Meeting Abstract

21.5  Thursday, Jan. 3  Hovering aerodynamics in hummingbirds: comparing a dynamically-scaled robot with live birds. TOBALSKE, B.W.**; WARRICK, D.R.; DICKSON, W.B.; ALTSHULER, D.A.; DICKINSON, M.H.; Univ. of Portland; Oregon State Univ.; California Institute of Technology; Univ. of California, Riverside; California Institute of Technology tobalske@up.edu

To help elucidate the mechanisms that permit hovering in hummingbirds, we used digital particle image velocimetry (DPIV) to measure flow in the near-field and wake of the wings of live birds and of a dynamically-scaled, flat-plate, robotic wing (Re ~ 4000) that was equipped with force transducers at the wing root. Flow about the wings of live birds was laminar and lacked a stable leading-edge vortex (LEV) through most of the translational phases of the wingbeat In contrast, at mid-upstroke and mid-downstroke, the robotic wing always exhibited LEV�s and trailing-edge vortices (TEV�s). This reveals that a flat plate is not fully adequate for modeling flow about a cambered, flexible bird wing. Our measures of bound circulation on the robotic wing lead to estimates of aerodynamic force that matched measured force for upstroke but were 60% of measured force for downstroke, which suggests that LEV�s and TEV�s were not stable during downstroke. Estimates of force from circulation in the trailing tip vortices matched measured forces at the wing root when circulation was measured within one chord length (c) of the wing tip, but these estimates decreased to 50% of measured forces at 2c. The observed decay of circulation at 2c is likely due to instability of the shed vortices. Such instability should be considered when estimating forces using flow patterns in the wakes of flying birds. NSF IOB-0615648.