Numerical study of the wake evolution behind a shaftless pump-jet propulsor

To identify the evolution of the vortices hidden in the wake behind a shaft-less pump-jet propulsor, the delayed detached eddy simulation is performed on a computational domain comprising 22.8 million cells to analyze the wake evolution behind a shaftless pump-jet propulsor under various uniform freestream conditions. Convergence analysis and validation against available data confirm the reliability of the numerical approach. Vortex structures are visualized by using the Q-criterion iso-surfaces, while instantaneous and phase-locked averaged flow variables are employed to elucidate the detailed wake dynamics. The numerical results reveal a novel wake evolution mechanism characterized by two wake transition points: Before the first wake transition point, duct-induced vortices rapidly dissipate, leaving the wake dominated by blade/stator trailing vortices and the hub vortex. Between the two wake transition points, the wake is primarily governed by the scythe-shaped vortices (evolved from earlier vortex structures) and the persistent hub vortex. Beyond the second wake transition point, the legs of the scythe-shaped vortices merge with each other and detach from the knee region, triggering instability and fragmenting into smaller-scale structures, which eventually turn into the far-field wake. This study provides valuable insights for the hydrodynamic optimization of the shaftless pump-jet propulsor.