Cameras capture breakdown of tip vortices during bird flight

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High-speed cameras have been used to observe the vortices created as a bird flaps its wings. Scientists at Stanford University in California found that vortices broke down after two or three wing beats, which has never been seen before and challenges theories for calculating lift.

Using four cameras running at 1,000 frames per second, a high-speed laser and a willing slow-flying parrot equipped with custom 3D printed laser goggles, the researchers captured images of the wingtip vortices, rotating air left behind a wing as the bird generates lift.

The study, which could have implications for the development of flapping wing micro air vehicles, was published on 6 December in the journal Bioinspiration and Biomimetics.

Birds shed well-defined vortices in their wake as they fly, which are shaped by the aerodynamic lift forces generated by the wings and body. The vortices are tiny air tornadoes resulting from how the bird manipulates the air with its wings.

‘We measured the three-dimensional dynamics of the tip vortex in great detail, and what we found was surprising,’ explained senior researcher Dr David Lentink. ‘The bird’s tip vortices broke down violently and quickly about two or three wing beats after they were generated. Although this breakdown is commonly seen in the tip vortices generated by airliners… it has never been observed in bird flight before.’

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 a laser sheet, was captured with 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.

‘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,’ Dr Lentink said.

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