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From dolphins to clams to flying creatures like hummingbirds and bats, many of nature's most fascinating creatures exhibit forms of fluid flow. When the 61st Annual Meeting of the American Physical Society (APS) Division of Fluid Dynamics takes place from November 23-25 at the San Antonio Convention Center, researchers from across the globe will describe cutting-edge research with applications in astronomy, engineering, alternative energy, biology, and medicine.
Reporters are invited to attend the conference free of charge. Registration instructions and other information may be found at the end of this news release. Several highlights from the more than 1,500 presentations at the meeting are listed below.
Flapping flight evolved at least four times in evolutionary history, in birds, 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 kinematics of bat flight, biologist Tatjana Hubel, a postdoctoral research scholar at Brown University, and her colleagues trained Lesser dog-faced fruit bats (Cynopterus brachyotis) to fly in a low-speed wind tunnel. As the bats flew, six high-speed cameras monitored both the motion of their wings and the air wake created by their flapping.
The researchers were able to simultaneously monitor the changing fluid structure of the wake generated by the bat's flapping wings and details of the wing motion, which "allows us to link the effect (the generated 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, then grows stronger, before dissipating during the latter half of the upstroke (with the wing beat cycle defined by the tip position). The vortex indicates that the bat's wings are generating lift; 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 that is similar to that seen in small birds.
Hubel's talk, "Wake structure and wing motion in bat flight," is at 4:36 pm on Sunday, November 23, 2008, in Room 103B of the San Antonio Convention Center.
The development of technologically sophisticated yet tiny aircraft--or micro air vehicles (MAV)--is a hot engineering topic, with researchers seeking new ways to add functional abilities to the craft. For example, says Humberto Bocanegra Evans, a graduate student in experimental fluid dynamics at New Mexico State University, "the creation of an MAV that has hovering capabilities would be of great help in reconnaissance missions into hostile territories or for the inspection of sites where human access is limited."
To explore the dynamics of hovering, Bocanegra Evans and his colleagues have focused on an animal that has perfected the technique: the hummingbird. He and his colleagues constructed a large scaled model of a rufous hummingbird, a common hummingbird species. In real life, the birds are about 3-4 inches long with a 4-inch wingspan; Bocanegra Evans's model has an 18.5-inch wingspan, with two 4.9-inch-long wings that are each controlled with two servo motors. One motor produces back and forth motions and the second changes the angle of attack of the wing. The robo-bird was "flown" in a large water channel seeded with tiny glass particles. A laser and camera tracked the particles as they flowed through the water as a result of the wing movements. From these particle trajectories, the scientists could determine the forces produced by the wings, "which will give us insight into how hovering is accomplished," Bocanegra Evans says. "By knowing the value of the forces at different wing positions, the production of lift and drag can be associated with specific kinematics."
Bocanegra Evans will discuss the preliminary results of the simulation in his talk, “Flight of a Rufous Hummingbird Robotic Model-PIV Measurements,” at 9:31 am on Monday, November 24, 2008, in Room 103B of the San Antonio Convention Center.
Since the 1930s, scientists have puzzled over the high speeds at which dolphins swim. Dolphins were not thought to be capable of generating enough thrust to overcome the drag they would experience at high speeds from the turbulent flow of water around their bodies--a conundrum known as Gray's paradox (after biologist Sir James Gray, who first noted it). That led biologists to speculate that dolphin skin must have unusual turbulence-reducing properties which let them swim fast.
Now, with the aid of a digital flow-tracking technology, fluid dynamicist Timothy Wei and his colleagues have, for the first time, directly addressed Gray's paradox--and proven it wrong.
Wei, a professor at Rensselaer Polytechnic University in New York, and his colleagues used Digital Particle Image Velocimetry (DPIV). In the technique, the fluid flow within a digital camera's field of view is determined by charting the movement of small particles distributed throughout the fluid. In experiments conducted at the Long Marine Laboratory at the University of California, Santa Cruz, the scientists measured the flows produced by two dolphins doing tailstands--a Sea World-worthy maneuver in which dolphins hold themselves vertically out of the water, using quick, strong oscillations of their tail. The data revealed that special skin wasn't necessary, because thrust produced by the dolphins' tails was "far more than necessary to overcome turbulent boundary layer drag," says Wei, who plans to use the DPIV technique to measure flow around other marine mammals. "These measurements create greater opportunities for addressing interesting and long-standing questions about swimming," he says.
Wei's talk, "DPIV measurements of dolphins performing tailstands," is at 8:13 am on Monday, November 24, 2008, in Room 102A of the San Antonio Convention Center.
Burrowing razor clams (Ensis directus) have the ability to tunnel into the sandy sea bottom with remarkable efficiency. According to Anette Hosoi, a professor of mechanical engineering at the Massachusetts Institute of Technology, the clams use 10 times less energy to dig and anchor themselves into the seafloor than the best man-made techniques currently used today.
