Performance of passively pitching flapping wings in the presence of vertical inflows

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The successful implementation of passively pitching flapping wings strongly depends on their ability to operate efficiently in wind disturbances. In this study, we experimentally investigated the interaction between a uniform vertical inflow perturbation and a passive-pitching flapping wing using a Reynolds-scaled apparatus operating in water at Reynolds number ≈3600. A parametric study was performed by systematically varying the Cauchy number (Ch) of the wings from 0.09 to 11.52. The overall lift and drag, and pitch angle of the wing were measured by varying the magnitude of perturbation from J Vert = −0.6 (downward inflow) to J Vert = 0.6 (upward inflow) at each Ch, where J Vert is the ratio of the inflow velocity to the wing’s velocity. We found that the lift and drag had remarkably different characteristics in response to both Ch and J Vert. Across all Ch, while mean lift tended to increase as the inflow perturbation varied from −0.6 to 0.6, drag was significantly less sensitive to the perturbation. However effect of the vertical inflow on drag was dependent on Ch, where it tended to vary from an increasing to a decreasing trend as Ch was changed from 0.09 to 11.52. The differences in the lift and drag with perturbation magnitude could be attributed to the reorientation of the net force over the wing as a result of the interaction with the perturbation. These results highlight the complex interactions between passively pitching flapping wings and freestream perturbations and will guide the design of miniature flying crafts with such architectures.