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Laser technique reveals bird-wing 'contrails'

07 Dec 2016

Particle imaging velocimetry shows how wing-beat vortices break up in similar manner to wake turbulence generated by airliners.

Researchers at Stanford University have used a high-speed laser imaging technique to directly view the wake turbulence generated by a bird in flight for the first time – and discovered that the models currently used to calculate lift generated by animal wing-beats are wrong.

David Lentink and colleagues from the California university say that the particle imaging velocimetry (PIV) experiments showed how airflow vortices created by a parrotlet’s wing-beat patterns broke down violently.

That “vortex breakdown” is similar to the effect seen in the wake turbulence created by airliners, and its challenge to the accepted theories of bird-flight dynamics could have implications for engineers developing “bioinspired” drones based on flapping wings.

Bird laser safety goggles
The research team were able to train the parrotlet to fly through a laser “sheet” setup to generate the PIV signals from which the vortex breakdown could be seen and analyzed. The Stanford team even 3D-printed a pair of custom laser safety goggles for the parrotlet to wear for protection.

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.”

Having analyzed the three-dimensional dynamics of the tip vortex in great detail, the researcher says that the findings were surprising. “The bird’s tip vortices broke down violently and quickly about two or three wing-beats after they were generated,” Lentink explained. And although this breakdown is commonly seen in the tip vortices generated by airliners, creating the familiar sight of long contrails behind the plane, it has never previously been observed in bird flight.

“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,” Lentink added. “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.”

Experimental setup
To conduct the experiments, the Stanford team used a double-pulsed Nd:YLF laser from UK-headquartered Litron emitting at 527 nm and with a repetition rate of 1 kHz.

That was complemented with two pairs of Phantom Miro high-speed cameras and PIV analysis software from German firm LaVision, to record 4000 frames per flight. A fifth camera was used to film the bird from behind, to record its wing-stroke angle and position in the laser sheet during flight.

Writing in the journal Bioinspiration & Biomimetics, the team suggested that the rapid breakdown of the bird’s wingtip vortex shown by the PIV experiments implies that the common “vortex ring” model of animal flight is only applicable in the near wake of slowly flying birds.

It is also incompatible with the “frozen turbulence” theory used by many, with Lentink and colleagues concluding that their work “confirms the limitations [of the lift models] for slowly flying animals”.

“Our study shows that there is a need to improve the predictive strength of aerodynamic lift models based on measured animal wakes, in particular for lower advance ratios, and possibly higher ones as well,” they write. “Whereas previous studies created vortex wake cartoons that helped convey the general topology of the vortex wake to a broad audience, these models may be overly simplified.”

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