Wake Properties of Rim Driven Thrusters

The growing importance of marine transportation in the world economy poses a number of challenges to be faced. Improving the efficiency of marine propulsion is a critical target for reducing the fuel consumption and in turn the emission into the atmosphere of pollutants and greenhouse gasses, besides producing advantages to the economy. A growing issue is also represented by the environmental impact of marine propulsion as a source of underwater radiated noise (URN), which is detrimental to the life of oceans. Therefore, marine engineers always strive to develop innovative, unconventional designs of propellers, aimed at improving their efficiency and decreasing their signature. Rim Driven Thrusters (RDTs) are a particular class of ducted propellers, where the blades rotate together with the rim within their nozzle. Ducted propellers operate within a nozzle and are able to achieve better performance, in comparison with non- ducted propellers. However, they still produce hub and tip vortices as well as leakage vortices, which are important in affecting their acoustic signature. In addition, these large vortices interact with downstream bodies, as rudders, damaging their performance, their structural integrity and contributing further to vibrations and sound. Thanks to their alternative design, RDTs do not need a hub, avoiding the onset of a large axial vortex in their wake. In comparison with conventional ducted propellers, they can also prevent the onset of tip and leakage vortices, generated at the outermost radial coordinates within the gap between the tip of the blades and the inner surface of the nozzle. This feature also limits the need of unloading the blades at their outermost radial coordinates, which is detrimental to the performance of conventional propellers. Therefore, the wake of RDTs is substantially different from that of conventional ducted propellers: it is populated at the outer radii by the junction vortices originating at the intersection between their blades and the rim and by the “root” vortices produced at the end of the propeller blades at their innermost radial coordinates, whose source is similar to that of the tip vortices of conventional propellers. Despite their potential, little is known on the flow physics and wake instability of RDTs, since only recently the development of more compact electric motors to be installed within their nozzle made their deployment technically feasible, increasing the interest towards this class of propellers. A better understanding would be beneficial for the proper exploitation of their special features and the development of more efficient and silent designs. The present study is aimed at addressing this problem by performing high-fidelity fluid dynamic computations, based on the Large Eddy Simulation (LES) technique, with the purpose of comparing the wake features of conventional ducted propellers and RDTs and evaluating the influence of the number of blades on performance and wake features. These state-of-the-art simulations will exploit the capability of LES of resolving all energy-carrying structures of the flow, limiting turbulence modeling to the only scales smaller than the grid spacing. Computational grids consisting of billion points, at least two orders of magnitude more extensive than those in the available studies on the subject, will be utilized, with the purpose of resolving a wide range of turbulent scales. Data from wall-resolved LES will be also exploited to reconstruct the acoustic signature by means of the Ffwocs Williams & Hawkins acoustic analogy and comparing the different sources of sound between conventional ducted propellers and RDTs.