Objectives In the context of the "Dual Carbon" strategy and 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, but there is a lack of in - depth research and experimental verification data on it. The aim of this study is 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 it. This can provide a basis for the design and optimization of toroidal propellers, promoting their application in the ship industry.

Methods Firstly, for a five - bladed toroidal propeller, the numerical simulation of its uniform flow hydrodynamic performance was carried out using the sliding mesh technology and the Reynolds stress turbulence model in STARCCM+ viscous flow CFD software. The grid convergence was analyzed by setting three sets of grids with different fineness levels and further refinement of the tip area. Secondly, hydrodynamic performance model tests of the toroidal propeller were conducted on in a cavitation tunnel. Finally, the pitch distribution, circulation distribution, and wake vortex structure of the toroidal propeller were analyzed by 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 studied 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 analysis of grid convergence showed that the grid fineness had little impact on the simulation results. The relative differences between the simulation results of 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 showed that they were in 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 adapt to the incoming flow, resulting in a lower blade pressure. The analysis of circulation distribution revealed that the toroidal structure at the tip enabled the toroidal propeller to provide 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 of the toroidal propeller created a low - pressure area inside the ring, which had 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 similar to the tip shape was formed; in the vortex - separation stage, the hollow vortex core developed into two clear vortices; in the vortex - dissipation stage, the secondary vortex gradually disappeared, and the main vortex 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 support for the design verification of toroidal propellers. The research on load and flow characteristics lays a foundation for further simulation optimization and parameter studies of toroidal propellers. In the future, the numerical simulation method can be used to optimize the design of tip parameters according to the flow characteristics inside and outside the tip of the toroidal propeller, reducing the tip - vortex intensity and achieving the goal of efficiency enhancement.