Hovering hummingbirds produce two trails of vortices - one under each wing per stroke - that help generate the aerodynamic forces required for the bird to power and control its flight, according to researchers at the University of California.
The result of the study challenges the current 1-vortex consensus.
The new finding could find wide application in aerospace technology and the development of unmanned vehicles for medical surveillance after natural disasters.
The researchers used high-speed image sequences - 500 frames per second - of hummingbirds hover-feeding within a white plume (emitted by the heating of dry ice) to study the vortex wake from multiple perspectives.
They also used particle image velocimetry (PIV), a flow-measuring method used in fluid mechanics, to quantitatively analyze the flow around the hummingbirds. PIV allowed the researchers to record the particles surrounding the birds and extract velocity fields.
The films and velocity fields showed two distinct jets of downwards airflow - one under each wing of the hummingbird. They also revealed that vortex loops around each jet are shed during each upstroke and downstroke.
The researchers therefore proposed that the hummingbird's two wings form bilateral vortex loops during each wing stroke, which is advantageous for maneuverability.
Hummingbirds each weigh 2-20 grams. Because they can hover with high precision, they are able to drink nectar from flowers without any jiggling movement to their bodies. Besides using upstrokes and downstrokes, hummingbirds can rotate their wings. 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 and a coauthor of the research paper.
"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," Princevac noted.
Sam Pournazeri, a former Ph.D. graduate student in Princevac's lab and a co-author on the paper, explained that 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. The result is a circular movement or vortex.
"Based on theories in fluid mechanics, this vortex should close either on the wing/body or create a loop around it. 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," he said.
Next, the research team plans to study the hummingbird in a wind tunnel to closely observe how the bird transitions from hovering to forward motion, and vice versa.
Their research results were published online last month in the journal Experiments in Fluids. (ANI)