When James Buchholz arrived for a Summer Faculty Fellowship at the U.S. Air Force Academy near Colorado Springs, Colo., last May, he was so excited to be back in the mountains, he immediately set out on a hike.
The Assistant IIHR Research Engineer drove to the Florissant Fossil Beds, where he walked among the fossilized redwoods and then struck out on a hiking trail. “Of course, in May in the mountains, you never know what you’re going to get,” he says now. About halfway up the trail, he found himself in the middle of a hailstorm. Luckily, the hail was soft and slushy. “I was perfectly happy,” he says. “It didn’t bother me.”
The assistant professor of mechanical engineering was surprised, though, when he got back to the visitor center and a woman asked to take his picture. He realized then that he was covered with hail. “I was white,” Buchholz says. “The top of my hat was white, my backpack, everything.
“It was a nice walk, though,” he adds.
Subsonic Wind Tunnel
Buchholz’s summer was off to an auspicious start. He was as eager to get started on the research as he was to be in the Rockies. Working with his Air Force adviser Tom McLaughlin, Buchholz planned to study the flow physics of vortices shed by flexible aircraft wings in stall flutter. It was an opportunity to expand the types of applications he had been studying and to learn something about flow control.
Buchholz was accompanied by University of Iowa PhD student Jim Akkala. Together, they designed and implemented an experiment to be conducted in the Air Force Academy’s Subsonic Wind Tunnel (with a maximum speed of Mach .6). Once they arrived at the academy, their research focus shifted to the problem of aircraft wing flutter. Once an aircraft wing is “perturbed” by turbulence or a bump, it can start to oscillate or flutter. The flutter can get worse, until the wing destroys itself — obviously a very bad outcome for anyone in the plane. Buchholz and Akkala set out to develop a model of the wing that could be tested in the wind tunnel.
Akkala was a key player in the success of the project, Buchholz says. “Jim was brilliant at designing these things. He’s extremely meticulous and was a huge factor in even getting this to work.” One issue was inducing the wing to flutter in the first place. “The problem is,” Buchholz says, “this wind tunnel is built like a tank.” Encased in steel and featuring interlocks on the doors, it was impossible to just reach in and give the wing a whack to get it started. Good luck was on their side, however, and the wing did eventually flutter.
“This was really serendipitous,” Buchholz says. “We realized later that very small changes to the airfoil, which we would normally consider to be inconsequential, made the difference between the airfoil flutter self-starting or not. Several times, we stood there watching hopefully, and nothing happened.”
Particle Image Velocimetry
Buchholz is especially interested in the physics of the vortices that develop on the leading edge of the wing. The formation and existence of these vortices result in very large aerodynamic forces, which can seriously damage wings or wind turbines. Using particle image velocimetry (PIV), they illuminated the flow around the model with a laser and took images of the micronscale droplets suspended in the flow. He and Akkala observed a very large source of vorticity contributing to a secondary counterclockwise vortex that is generated behind the clockwise leading-edge vortex. “They basically destroy each other,” Buchholz says. “One’s rotating one way; the other one’s rotating the other way. They mix and essentially cancel each other out.” “We realized that very small changes to the airfoil made the difference between the airfoil flutter self-starting or not.” Buchholz had already observed this secondary vorticity flux in his lab at the UI. He says the major step forward from the summer work was the characterization of the equivalent phenomenon in the case of stall flutter (a fairly different configuration), which Buchholz says has given us important information about how ubiquitous the phenomenon is. There are some differences from the flapping wing, but also many key similarities, which he says is good, because it suggests similar behaviors in other applications as well.
Next, they’d like to try to control the flow using the unique observations they’ve made of the flow itself. “We’ve identified this generation of secondary vorticity, which turns out to be very large,” Buchholz says. “Can we harness that somehow to regulate the strength of that dynamic stall vortex?” It’s a new idea, and one that merits further exploration. Buchholz hopes to continue the work next summer back at the Air Force Academy.
The ultimate goal is to understand and better predict the phenomenon. “That’s a lofty goal,” Buchholz says. “A lot of people have been working on that problem for a lot of years.”