22 Oct 2003
Particle imaging velocimetry reveals how birds produce enough lift to support their body weight in flight.
Researchers in Sweden have used particle imaging velocimetry (PIV) to analyze the air vortexes formed in the wake of a flying nightingale. After studying the bird in a specially-constructed wind tunnel, the scientists believe they can now explain how a bird produces enough lift to counterbalance its body weight.
The research has solved the long-standing “wake momentum paradox”. In the 1980s experiments on the subject suggested that a bird in flight generates vortexes with just half of the momentum needed for flight; the other half was unaccounted for. Hedenstrom’s approach resolves the fine structure of the vortexes and reveals where the elusive 50% of the momentum comes from.
“We were interested in how the wake vortex topology and properties such as vorticity and circulation change with speed,” Anders Hedenstrom from Lund University told Optics.org. “The topology of the wake structures was far more complicated than previous aerodynamic models suggested.”
Before the experiments could even begin, Hedenstrom and colleagues spent over two months training a nightingale to fly in the center of the tunnel at wind speed ranging between 4 – 11 meters per second. Once the bird was trained, the tunnel was filled with a thin fog of 1 micron water particles.
The team then fired two 120 mJ 532 nm pulses from an Nd:YAG laser into the tunnel in quick succession. “The laser light sheet was aligned with the tunnel flow and placed approximately one meter downstream from the flying bird,” explained Hedenstrom. “In this way, we get a series of pictures of how the particles of fog move.” This essentially gives the researchers a cross-section of the structure of the wake.
“The “missing” vorticity was found in an unexpected place,” said Hedenstrom. “We found that the transition between the upstroke and the downstroke [of the bird’s wings] contained a mosaic of positive and negative vorticity. Adding together all the positive vorticity we get a 100% weight support.”
Hedenstrom now plans to study the flight of other birds including house martins and swifts.