Digital Velocimetry Exposes Hummingbird Flight
Annie L. Fischer
Hummingbirds are fascinating to watch because, unlike other birds, they can hover in one spot for periods of time. The birds have been likened to insects in the way they flap their wings, but researchers from Oregon State University in Corvallis, the University of Portland and George Fox University in Newberg, all in Oregon, have dispelled this notion using digital particle imaging velocimetry to measure the wake of hummingbird flight.
Researchers have used digital particle imaging velocimetry to study hummingbird flight. They placed the hovering birds in a wind tunnel seeded with droplets of olive oil, which were illuminated with pulses of light from an Nd:YAG laser.
Digital particle imaging velocimetry is commonly used to study the flow dynamics of liquids and gases. In their work, the researchers placed hummingbirds in a wind tunnel that was seeded with droplets of olive oil, and took a series of paired images as the birds hovered at a feeder. Their tiny wings stirred the air, moving the floating oil particles, which were captured by the imaging technique.
Douglas R. Warrick, assistant professor of zoology at Oregon State, compared the first views to the experience of an astronomer who peers through a new and larger telescope. "The universe just gets bigger; ours just got bigger in the opposite direction," he said.
The digital particle imaging velocimetry system from LaVision GmbH of Göttingen, Germany, includes a 1376 × 1040-pixel black-and-white interline CCD camera from PCO AG of Kelheim, Germany. The camera can capture successive images 500 ns apart. A 50-mJ dual-cavity pulsed Nd:YAG laser from New Wave Research Inc. of Fremont, Calif., lighted the flow field as the images were taken. The images were then sent to a computer running LaVision image processing software, which tracked the captured oil particles and processed a dozen vector fields in about a minute.
Another camera, a PCI-interface CCD from Redlake MASD LLC of San Diego, took simultaneous digital video at 500 frames per second from above the bird to allow a more thorough view of its wing movements.
According to Warrick, the main challenge for the investigators was capturing images of a flitting hummingbird in a field of view measuring only 16 × 12 cm. "But with the feeder in a fixed position," he said, "they will fly exactly where we want them, at any speed, from 0 (hovering) to 12 m/s."
The researchers collected hundreds of images over a period of months, from which they calculated airflow vector fields. From that data, they reconstructed the airflow created by the up-and-down movement of the bird's wings. They found that, unlike insects, which support half their weight with each stroke, hummingbirds support 75 percent of their weight during the downstroke and 25 percent with the upstroke. Other birds use only the downstroke to support their weight.
Warrick and his team hope to upgrade to a 3-D visualization of flow and to apply it to the maneuvering flight of hummingbirds, as well as to their closest relatives, the swifts. They also plan to apply the technique to the study of other functions of other varieties of birds.
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