RANOLA, J.O.**; TOBALSKE, B.W.; WARRICK, D.R.; POWERS, D.R.; Univ. of Portland; Univ. of Portland; Oregon State Univ.; George Fox Univ.: Circulation in the Wake of the Flying Hummingbird: Effects of Thresholding and Vortex Decay

Recent physical modeling has shown that measures of circulation in the wake of flying animals may underestimate bound circulation on their wings due to rapid, turbulence-induced vortex decay. An upper size limit for accurate measures was suggested to be at a ring Reynolds number (Re) of 17000. We used digital particle image velocimetry (DPIV) to test for effects of vortex decay on measures of circulation in the wake of one of the smallest bird species, rufous hummingbird (Selasphorus rufus, 3 g, n = 6) hovering and flying at speeds from 2 to 12 m/s in a wind tunnel. Over this speed range, Re varied from 12,000 to 84,000. We also tested the effects of threshold upon measured circulation. We sampled vortex cores in a parasagittal plane that intersected the midwing of the bird, with an interrogation area between 2 and 25 chord lengths behind the animal. Our results reveal that the decay of circulation and peak vorticity is a function of Reynolds number, with wake decay being significant at Re > 28,000 ( > 4 m/s). For example, at Re = 56,000 (8 m/s), measured circulation declined by a factor of 3 over a distance of 20 chord lengths. Also, the currently accepted method for wake analysis, which thresholds using 20% of the peak value, was more conservative in the region of interest than thresholding using 3 standard deviations away from the average free-stream noise. This discrepancy was most apparent at Re < 28,000 (< 4 m/s), which indicates that effects of thresholding change as a function of flight velocity. Clearly, wake decay must be taken into account when estimating bound circulation in any bird species, and the effects of thresholding should be explored when bracketing measures of circulation. NSF IBN-0327380