## 1 Introduction
However, it is often quite costly to build an experimental setup and sometimes even impractical. Therefore, Computational Fluid Dynamics (CFD) has become a popular choice in dealing with such problems due to its extraordinary convenience and low cost. Among various numerical methods, the RANS methods stand out and can be found in extensive research studies. [PERSON] et al. [6] investigated the performance of an RDT with a commercial RANS solver after validating the numerical model by comparing the simulated results of a Ka 4-55  
Figure 1: Schematic layout of an RDT. propeller with existing experimental data. From their results, rim-driven thruster propellers could produce a larger thrust by absorbing a higher torque from the motor compared to conventional shaft-driven propellers, but this resulted in a lower efficiency. However, this situation can be alleviated by applying thinner propeller blades. Moreover, a long rim can slightly increase friction torque and affect the performance of the propeller blades as a circumferential velocity will be introduced to the local flow, causing a reduced tangential velocity and angle of attack for the propeller. Due to the fact that no design guidelines were available for propeller design in hub-less RDTs, [PERSON] et al. [7] designed a propeller based on current Ka-series propeller geometry. Three configurations of hub-less RDTs were modeled and calculated numerically, i.e., 5 blades, 7 blades, and 5-4 contra-rotating blades. They concluded that the hydrodynamic performance of RDTs is similar to that of ducted propellers, and, because there is no hub in the hub-less RDT, no hub vortex exists and therefore cavitation is diminished. The rim gap is a characteristic structure in RDTs and, even though the dimensions of the gap are usually small, its influence on the performance of the thruster can hardly be ignored. [PERSON] et al. [8; 9] studied the effects of the gap flow with and without axial pressure difference using RANS simulations. When there was no pressure gradient in the axial direction, the angular momentum current analyzing method was introduced to analyze the gap flow. The developed method was validated by experiments in the large cavitation tunnel of CSSRC. Compared to the calculations with empirical formulas, the predicted results by the numerical method were improved significantly. However, when a pressure difference existed between the rim ends due to the rotational movement of the propeller, the flow regimes in the gap flow changed significantly, resulting in a higher torque friction. The thrusts produced by the surfaces surrounding the gap, including the duct inner surface, rim outer surface, and rim end surface, were balanced out. The propeller loading distributions and steady wake flow fields were also investigated by studying four cases of different propellers. The propeller blade radial circulation and chord-wise circulation density distributions were analyzed. It was found that the maximum radial circulation occurs at the blade tip, which is different from conventional shaft-driven propellers, and the right adjustment of the blade loading distribution could restrain root and tip region vortices. As an innovative invention, the RDT is quite similar to a conventional DP in its structural design. However, a detailed comparison study of hydrodynamic performance between them is difficult to find. Therefore, the present work aims to find out the difference in hydrodynamic performance between the DP and the RDT with the aid of numerical simulations. A verification and validation study is carried out for the CFD model to ensure the numerical accuracy. The CFD model is then applied to the performance prediction of the DP and the RDT. Based on the results, the local and global hydrodynamic characteristics of the DP and the RDT are compared and analyzed, including the open water performance and several important flow features. The model setup and validation is given in Section 2.