Vortex Wakes Of Bird Flight: Old Theory, New Data AndFuture Prospects

Abstract

Flying birds leave a vortex wake. Fluid dynamic theory in the form of Helmholtz’ theorems dictate the allowable topologies, and Kelvin’s circulation theorem requires that changes in wake circulation are directly proportional to force changes on the wing/aerofoil that generated the wake. Much bird flight research has therefore been focused on the properties of trailing wake vortices behind birds, since an accurate quantitative description of these will reveal also the aerodynamics of bird wings. The first vortex theory of bird flight assumed the periodic shedding of discrete vortex loops, each one generated during a downstroke, while the upstroke was considered aerodynamically functionless. This view received some support from early visualization experiments of take-off flight or very low speeds, while experiments at a higher speed (U = 7 m/s) in one species showed undulating wing-tip vortices of similar circulation on both down and upstroke. The necessary force asymmetry between downstroke and upstroke was obtained by wing flexing during the upstroke. Then followed an almost 20-year drought, with no further quantitative experiments, until recently when digital particle imaging velocimetry (DPIV) was successfully deployed in a low-turbulence wind tunnel, and where the same small (30 g) bird could be studied across a large range of flight speeds (4–11 m/s). These new experiments revealed a much more complicated wake pattern than previous data suggested, mainly due to the improved experimental resolution. The bird generated structures most closely resembling vortex loops at slow speeds, which gradually transformed into something similar to a constant circulation wake at the highest speeds. However, the wakes were never as clean as the idealized cartoon models of the vortex theory of bird flight, and previous paradoxical results were shown to be attributable to the resulting difficulty in accounting for all wake components. New DPIV data on other species indicate that these findings are quite general.