To find out how the clams accomplish this, Hosoi, along with graduate student Amos Winter, visualized them in action. The scientists found that the clams open and close their shell in a synchronized manner as they dig. "By pushing itself UP with its foot before it digs down," Hosoi says, "the clam clears a space around the leading edge of the shell. Given the extra space, the sand at the leading edge of the shell can unpack." This "fluidizes" the normally solid seafloor in a small region surrounding their shells, so that it flows like a fluid.
Hosoi and her colleagues plan to design a robotic digger based on the method used by the clams. The goal, she says, "is to understand the fundamental mechanisms of optimized burrowing and then apply that to new anchoring technologies."
Winter will discuss the research in his talk, "Drag reduction mechanisms employed by burrowing razor clams (Ensis directus), at 9:44 am on Monday, November 24, 2008, in Room 203B of the San Antonio Convention Center.
Modern aircraft have been fabulously successful 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 very complicated because both the structure and airflow change and adapt to each other in a highly nonlinear way. Bats' wing bones are even flexible, unlike those of birds, which gives the mammals added control but is an additional challenge for scientists trying to understand them. Kenny Breuer's research group at Brown University is designing a series of fundamental experiments that will allow scientists to isolate, observe and analyze a variety of specific flow-structure interactions that are important in understanding bat flight and, in general, the aerodynamics of compliant structures. Ultimately, Breuer expects that experiments like these will yield insights enabling new generations of flying machines that are impossible to consider today.
In his talk at the 61st Annual Meeting of the APS Division of Fluid Dynamics in San Antonio, Arnold Song, who is one member of this group, will describe the basic motions -- and their aerodynamic implications -- that he and his colleagues at Brown have 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 being pummeled by hurricane-force winds. The ribbon's behavior is more complicated, but also essential for understanding how bat wings or other compliant structures generate lift so efficiently.
Song's talk, "On Vortex Induced Motion in Compliant Structures," will be held at 10:30 a.m. on Sunday, November 23, 2008, in Room 204B of the San Antonio Convention Center.
Congenital heart diseases affect one percent of newborn babies and babies who are lost in the prenatal period, and the malformation of the circulatory system is responsible for 10 percent of stillbirths.
To give scientists new insight into the developing heart, math biologist Laura Miller, an assistant professor of mathematics at the University of North Carolina, along with mechanical engineer Arvind Santhanakrishnan and graduate student Anil Shenoy, constructed several dynamically scaled models of the embryonic heart within rigid plexiglass walls. The models represent the morphologies of different stages in heart development, and provide insight into the flow within the developing heart. Corresponding numerical simulations by Miller couple the electrophysiology to the flow patterns within a beating heart.
These physical and numerical models will help scientists gain a better understanding of the relationship between the fluid dynamics, electrophysiology, and biochemistry of the embryonic heart, which may allow doctors to detect the beginnings of congenital heart disease.
"The ultimate goal of the research would be to enable doctors to use electrocardiograms and ultrasounds for the early detection of congenital heart disease. It may also be possible to use techniques such as microfluidic surgery to correct problems in heart chamber and valve formation," Miller says.
Miller will describe the heart models in her talk, "Fluid-structure interaction and electrophysiology of the embryonic heart," at 10:43 am on Sunday, November 23, 2008, in Room 103A of the San Antonio Convention Center.
Airliner ventilation systems are designed to limit passengers' exposure to airborne particles -- from ill travelers' contagious germs to terrorists' aerosol biohazards. Vents in a plane's center ceiling direct air out and down toward the floor below the windows, creating a swirling flow pattern within each row of seats that effectively confines contaminants to a single row or, at worst, its next-row neighbors.
But new research at Purdue University has shown that anyone -- a flight attendant or a passenger, for instance -- merely walking down an airliner's aisle will disrupt this carefully designed flow pattern by creating a wake of eddies that can spread contaminants as far as 10 rows away. Moreover, lead scientist Michael Plesniak says, the eddies' interaction with the ventilation system's swirling flow creates a stagnant zone "at just the wrong place." The height of the stagnant zone is exactly where seated passengers breathe. Future research aims to devise ways for breaking up this stagnant zone and reducing the ability of wakes from people moving around the cabin to disperse contaminants.
In his presentation to the 61st APS Division of Fluid Dynamics in San Antonio, Plesniak will describe his team's research and how it is also helping to create a computational turbulence simulation software tool that airplane manufacturers and safety regulators could use to model complex, realistic cabin ventilation scenarios more rapidly and economically than is now possible. The tool could also be used to design ventilation systems for interiors of buildings, subways and tunnels.
Plesniak's talk, "Experimental Model of Contaminant Transport by a Moving Wake Inside an Aircraft Cabin," will be given at 5:06 p.m. on Monday, November 24, 2008, in Room 002A of the San Antonio Convention Center.