Comparison study of the vortical structures in the wake of a rim-driven thruster and a ducted propeller in bollard conditions

This paper studies the vortical structures found in the wake of a ducted propeller (DP) and a rim-driven thruster (RDT) in bollard conditions. Despite a good understanding of the vortical structures in the wake of DPs, an understanding of these structures and their dynamics in the wake of RDTs is largely lacking. At bollard conditions, noise levels are higher compared to other loading conditions due to the higher pressure fluctuations in the wake. Hence, understanding the flow structures in the wake is a necessity to understand the noise production and propagation. Both wake flows were analysed using visualization of the instantaneous vortical structures, frequency spectra at several locations in the flow field and proper orthogonal decomposition (POD). It was shown that the centre of the wake in both the DP and RDT contained several spiralling structures behind the central hub. These structures contained a wide range of frequencies, making the wake very dynamic with a broad range of different coherent fluctuations. In case of the RDT, these structures persisted longer in the flow field before breaking up into smaller turbulent structures. In the DP wake, very strong tip vortices were formed in the gap flow between the duct and the propeller blades, which were not present in the RDT. These tip vortices interacted with the flow around the duct to form a shear layer with large pressure fluctuations. As these tip vortices induce higher sound levels, the RDT shows a significantly better noise performance compared to the DP.