|M.Sc Student||Achache Yonathan|
|Subject||On the Annual Cycle of a Hovering Hummingbird Wing|
An Aerodynamics and Fluid Mechanics Perspective
|Department||Department of Autonomous Systems and Robotics||Supervisors||Professor Amir Gat|
|Dr. Yossef Elimelech|
The diverse hummingbird family (Trochilidae) has unique adaptations for nectarivory, among which is the ability to sustain hover-feeding. As hummingbirds mainly feed while hovering, it is crucial to maintain this ability throughout the annual cycle - especially during flight-feather moult, in which wing area is reduced. The goal of this work is to quantify the aerodynamic characteristics and flow mechanisms during the wing's annual cycle. To do so, time-accurate aerodynamic loads and flow field measurements were correlated over dynamically scaled wing models of the Calypte anna's hummingbird. First, we present measurements recorded over a model of a complete wing in order to evaluate the baseline aerodynamic characteristics and analyse the flow mechanisms involved. Then, we apply the same experimental methodology and analysis over the wing at different natural stages of moult.
Results indicate about a drop of more then 20% in lift production during the earlier stages of moult. The largest drop in wing effectiveness was recorder as the wing replaced it's medial flight feathers. Furthermore, the outer primaries and leading-edge integrity was shown to be highly significant to the wing's ability to generate lift, explaining the high feather overlap at the wingtip and the sequential replacement of the feathers, resulting in very small wing surface reduction at the later stages of moult (less then 2%). Time-accurate aerodynamic loads were also utilised to evaluate the time-evolution of the specific power required from the flight muscles and the overall wingbeat power requirements. These power analyses suggested a distinct link to behavioural repertoire and body mass reduction characterising moulting hummingbirds. The latter mechanism is employed to meet hovering power requirements under reduced wing area during moult.
Flow field measurements described the vorticity concentration that had developed from the wing leading-edge, which, differs from the attached vorticity structure that is typically found over insects wings; firstly, the vorticity concentration over the Calypte anna wing is more elongated along the wing chord and secondly, it encounters high levels of fluctuations rather than a steady vortex found over insects wings. Modal investigation associated the periodic shedding of these vorticity structures to the time-accurate load measurements. Apparently, lift characteristics resemble those of insects for similar aspect ratio wings. However, a 20% increase in lift-to-torque ratio was obtained for the hummingbird wing model. An increase shown to be associated to the wing camber.