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Small birds, big science

by Travis Tate

They float and flutter away in mere seconds, but a lot happens in the fleeting moments people observe a hummingbird at a flower.

A recent study—done with the help of XSEDE-allocated resources—has figured out the mathematical mechanisms behind their flight.

In a paper published by the Journal of The Royal Society Interface, Jialei Song, Haoxiang Luo, and Tyson L. Hedrick attempted to quantify the way in which the hummingbird is able to hover. The authors used XSEDE’s environment of supercomputers, data storage, and tech support to help complete the work.

“This work is primarily based on large-scale computational fluid dynamics modeling for hummingbird flight. For a hummingbird with only a 10 centimeter wingspan, the unsteady aerodynamics is complex enough to require millions of mesh points to resolve the many, many small vortices stirred up by the wings,” says Luo, an assistant professor of mechanical engineering at Vanderbilt. “Therefore, high-performance computing is a must for this work, and XSEDE provides the essential computing resources we needed.”

Hedrick is an associate professor in biology at the University of North Carolina at Chapel Hill; Song is a research assistant under Luo.

“Our work is sponsored by NSF, so it is natural to apply for an allocation from XSEDE,” Luo continued. “It is free and it provides high-quality service support.”

Modeling that flight first asked the researchers to film a female ruby-throated hummingbird—the smallest breeding bird in the Eastern United States—with four-high speed cameras, each shooting 1,000 frames per second with a 1/5,000th shutter.

Each flapping cycle—one downstroke and one upstroke—took about 25 frames. Then they constructed a computational fluid dynamics (CFD) model based on the wing motion extracted from the video.

When the XSEDE-allocated Lonestar supercomputer at the University of Texas at Austin was used to help digest the data from the intense simulation, the researchers learned that the wing’s downstroke does a majority of the work while the hummingbird floats, even though the bird reverses its wings during the upstroke and sweeps them in a similar way as in the downstroke. This difference between the downstroke and upstroke is what the authors call asymmetry.

The researchers were able to discover by examining each stroke how the forces on the wing vary at each moment, and determined the downstroke generates more than two times the amount of vertical force as the upstroke.

There are a number of aerodynamics facts to keep in mind during each moment of the full flapping motion. These include the drag caused by the wing, rotation around the long axis, the interaction between the wing and the draft following a preceding stroke (like the wake of a moving body of water), in addition to speed and angle of attack (the inclination angle between wing surface and its direction of translation).

Each of these factors plays a part in keeping those hummingbirds aloft.

According to the paper, “Our result confirms and provides specific data for the lift asymmetry that was previously suggested based on the measurement of the circulation around the wing…the downstroke produces 150% higher vertical force than the upstroke. Among the factors, the wing area contributes 10% greater force, the drag-based effect contributes 60%, the wing-wake interaction contributes 30% and the remaining 50% can be attributed to the combined wing speed, angle of attack and wing rotation.”

Essentially, the downstroke differs from the upstroke in more ways than one—the wing is twisted and sweeps through air in a manner that allows for the path of easier resistance on the upstroke, while the full power, pitching, and speed of the wing is used during the downstroke.

Thanks to having access to the power of XSEDE, the authors were able to discover these specific numbers, and for that the authors are grateful.

“XSEDE is critical for our research,” Luo says.

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