The role of effective angle of attack in hovering pitching-flapping-perturbed revolving wings at low Reynolds number

Abstract

Since the performance of revolving wings is limited at a low Reynolds number (Re), the pitching-flapping-perturbed revolving wing (PFP-RW) is proposed as an approach for an augmentation of lift and efficiency. However, the underlying physics of the effective angle of attack (αe) in aerodynamic force generation is underexplored. Here, as a follow-up of our previous study [L. Chen et al., “Unsteady aerodynamics of a pitching-flapping-perturbed revolving wing at low Reynolds number,” Phys. Fluids 30, 051903 (2018)], we further investigate the role of αe in aerodynamic force generation and the corresponding leading-edge vortex (LEV) behaviors of a hovering PFP-RW at Re = 1500. Results show that the efficiency of a PFP-RW with sinusoidal flapping motion can be improved by a sinusoidal modulation of the αe profile while retaining the αe amplitude. Instead of the αe amplitude, if the flapping amplitude is fixed during the sinusoidal modulation of the αe profile, the lift of a PFP-RW is significantly enhanced despite a slight reduction in efficiency. For PFP-RWs with different pitching-flapping perturbations but an identical αe profile, a general trend of resultant-velocity-normalized lift and drag coefficients in the wind frame is predominantly retained, except for the deviations during the late downstroke. The breakdown of the general trend is attributed to the wing-LEV interaction, the effect of which is not reflected by the instantaneous resultant velocity. For PFP-RWs with small-amplitude sinusoidal αe profiles, the variation of pitching-flapping perturbations can further lead to novel LEV behaviors, e.g., a dual-LEV structure, during its formation. Our findings can provide guidance for the control of pitching-flapping perturbations on PFP-RWs.