Objectives In the context of the "Dual Carbon" strategy and the IMO's new regulations, the development of high-efficiency propellers is crucial for the ship industry to achieve energy conservation and emission reduction. The toroidal propeller, an unconventional propeller, has the potential to control tip flow and enhance efficiency. However, there is a lack of in-depth research and experimental verification data on its performance. This study aims to explore the flow characteristics of the special toroidal structure at the tip of the toroidal propeller and develop a reliable numerical simulation method for its analysis. The results of this study will provide a basis for the design and optimization of toroidal propellers, promoting their application in the shipping industry.

Methods First, for a five-bladed toroidal propeller, numerical simulation of its uniform flow hydrodynamic performance were conducted using the sliding mesh technology and the Reynolds stress turbulence model in STAR-CCM+ CFD software for viscous flow analysi. Grid convergence was assessed by using three different grid sets with varying levels of fitness, with further refinement applied to the tip area. Second, hydrodynamic performance model tests of the toroidal propeller were performed in a cavitation tunnel. Finally, the pitch distribution, circulation distribution, and wake vortex structure of the toroidal propeller were analyzed based on the numerical simulation results. The pitch distribution was compared with that of a conventional propeller to explore its influence on hydrodynamic performance. The circulation distribution was examined to understand the loading characteristics at the tip. The wake vortex structure was observed by releasing streamlines and analyzing the pressure distribution at different positions.

Results The grid convergence analysis showed that the grid fineness had minimal impact on the simulation results. The relative differences between the the three sets of grids were small. Moreover, local grid refine of the tip had little effect on the open-water results. The comparison between the numerical simulation results and the experimental data of the toroidal propeller's hydrodynamic performance revealed good agreement. The error of the numerical simulation method was within 4%, indicating its reliability. In terms of load characteristics, the rear blade had a larger pitch ratio to better adapt to the incoming flow, resulting in a lower blade pressure. The analysis of the circulation distribution showed that the toroidal structure at the tip allowed the toroidal propeller to generate more load at the tip compared to conventional propellers, with a non-zero circulation at the tip. Regarding the flow characteristics, the toroidal structure at the tip created a low-pressure area inside the ring, resulting in a "water-suction" effect. The wake structure of the toroidal propeller could be divided into two main vortex structures and three stages. In the vortex-shedding stage, a U-shaped vortex core formed, resembling the tip shape; in the vortex-separation stage, the hollow vortex core developed into two clear vortices; and in the vortex-dissipation stage, the secondary vortex gradually disappeared, leaving the main vortex to dominated the development of the wake vortex.

Conclusions The numerical simulation method established in this paper can accurately predict the hydrodynamic performance of toroidal propellers. The model test results provide valuable data to support the design verification of toroidal propellers. The research on load and flow characteristics lays a foundation for further optimization simulations and parameter studies of toroidal propellers. In the future, the numerical simulation method can be used to optimize the design of tip parameters, focusing on the flow characteristics both inside and outside the tip of the toroidal propeller. This could reduce the tip-vortex intensity and enhance efficiency.