The research objective of this proposal is to understand the unsteady, 3D flow structure produced by a flapping wing in hover, how it relates to the forces, and the effect of key parameters such as stroke amplitude. In nature, flapping wings have low aspect ratios (ARs), and due to the high angles of attack and low Reynolds numbers flow separation and roll-up into vortices occurs.
Intellectual Merit Wings in hover are known to generate highly 3D, interconnected vortex loops, which shed each half-stroke and create an overall downward jet-like flow to sustain the body weight. Prior experiments using animals or mechanical models, coupled with flow visualization or 2D digital particle image velocimetry (DPIV), have shed light on the loop structure, but the 3D vortex topology and its relationship to the lift force remain open topics. Computational studies focusing primarily on animal configurations provide greater detail and insight. However, there is a lack of experimental, 3-component 3D DPIV data, and the effects of parameters such as half-stroke amplitude, AR, and motion program on the flow structure are not well understood. A key question is: why do hovering animals flap with amplitudes of 3-5 chord lengths? The approach is to perform experiments using a 2-degree-of-freedom flapping-wing model with simplified wing geometries and motions. The goals are to understand the 3D vortex topology, its variation with half-stroke amplitude, AR, and velocity program, and how it relates to the forces. Of interest are changes in the vortex structure for different cases. Diagnostics include dye visualization, stereoscopic DPIV (SDPIV) for 3-component velocity fields, and force measurements.
Broader Impacts: This research will provide valuable insight into the design of highly-maneuverable bio-inspired micro air vehicles (MAVs). They can collect scientific information in complex environments, e.g. urban settings, caves, and disaster sites such as collapsed buildings after an earthquake, where conventional drones cannot operate. Moreover, they could operate efficiently in swarms to track the spread of a toxic gas plume, for example. The results of the proposed research will yield a greater understanding of the unsteady vortical flow and how kinematics and AR affect maximum lift, yielding design strategies for flapping-wing MAVs. The educational and outreach component, which focuses on engaging and mentoring students at many levels through hands-on research in bio-inspired propulsion, will motivate them to pursue higher education and careers in STEM. This addresses the need for a national workforce of exceptional scientists and engineers.
