Effect of inflow velocity on cavitation state transition and flow mechanism in a pump-jet propulsor

Abstract

Cavitation state transition and the underlying flow mechanism are core issues in fluid physics, especially in the underwater propulsion systems. Inflow velocity variation alters local flow structure and pressure distribution, regulating cavitation inception and critical collapse in pump-jet propulsors (PJPs). This study focuses on the fluid physics of cavitation state transition, investigating the effects of inflow velocity on cavitation inception (cavitation number σn = 3.0) and critical collapse state (σn = 1.5) in a PJP using improved delayed detached eddy simulation coupled with the Schnerr–Sauer cavitation model. The results reveal a critical advance coefficient J = 0.8 at σn = 1.5, which serves as the boundary for cavitation state transition: for J ≤ 0.6, the flow field is dominated by strong backflow in the mid-chord region, leading to severe cavitation with chaotic vortex structures, characterized by cavitation volume Cv = 0.0106 and coverage area Cs = 93.41% at J = 0.2; for J ≥ 0.8, the flow transitions to a forward-flow dominated state, vortex structures recover to organized horseshoe and tip vortexes, and Cs = 48.69% at J = 0.8. At σn = 3.0, cavitation is strongly suppressed, with Cv at J = 0.2 only 0.93% of that at σn = 1.5, and the flow field remains stable despite occasional excitation force spikes. A key new finding is that inflow-driven flow pattern transition governs cavitation regime shift, where backflow-vortex-collapse coupling causes sudden hydrodynamic degradation. Furthermore, the interquartile range of rotor excitation force, which reflects flow unsteadiness, is approximately 2.4 times higher at J = 0.2–0.6 than at J = 0.8, confirming the close correlation between flow stability and cavitation state. This study provides theoretical guidance for cavitation suppression in underwater propulsion systems.