A comparative DES study of wake vortex evolution for ducted and non-ducted propellers

A comparative numerical analysis was conducted on the evolution of wake vortices generated by ducted and non-ducted propellers in open-water tests. The simulations employed detached eddy simulation (DES) method with a focus on the detailed analysis of the composition of vortices and spatial evolution mechanism under various loading conditions. Compared with non-ducted propellers, the interference between the duct and the propeller directly changed the morphology of vortices and wake contraction, altered the energy distribution in the internal flow fields and thereby strengthened the self- and mutual induction and accelerated the primary merging and grouping processes. The results indicated that the shear layer vortices shedding from the duct displayed a strong interaction with the distorted tip vortices in an irregular manner and ejected the secondary coherent structures. Numerous secondary vortices were observed during the transition regime of the dynamic evolution. Followed by the energy dissipation and instabilities in the wake vortices, the wake vortex structures gradually broke down. The spectra of kinetic energy (KE) analyses of the wake field quantitatively confirmed the evolution of wake vortical structures in the temporal domain. The ducted propellers possessed a characteristic frequency that corresponded to its shaft frequency, blade passing frequency (BPF) and their harmonics.