Looking to smooth out aircraft performance and save on use of fuel, engineers at Case Western Reserve University have rebuilt and modernized a wind tunnel that will help develop next-generation aircraft.

The 45-foot-long tunnel ranges from eight feet wide at its mouth to just over two feet wide in the middle, where experimental models are tested.

Edward White, assistant professor of mechanical and aerospace engineering; Sam Cordero; Wayne Schmidt; and John Webber from the CWRU Engineering Services Fabrication Center and undergraduate and graduate students worked for 18 months to rebuild the facility. Today, it's ready to tackle tough analyses—like how airflow over wings and turbine blades undergoes laminar-to-turbulent transition, the process that leads to nearly 25 percent of the drag force on commercial aircraft.

"Ninety percent of the tunnel's structure is new and all of its features-including flow quality treatments, instrumentation, operability and noise and vibration levels-are state-of-the-art, creating a completely modular tunnel with a minuscule turbulence level," said White. "Now we can observe details of the laminar-turbulent transition process from start to finish. Understanding transition is important because of the tremendous performance improvements and fuel savings that could result by eliminating drag from turbulent flow. We hope that savings on running aircraft would filter through to lower airfare costs to consumers."

White's two main objectives during reconstruction were to get the tunnel to operate with a much steadier, low-turbulence airflow and to provide computer-automated control of experiments, which often involve measurements at hundreds of thousands of locations around the experimental model.

"Improvements in automation allow us to perform an experiment in an afternoon that just a few years ago would have taken weeks, if attempted at all," White said. "This means that we're now in a position to tackle some problems that have resisted solution for a long time."

The new tunnel is located in the fluid mechanics laboratory at CWRU. Originally constructed in the late 1940s as part of the propulsion laboratory, the wind tunnel was used only sporadically through the 1960s and 1970s. It was last used to perform research in the early 1980s, then sat idle until now.

With reconstruction complete, the tunnel now provides airflows of up to 70 miles per hour. While this isn't nearly as fast as the aircraft it simulates, White explains that the tunnel's most important measure of performance is turbulence, the level of unsteady velocity fluctuations about the flow's average velocity.

"Turbulence in the wind tunnel is only 0.05 percent of the average air speed. Wind turbulence outside ranges from five to 30 percent, so the flow in the tunnel is at least 100 times cleaner than the wind outside. At this low level in the lab, we can introduce specific disturbances that simulate real flight situations to see how they lead to turbulence. If the turbulence level were any higher, we wouldn't have good enough control over the experiment to know for sure what was leading to transition or how we might prevent it."

Current experiments focus on what might first appear mundane, the effect that very small rough spots on a wing's surface have on the production of turbulence and drag. Although roughness on a wing's surface might be small, its importance is not.

"If we were able to produce and maintain a perfectly smooth wing, turbulence would be prevented over most of the wing and airplanes would immediately reduce their fuel consumption by 25 percent," says White, "Of course, producing a perfect surface is impossible. And, even if it were possible, maintaining it would not be. Instead, we're looking to achieve the same sort of savings within the reality of how planes are manufactured and the sorts of environments in which they actually fly."

Research in the tunnel is funded by grants from the U.S. Air Force Office of Scientific Research.