Experimental and numerical investigations on the aerodynamic lift characteristics and flow features of water-treading hovering of two-dimensional elliptic airfoil*

This study investigates the aerodynamic performance of a 2D hovering airfoil through experiments and numerical simulations. Here, the water-treading type of hovering kinematics, combining the surging and pitching motions at a hovering frequency of 0.833 Hz, is considered. The airfoil has a rectangular planform with an elliptical cross-section, a chord length of 0.04 m, a thickness of 0.005 m, and an aspect ratio of 10. Three different pitching kinematics: (i) pure sinusoidal, (ii) and (iii) a smoothened trapezoidal waveform with base lengths of 0.2 and 0.4 times the duration of one hovering cycle, respectively, are followed to access the airfoil aerodynamic performance. The pitching amplitude is kept at 30°. It is observed that the time-averaged lift coefficient over a hovering cycle is 11.62% and 9.09% higher for case (iii) hovering mode compared to case (i) and case (ii), respectively. However, the time-averaged aerodynamic power coefficient for case (iii) hovering mode is 62.96% and 57.14% higher than case (i) and case (ii), respectively. The best energy efficiency hovering mode is seen in case (i) by comparing the ratio of the aerodynamic lift to power coefficients. The near-wake flow features at different hovering modes will be thoroughly discussed in the presentation.