Two-dimensional and three-dimensional numerical simulations of cycloidal propellers in hover

The cycloidal propellers for micro aerial vehicle scale cyclocopter in hovering status were studied in this paper based on the URANs solver using 2D, 2.5D, 3D half blade and 3D full blade model. The results from all numerical models were validated with the experimental results. It was found that 2.5D model cannot produce more accurate results than 2D model, hence results from 2D model were employed to discuss cycloidal rotor with infinite blade span. It was also indicated that the 3D half blade model produced the same results as 3D full blade model, but was more efficient than 3D model. The numerical simulation results of cycloidal rotor with finite (3D model) and infinite span (2D model) were compared. The results indicated that for the 2D cycloidal rotor with large blade pitching amplitude, there were leading edge and trailing edge vortices due to dynamic stall, which resulted in parallel blade vortex interactions. The parallel blade vortex interactions will also induce the fluctuation of aerodynamic forces. For the 3D blade with small aspect ratio, the flow was dominated by 3D dynamic stall and blade vortex interactions. The 3D flow due to finite blade span resulted in smooth dynamic stall and can weaken the parallel blade vortex interactions induced by dynamic stall vortices, hence no strong aerodynamic force fluctuation was observed. The perpendicular blade vortex interactions caused by blade tip vortices can induce cross flow when the azimuth angle of the rotor is between 270 degrees and 360 degrees, which reduces the strength of downwash in the region where the rotor azimuth angle is between 180 degrees and 360 degrees. This results in much smaller side force. Although sometimes the time-averaged aerodynamic forces obtained by 2D and 3D model were quite close to each other, the physics lying behind is quite different. Hence, it was not correct to use the 2D models to discuss the principles of cycloidal rotors with finite blade span.