Experimental investigation of cavitation evolution and the flow structure in a submerged water Jet with a nozzle-mounted cap

초록

The effects of a nozzle-mounted cap on the cavitation evolution and erosion aggressiveness of a water jet are experimentally investigated using high-speed imaging, ultrasonic Doppler velocimetry (UDV), particle image velocimetry (PIV), and erosion measurements. High-pressure submerged water jets exhibit highly unsteady flow structures that strongly depend on the nozzle geometry and pressure conditions. By systematically varying the cap diameter and length under identical injection pressures, the spatial development of cavitation clouds, dominant shedding frequencies, entrainment characteristics, and jet spreading behavior are examined. The results show that geometric confinement introduced by the cap significantly modifies jet development, leading to extended regions of coherent cavitation shedding and modified inward directed entrainment flow near the cap exit. A frequency analysis reveals that caps with intermediate geometric confinement (an intermediate range of Dc/d = 22 and Lc/d = 20 under the present conditions) sustain dominant cavitation oscillations over longer downstream distances, whereas excessive or insufficient confinement results in premature frequency attenuation. The combined results indicate that cap-induced entrainment plays a key role in regulating cavitation coherence and jet aggressiveness. This study provides insights into the role of the nozzle-cap geometry in controlling the behavior of a cavitating jet.

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