Researchers measure vortex breakdown in a bird’s wake for the first time

New research has shed light for the first time on how the breakdown of strong vortices birds create by flapping their wings limits our ability to calculate the lift they generate to fly.

Using a high speed laser, four cameras running at 1,000 frames per second, and a willing slow-flying parrotlet equipped with its own custom 3D printed laser goggles, engineers at Stanford University in California studied the tip vortices the bird generates to stay in the air.

Their study, which challenges three accepted theories for calculating lift, and may have implications for the development of flapping-wing micro air vehicles, is published in the journal Bioinspiration & Biomimetics.

Senior researcher Dr David Lentink said: “Freely flying animals shed well-defined vortices in their wakes, which are shaped by the aerodynamic lift forces generated by the wings and body. The vortices are little ‘air tornadoes’, which result from how the animal – in this case a bird – manipulates the air with its flapping wings to generate aerodynamic lift.

“We measured the three-dimensional dynamics of the tip vortex in great detail, and what we found was surprising.

“The bird’s tip vortices broke down violently and quickly about two or three wingbeats after they were generated.

“Although this breakdown is commonly seen in the tip vortices generated by airliners – when wingtip vortices merge with engine contrails they become visible, these thousands of meters long white stripes tend to break up violently into donut-shaped contrail plumes far behind the jet – it has never been observed in bird flight before.”

However, none of the research would have been possible without the creation of the custom laser safety goggles for the parrotlet.

First author, Mr. Eric Gutierrez, who designed the goggles said: “As part of the experiment, we needed the birds to fly through a laser sheet to study the vortices in their wakes. Of course, their eyes need protecting from the lasers just like my own eyes, so I developed laser goggles for the birds to wear.

“We salvaged the lenses from professional laser goggles we wear ourselves for visualising airflow with a laser sheet. Then we trained the parrotlet to fly between perches while wearing the ‘aviator goggles’, in return for a few seeds as a reward. The extra protection of the laser goggles, which only weigh one gram, enabled us to capture the up-close images used to reach our conclusions so without them, obtaining these results would have been much more difficult.”

To visualise the breakdown of the tip vortices generated by the bird in the lab, the researchers released a small amount of odourless particles, which tracked the motion of the tip vortices. The motion of the particles, generated as the bird flew through the laser sheet, was captured on high speed video and analysed to calculate the velocity of the air whirls.

It was the first time vortex breakdown has been observed and measured accurately in the wake of a bird.

Dr Lentink said: “Based on these velocity measurements, it is theoretically possible to calculate the lift force generated by the bird at the moment it sets the air in motion. However, we found that the three most commonly published lift models, and their many variations, failed to predict the lift generated by the bird. We thus found that not only the vortices, but also the well-accepted theories for calculating lift, broke down.”

The published version of the paper is avialable here: Lift calculations based on accepted wake models for animal flight are inconsistent and sensitive to vortex dynamics (Gutierrez et al, Bioinspiration and Biomimetics). 

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