erosion-ring area by about 26% relative to the cylindrical lip. This enhanced performance is traced to a fundamental reorganization of the near-nozzle flow field—the horn geometry generates a stronger low-pressure region near the exit, induces localized backflow, intensifies the shear layer, and promotes vortex coalescence, leading to larger, more coherent cavitation clouds and a lower shedding frequency. Crucially, this reorganization strengthens the acoustic–hydrodynamic coupling with the organ-pipe resonator, amplifying the internal pressure response from 538 to 1565 Pa. These results demonstrate that nozzle-lip geometry is not merely a passive boundary condition, but an active control parameter that governs vapor-cloud organization, resonant feedback, and erosion intensity in co-flow self-excited cavitating jets, offering a design pathway for more efficient cavitation-peening nozzles.