Pitch perturbation effects on a revolving wing at low Reynolds number

Low-aspect-ratio revolving wings find applications in miniature robotic flyers and turbo -machinery. At a high angle of attack, the flow separates from a wing's leading edge, forming a leading-edge vortex. On a revolving wing, the leading-edge vortex (LEV) is stabilized by strong rotational acceleration and is known to be the primary source of high and stable aerodynamic forces acting on the wing. While previous studies have characterized the performance of revolving wings, the effects of perturbations on the flow profile and performance have not been explored. This study combined experiments and computational fluid dynamics simulations to investigate the lift, drag, and power coefficients of a wing revolving at a Reynolds number of Re = 2500 and undergoing pitch perturbations. Perturbations were systematically varied to investigate the effects of the wing's initial angle, the amplitude and duration of perturbation, and the location of the pitch axis. Simulations provided insights into the flow structures around the wing and their effects on the wing-surface pressures and the aerodynamic forces. Finally, a quasisteady model was used to decompose the effects of wing rotation and pitch perturbations on the lift and drag forces. The decomposition revealed a consistent dependency of the variations in the lift on the pitch angular velocity across all perturbations, irrespective of the initial angle. The transition from the LEV-dominant to the rotation-dominant flow occurs at very low pitch rates, the values of which were found to depend on the initial angle of the revolving wing. For lower initial angles, the transition occurred at lower pitch rates. During the rotation-dominant perturbations, changes in the vortical structures around the wing were found to have a minimal effect on the lift and drag compared to the quasisteady effects. Moreover, the oscillations in the lift and drag during the perturbation could be reduced by appropriately shifting the pitch axis location. These findings highlight the dominant role of inviscid effects on the variations in loads acting on a revolving wing during pitch perturbations.