By Elizabeth Myers
A national and international collaboration has brought researchers from the University of Minnesota and the University of Melbourne to the Subsonic Aerospace Wind Tunnel at New Mexico State University to study turbulence in an experiment described as a world’s first in terms of scale.
Jason Monty, a post-doctorate from the University of Melbourne, is working with Todd Novak, a graduate student from the University of Minnesota, and Michael Harris, a mechanical engineering undergraduate at NMSU.
The researchers are measuring wall-bounded turbulence, the turbulence on the surface of tunnels. The floor of the wind tunnel was covered in Braille paper to mimic “random roughness,” creating a rough surface on the walls of the tunnel so that the effect on turbulence can be measured and studied. Rough wall turbulence is the same kind of turbulence that exists around a ship or a plane.
“All forms of transport have a turbulent boundary layer on the surface,” Monty explained. “The problem with it is that nobody understands turbulence.” Monty cited the physicist Richard Feynman, who described turbulence as the last unsolved problem in classical mechanics.
A better understanding of rough wall turbulence could lead to more efficient travel for ships and airplanes. For instance, ships on long voyages are significantly slowed by barnacles attaching and increasing the roughness of the surface, creating resistance.
“The problem is that the boundary layer is responsible for all the drag or air resistance on a body. You need to understand it to control drag and resistance,” Monty said. The NMSU wind tunnel is ideal for this sort of experiment, as its long flat surface is similar to a ship hull, Monty said.
A traverse, an aerodynamically shaped metal bar that stretches across the wind tunnel and holds the measurement sensors, takes measurements as it slowly moves up and down during wind tunnel operation. The traverse is entirely computer controlled and is capable of movements as small as one micron. The tiny five micron flow sensors used can detect the smallest changes in turbulence.
The project is currently being funded by a combination of professor start-up and aerospace engineering funds, said James Allen, assistant professor of mechanical engineering. Monty and Novak paid their own way to NMSU.
“The Australian group has a lot of interest in rough wall flows and international expertise in measurement skills, but no dedicated facility,” Allen said. “We’re currently negotiating for Jason to do experiments for six months of the year here while being funded out of Australia. This reflects the quality of the facility.”
“This is the first experiment with any kind of roughness in such a long tunnel,” Monty said. “Most people work with a two or three meter tunnel.” NMSU’s wind tunnel is about 21 meters long, or about 70 feet.
The wind tunnel reopened in January after receiving some upgrades, as well as new instrumentation for flow measurement and renovation of the building housing the tunnel. It is used for teaching and research in the state’s only aerospace engineering program, which was launched in fall 2006.
“This is definitely a world class facility. It’s as good as any I’ve seen,” Monty said.