February 26, 2013
Hovering Hummingbirds Generate Two Trails Of Vortices Under Their Wings
[ Watch the Video: Hummingbird Hovering ]
April Flowers for redOrbit.com - Your Universe Online
Until now, scientists believed the bird's remarkable flight generated a single trail of vortices in its wake that helps the bird hover. A research team, led by the University of California, Riverside, conducted experiments in the laboratory that suggest the hovering hummingbird produces two trails of vortices — one under each wing per stroke — that helps to generate the aerodynamic forces required for the bird to power and control its flight.
The results of this study, published online last month in Experiments in Fluids, could have wide applications in aerospace technology and the development of unmanned vehicles for medical surveillance after natural disasters.
The research team created a white plume by heating dry ice and then used high-speed image sequences — 500 frames per second — of hummingbirds hover-feeding to study the vortex wake from multiple perspectives. Additionally, they use particle image velocimetry (PIV), a flow-measuring method generally used in fluid mechanics, to do a quantitative analysis of the flow around the birds. The scientists were able to record the particles surrounding the birds and extract velocity fields using PIV.
Two distinct jets of downward airflow — one under each wing of the hummingbird — were shown by the films and velocity fields that also revealed vortex loops around each jet, which are shed during each upstroke and downstroke of the wings.
The scientists suggest the hummingbird's two wings form bilateral vortex loops during each wing stroke. This is extremely advantageous for maneuverability.
"Previous studies have indicated that slow-flying bats and faster flying birds produced different structures in their wakes," said Douglas Altshuler, formerly an assistant professor of biology at UC Riverside, whose lab led the research. "We have been investigating the wake structure of hovering hummingbirds because this allows us to decouple the effects of different types of wings – bat versus bird – from different forward flight speeds."
Hummingbirds are tiny, weighing between 2 and 20 grams each. They are able to drink nectar from flowers without any jiggling movement of their bodies because they can hover with high precision. The birds are able to rotate their wings, in addition to using up and down strokes, they can even flap their wings from front to back with a 180-degree amplitude.
"We began this study to investigate how the hummingbird used its tail while hovering," said Marko Princevac, an associate professor of mechanical engineering. "After all, many insects also hover, but they have no tail. Instead, however, our research showed something interesting about the hummingbird's wings: the bilateral vortex structure. Hummingbirds hovering should cost a lot of energy but these birds are able to hover for long periods of time. Ideally, unmanned vehicles need to be operated with a very limited energy supply, which is why understanding how the hummingbird maximizes its use of energy is tremendously beneficial."
In a downstroke, the air pressure difference developed as a result of wing movement creates flow from the bottom to the top of the wing, resulting in a circular movement or vortex, Sam Pournazeri, a former PhD graduate student in Princevac's lab, explained.
"Based on theories in fluid mechanics, this vortex should close either on the wing/body or create a loop around it," he said. "It's these loops that provide circulation around the wings and cause the hummingbird to overcome its weight. Hovering requires the bird to create a lift that cancels its body weight. Although the two-vortex structure we observed increases the hummingbird's energy consumption, it provides the bird a big advantage: a lot more maneuverability."
In the future, the team plans to study the hummingbird in a wind tunnel to observe how the bird transitions from forward motion to hovering and back again.
"Current technology is not successfully mimicking how living things fly," Princevac said. "Drones don't hover, and must rely on forward motion. Research done using hummingbirds, like ours, can inform the development of the next generation of drones."