Influence of the number of blades on the wake development of rim-driven thrusters

The wake systems of rim-driven thrusters (RDTs) with different numbers of blades, producing the same overall thrust, are compared by using a large eddy simulation methodology. For all cases, the shear produced at the outer boundary of the wake results in a quick breakup of the helical vortices shed at the outermost radial coordinates, while those at the innermost radii keep coherent up to the outlet section of the computational domain, located five diameters downstream of the propeller plane. The vortices within the trailing wake of each blade undergo merging into larger structures, while, in contrast with the wake development process typical of conventional propellers, no mutual inductance phenomena between the vortices shed by neighboring blades were observed, even as the number of blades increases. Therefore, more blades experiencing lower loads result in less intense vortices, shear layers, and turbulent stresses, since the decreasing azimuthal distance between trailing wakes and vortices from neighboring blades was not found to trigger more intense interactions and a faster diffusion. These results suggest that more blades are beneficial to the wake properties of RDTs, at least in the range of blade numbers considered in this study, from three to six.