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

Abstract

Visualization of the vortex wake of a flying animal provides understanding of how wingbeat kinematics are translated into the aerodynamic forces for powering and controlling flight. Two general vortex flow patterns have been proposed for the wake of hovering hummingbirds: (1) The two wings form a single, merged vortex ring during each wing stroke; and (2) the two wings form bilateral vortex loops during each wing stroke. The second pattern was proposed after a study with particle image velocimetry that demonstrated bilateral source flows in a horizontal measurement plane underneath hovering Anna’s hummingbirds (Calypte anna). Proof of this hypothesis requires a clear perspective of bilateral pairs of vortices. Here, we used high-speed image sequences (500 frames per second) of C. anna hover feeding within a white plume to visualize the vortex wake from multiple perspectives. The films revealed two key structural features: (1) Two distinct jets of downwards airflow are present under each wing; and (2) vortex loops around each jet are shed during each upstroke and downstroke. To aid in the interpretation of the flow visualization data, we analyzed high-speed kinematic data (1,000 frames per second) of wing tips and wing roots as C. anna hovered in normal air. These data were used to refine several simplified models of vortex topology. The observed flow patterns can be explained by either a single loop model with an hourglass shape or a bilateral model, with the latter being more likely. When hovering in normal air, hummingbirds used an average stroke amplitude of 153.6° (range 148.9°–164.4°) and a wingbeat frequency of 38.5 Hz (range 38.1–39.1 Hz). When hovering in the white plume, hummingbirds used shallower stroke amplitudes (\( \bar{x} \) = 129.8°, range 116.3°–154.1°) and faster wingbeat frequencies (\( \bar{x} \) = 41.1 Hz, range 38.5–44.7 Hz), although the bilateral jets and associated vortices were observed across the full kinematic range. The plume did not significantly alter the air density or constrain the sustained muscle contractile frequency. Instead, higher wingbeat frequencies likely incurred a higher metabolic cost with the possible benefit of allowing the birds to more rapidly escape from the visually disruptive plume.

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Acknowledgments

We thank Benny Goller for assistance with data collection and thoughtful discussions about the research. David Lentink, Dimitri Skandalis and three anonymous reviewers provided helpful comments on the manuscript. Lev Darkhovsky and Jonathan Scott assisted with animal care. This research was supported by grants from the United States National Science Foundation (IOS 0923849) and the Natural Sciences and Engineering Research Council of Canada (402677).

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Correspondence to Douglas L. Altshuler.

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Sam Pournazeri and Paolo S. Segre contributed equally to the work.

Electronic supplementary material

348_2012_1439_MOESM1_ESM.mp4

The online video (S1) provides the image sequence of bird 6 during trial 5 from the rear-left perspective of the hovering hummingbird. The video shows the sequential development of a vortex loop on the left side of the bird. Frames 59-65 are depicted in figure 1. Frame rate speed has been reduced by 50 times. (MP4 4616 kb)

348_2012_1439_MOESM2_ESM.mp4

The online video (S2) provides the image sequence of bird 6 during trial 3 demonstrating the frontal perspective of the bird while hovering. The development of vortex loops on the left and right side of the bird as well as the presence of wingtip and root vortices on the left side of the bird can be observed. This sequence is depicted in figure 2, with frame 101 in panels a and b, and frame 165 in panels d and e. Frame rate speed has been reduced by 50 times. (MP4 4615 kb)

348_2012_1439_MOESM3_ESM.mp4

The online video (S3) provides the image sequence of bird 5 during trial 2 from an off axis rear perspective with a wide field of view of the hovering hummingbird. This video provides a view of the shape of the vortex tube connecting the vortices. Frame 58 is depicted in figure 2 (g,h). Frame rate speed has been reduced by 50 times. (MP4 4271 kb)

348_2012_1439_MOESM4_ESM.mp4

The online video (S4) provides the image sequence of bird 6 during trial 9 from the lateral perspective of the bird while hovering. Evolution of distal vortices and the wingtip paths during the down- and up-strokes can be observed. Frame 37 is depicted in figure 2 (j,k). Frame rate speed has been reduced by 50 times. (MP4 4437 kb)

348_2012_1439_MOESM5_ESM.mp4

The online video (S5) provides the image sequence of bird 4 from the rear perspective during hovering. This bird used the highest wing stroke amplitude and lowest wingbeat frequency when hovering in the plume. This video provides views of the wing tip and reversal vortices on the animal’s left side. Frames 347-355 are depicted in figure 3. Frame rate speed has been reduced by 50 times. (MP4 10831 kb)

348_2012_1439_MOESM6_ESM.m4v

The online video (S6) provides a 3D animation of the merged ring vortex model constructed using the available kinematic data. (M4V 4634 kb)

348_2012_1439_MOESM7_ESM.m4v

The online video (S7) provides a 3D animation of the indented merged vortex loop model constructed using the available kinematic data. (M4V 4609 kb)

348_2012_1439_MOESM8_ESM.m4v

The online video (S8) provides a 3D animation of the bilateral vortex loops model constructed using the available kinematic data. (M4V 4596 kb)

348_2012_1439_MOESM9_ESM.m4v

The online video (S9) provides a 3D animation of the concentric vortex rings model constructed using the available kinematic data. (M4V 4639 kb)

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Pournazeri, S., Segre, P.S., Princevac, M. et al. Hummingbirds generate bilateral vortex loops during hovering: evidence from flow visualization. Exp Fluids 54, 1439 (2013). https://doi.org/10.1007/s00348-012-1439-5

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  • DOI: https://doi.org/10.1007/s00348-012-1439-5

Keywords

  • Vortex
  • Particle Image Velocimetry
  • Vortex Ring
  • Vortex Loop
  • Wingbeat Frequency