Flow field characteristics and three-dimensional coherent structures of free and impinging cavitating water jets: Effects of nozzle configuration

In engineering applications leveraging the erosive capabilities of submerged cavitating jets, impinging jets are prevalent, and optimizing nozzle configuration can enhance cavitation-induced erosion. This study investigates the effects of nozzle configuration on flow structures and erosion intensity in both free and impinging submerged cavitating jets. Two nozzle configurations are analyzed: one with a cylindrical expansion section and one without. High-speed visualization, erosion testing, and numerical simulations with the stress-blended eddy simulation turbulence model and Schnerr-Sauer cavitation model are integrated to resolve cavitation dynamics. Proper orthogonal decomposition (POD) is further used to analyze the three-dimensional coherent structures. In results, the expansion section enhances both cavitation and erosive intensity, producing periodic cloud shedding through generating upstream-directed reentrant motion that is accompanied by vortex fragmentation. Impinging jets from a nozzle with an expansion section produce structured cavitation cloud collapse with extended high-pressure regions. Wall impingement induces axial-to-radial vortex reorientation, reducing vorticity density in the expansion region. POD analysis indicates that the expansion section concentrates energy in low-order modes, reflecting stronger large-scale fluctuations, while spectral entropy highlights the frequency modulation effect of the expansion section. Moreover, wall constraint in impinging jets promotes small-scale fluctuations.