Wake patterns of the wings and tail of hovering hummingbirds

Abstract

The flow fields of slowly flying bats and fasterflying birds differ in that bats produce two vortex loops during each stroke, one per wing, and birds produce a single vortex loop per stroke. In addition, the circulation at stroke transition approaches zero in bats but remains strong in birds. It is unknown if these difference derive from fundamental differences in wing morphology or are a consequence of flight speed. Here, we present an analysis of the horizontal flow field underneath hovering Anna's hummingbirds (Calypte anna) to describe the wake of a bird flying at zero forward velocity. We also consider how the hummingbird tail interacts with the wake generated by the wings. High-speed image recording and analysis from three orthogonal perspectives revealed that the wing tips reach peak velocities in the middle of each stroke and approach zero velocity at stroke transition. Hummingbirds use complex tail kinematic patterns ranging from in phase to antiphase cycling with respect to the wings, covering several phase shifted patterns. We employed particle image velocimetry to attain detailed horizontal flow measurements at three levels with respect to the tail: in the tail, at the tail tip, and just below the tail. The velocity patterns underneath the wings indicate that flow oscillates along the ventral-dorsal axis in response to the down-and up-strokes and that the sideways flows with respect to the bird are consistently from the lateral to medial. The region around the tail is dominated by axial flows in dorsal to ventral direction. We propose that these flows are generated by interaction between the wakes of the two wings at the end of the upstroke, and that the tail actively defects flows to generate moments that contribute to pitch stability. The flow fields images also revealed distinct vortex loops underneath each wing, which were generated during each stroke. From these data, we propose a model for the primary flow structures of hummingbirds that more strongly resembles the bat model. Thus, pairs of unconnected vortex loops may be shared features of different animals during hovering and slow forward flight.

Hummingbirds generate bilateral vortex loops during hovering: evidence from flow visualization

“…These new but still simplified models are based upon previous hypotheses of the single (Rayner 1979) and bilateral (Altshuler et al 2009) vortex loops and other possible explanations for the flow patterns observed here (Fig. 5).…”

“…It had been proposed that a hovering hummingbird generates one vortex ring per stroke (Rayner 1979;Ellington 1984;Pennycuick 1988;Rayner and Gordon 1998), which would match the vortex shedding pattern proposed for larger birds during slow flight (Spedding et al 1984(Spedding et al , 2003. Altshuler et al (2009) made PIV measurements in a horizontal plane beneath the hummingbirds close to the tail. These measurements revealed source flows induced by vortices, which appeared on each side of the animal at an interval of double the wingbeat frequency.…”

“…Previous PIV measurement of the horizontal plane underneath hovering Anna's hummingbirds, C. anna, provided vector topologies that implicated the presence of bilateral vortex loops in the wake (Altshuler et al 2009). However, only the source flows caused by the jets could be distinguished and not the associated vortices.…”

“…Previous PIV studies with hovering hummingbirds used 2D approaches that included descriptions of flows around cross-sections of the wing and thin slices from the lateral, rear, and underneath perspectives (Warrick et al 2005(Warrick et al , 2009Altshuler et al 2009). Proof of the bilateral vortex loop model requires a perspective of the 3D flow, which can be obtained by visualization of the flow field.…”

Hovering performance of Anna's hummingbirds ( Calypte anna ) in ground effect

“…It is well known that aircraft in ground effect experience pitch instability, which is usually corrected by a highly elevated tail plane in order to shift the centre of pitch downstream while maintaining the aerodynamic centre of height upstream (for details, see [33]). Altshuler et al [34] proposed that hummingbirds use their tails to deflect the flow created by the wings in order to maintain pitch stability Figure 6. Comparison of average weight-normalized mechanical power estimates for Anna's hummingbirds with theoretical estimates derived from two helicopter models for hovering in ground effect (IGE) and out of ground effect (OGE) ( [28,29]; as cited in [1,2]), with vertical thrust assumed to be constant.…”