As the bat flies
Graduate, undergraduate, and postdoctoral researchers from Brown took the stage on Sunday during the 61st annual meeting of the American Physical Society (APS) Division of Fluid Dynamics, November 23 to 25, in San Antonio, Texas.
At the conference, researchers from across the globe describe cutting-edge research with applications in astronomy, engineering, alternative energy, biology, and medicine. The Brown projects originate in the Fluid Mechanics Laboratory led by Professor of Engineering Kenny Breuer (who attended both presentations) in collaboration with Professor Sharon Swartz in ecology and evolutionary biology.
Tales told by flapping bat wings
Biologist Tatjana Hubel, a postdoctoral research scholar, and her Brown colleagues have trained lesser dog-faced fruit bats (Cynopterus brachyotis) to fly in a low-speed wind tunnel. As they fly, six high-speed cameras monitor both the bats’ wing motion and the air wake it creates.
Flapping flight evolved at least four times in evolutionary history, inbirds, insects, pterosaurs, and bats. Of the four, bats have the most flexible wings, controlled with extremely complex motions that scientists are only just beginning to understand.
To study the dynamics of bat flight, Hubel’s group simultaneously monitored the changing fluid structure of the wake generated by the bat’s flapping wings and details of the wing motion. The latter “allows us to link the effect (the aerodynamic forces) with the cause (the actual motion of the wing) for the first time,” Hubel says.
The studies revealed that a vortex is generated at the tip of the bat’s wing during the first third of its downstroke; it grows stronger before dissipating during the latter half of the upstroke. The vortex indicates that the bat’s wings are generating lift, says Hubel. The fact that the vortex dissipates, she says, shows that there is a part of the bat’s flapping cycle when its wings are producing no lift at all – a pattern similar to that seen in small birds.
Hubel’s talk, “Wake Structure and Wing Motion in Bat Flight,” took place Sunday afternoon, November 23. Read the abstract.
Are flexible flying machines in our future?
Modern aircraft perform well with rigid wings and rotors. But just imagine the flying machines that would be possible if we could understand and harness the most efficient and acrobatic airfoils in nature: the flexible wings of the bat.
The aerodynamics of “compliant” structures, such as bat wings, are complicated because both the structure and airflow change adapt to each other in a nonlinear way. Bats’ wing bones are flexible, unlike those of birds; this quality gives the mammals more control but is an additional challenge for scientists trying to understand the way they work.
Kenny Breuer’s research group is designing a series of fundamental experiments that will allow scientists to isolate, observe, and analyze specific flow-structure interactions that are important in understanding bat flight and, more generally, the aerodynamics of compliant structures. Ultimately, Breuer expects that experiments like these will yield insights enabling the development of flying machines that are impossible to consider today.
At the APS conference, graduate student Arnold Song and undergraduate Max Tuttman ’09 described the basic motions – and their aerodynamic implications – that their research group has discovered so far by measuring how paddles and stretched ribbons of sailcloth vibrate in manmade breezes in a wind tunnel. As the airflow increases, for example, a paddle on a post first twists and then flaps, like a stop sign pummeled by hurricane-force winds. The ribbon’s behavior, while more complicated, is also essential for understanding how bat wings or other compliant structures generate lift so efficiently.
The students’ talk, “On Vortex Induced Motion in Compliant Structures,” took place Sunday morning, November 23. View the abstract.