Influence of propeller boss cap fins on the wake of a marine propeller

Large eddy simulation (LES) on a cylindrical grid consisting of approximately 7 billion points is used to analyse the wake flow of a marine propeller equipped with propeller boss cap fins (PBCFs). Computations of the same propeller without this energy saving device (ESD) were also conducted, for comparison purposes. The results of the simulations demonstrate almost an order of magnitude mitigation of the minima of pressure and the maxima of vorticity at the core of the hub vortex. Meanwhile, a significant recovery of both streamwise and azimuthal momentum is achieved, with beneficial effects on the efficiency of propulsion. Additional, smaller vortices are shed by the PBCFs, having a negligible impact on the wake flow, if compared with the benefits allowed by the mitigation of the hub vortex. This is also the case of the turbulent stresses. The only exception is the region just downstream of the propeller plane, upstream of the instability of the hub vortex, where turbulence levels at the core of the hub vortex are still relatively low. However, within a few diameters downstream of the rotor, the instability of the stronger hub vortex shed by the propeller without PBCFs is the source of turbulent stresses more than an order of magnitude higher than those experienced in the wake of the propeller equipped with PBCFs. Meanwhile, the influence of the PBCFs on the outer region of the wake and especially on the tip vortices was found to be practically negligible.