Human understanding of the mechanisms of animal flight is entering into an exciting new phase. Over the past century, many studies have sought to estimate the aerodynamic function of flapping wings from patterns of wing motion measured using high-speed film and video. A new technique, digital particle image velocimetry (DPIV) directly reveals patterns of airflow, and thereby permits direct measurement of the aerodynamic mechanisms produced by these wing movements. A surprising result that immediately emerged from the recent DPIV studies of the principal investigator is that hovering hummingbirds, long thought to use their wings in a symmetrical manner similar to insects, depend much more upon their downstroke than their upstroke. This finding has the potential to completely revise thinking about the evolution of hovering in birds. To extend this observation, this research project will explore in detail the aerodynamics of hovering and slow-speed maneuvering in hummingbirds by examining near-field flow. This research will be the first use of stereo DPIV in flying birds, revealing flow in three dimensions - an advance vital for characterizing unsteady flow patterns. These measurements will allow full tests for the presence of unsteady aerodynamic mechanisms in birds, which have been reported for insects. To determine the functional significance of putative unsteady mechanisms to the force balance in hovering, the investigators will program observed 3D hummingbird kinematics into a dynamically scaled robot, test for the congruence in near-field/far field flow patterns, and measure moments acting about the wing of the robot. Sampling in the near field and wake of the robot will enable tests of the validity of using far field vortex-wake structures to estimate time-averaged forces during flapping flight. Ultimately, the results of this research will improve basic understanding of the extent to which hovering hummingbirds converge with insects in wing function, refine robotic simulations of animal flight, and determine the kinematic and aerodynamic basis of stability and maneuverability in hummingbirds. More broadly, the research will elucidate biological manipulation of unsteady and quasi-steady aerodynamics, thereby providing engineers with a useful model for the development of autonomous micro-air vehicles, and providing computational fluid dynamicists with precise kinematic and DPIV data to incorporate into models of hummingbird flight. The computational models may, in turn, be used to test hypotheses regarding the evolution of hummingbird's unique flight style. Undergraduate collaborators are involved in all aspects of this cross-disciplinary research, and the DPIV instrumentation used in this study will foster collaborations across regions of the United States. Because hummingbirds in particular generate great public fascination, the project scientists will continue active collaboration with professional artists and science writers to disseminate DPIV images and insights about hummingbirds through artistic venues and the popular press.
