# Advancements and Applications of Rim-Driven Fans in Aerial Vehicles: A Comprehensive Review
## 9 FEA and CFD
By investigating blade tip geometry modification, a study aimed to increase the strength of the centerline flow region to improve thrust performance in static fluid [95]. The study utilized CFD for performance evaluation. Results showed that the reverse flow in the centerline, induced by the rotating blades from both the upstream ambient air and downstream domain, reduced overall thrust performance. Blade tip geometry modification was proposed to mitigate this issue, resulting in improved performance. [PERSON] and colleagues designed RDT propellers both with and without hubs [36]. While the efficiency profiles of these propellers were similar, the hubless variants generated higher levels of thrust and torque. This was primarily attributed to a slight reduction in the flow velocity over the propeller disk and the presence of larger blade radii in the hubless configuration. In open water, a CFD investigation by [PERSON] et al. [96] determined that hub-type RDTs are less hydrodynamically efficient than hubless RDTs (Figure 4). The hub restricted the water flow area, resulting in a faster axial water flow speed and a lower aft side water pressure compared to the RDT without a hub, which decreased the blade and duct thrust. Four pairs of four-bladed rim-driven thrusters having different hub ratios were tested to study the effect of the hub, where the hub ratio refers to the ratio of the hub diameter to the propeller diameter. The hydrodynamic performances of the hubless and hub-type RDTs are compared both at the design point and off-design points [96]. Based on the computational results, it is concluded that the hubless RDT has a higher efficiency than the hub-type RDT. At the design point characterized by a hub ratio of 0.25 and an advance coefficient of 0.7, the efficiency of the hubless RDT surpasses that of the hub-type RDT by 2.2%. The absence of the hub also results in increased thrust and torque while reducing the thrust ratio (Figure 4). Figure 4: (i) (a,b) Hub-type and hubless RDT models, (ii) (a) thrust coefficient, (b) torque coefficient, and (c) efficiency of RDT at design point with changes of hub ratio [96]. [PERSON] et al. used a commercial RANS solver to investigate the effects of axial and radial gap flow on the torque of rim-driven thrusters [97] (Figures 5 and 6). Both simulation outcomes were robustly verified by experiments, demonstrating the simulation tools' efficacy. The study analyzes the independent effects of axial and radial clearance ratios on the torque of the outer surface and end faces and presents a new predicted method by fitting the numerical and analytical results. The study reveals the presence of circular flow, Taylor vortices, and an increase in the torque coefficient with increasing radial gap clearance. [PERSON] et al. provided a number of numerical solutions for assessing the hydrodynamic performance of RDTs across separate experiments conducted between 2011 and 2015 [85; 98] (Figure 7). The importance of considering the entire rotation of the device when using the frozen-rotor method to model rotor-stator interaction is highlighted. Under-predictions of torque and thrust were observed in the simulations, which were improved by the augmentation of analytical rim friction models. The lack of consideration for end effects which become significant at low aspect ratios and interactivity between axial and radial gap flows in the analytical models might explain the error in torque prediction. An unsteady simulation revealed an oscillating effect on the inflow angle to the blades caused by the rotor-stator interaction, which has design implications on pitch distribution selection. The  
Figure 5: Configuration of test setup of a rim-driven thruster [97] (Copyright (2023), with permission from Elsevier). Figure 6: Grid structure and detail with enlarged scale [97] (Copyright (2023), with permission from Elsevier). oscillating flow past the stators causes an oscillating inflow to the blades, which may cause a variation in the effective angle of attack, implying potential peak loads.