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January 1998

Supersonic research soars to new speeds

WEST LAFAYETTE, Ind. -- Purdue University researchers are helping to build the next generation of reusable space vehicles, which will travel at more than 15 times the speed of sound and ferry passengers to an orbiting space station.

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Steven Schneider, associate professor of aeronautics and astronautics at Purdue, has begun construction on a Mach 6 wind tunnel, which when completed will be the fastest and quietest research wind tunnel at any academic institution in the world. Designed to conduct experiments in airstreams traveling at six times the speed of sound, the nearly $1 million facility will begin taking data in December 1998.

The performance of a "quiet" wind tunnel is measured by its maximum Reynolds number, a combination of factors such as cross sectional area of the tunnel, air speed through it, and air density. The new Mach 6 tunnel will have a quiet-flow Reynolds number of 13 million-- nearly 30 times larger than that of Purdue's existing Mach 4 tunnel and one of the highest ratings anywhere in the world.

The research of Schneider and his colleague Steven Collicott, associate professor of aeronautics and astronautics, focuses on how air flows over and around objects that travel faster than the speed of sound, such as reconnaissance vehicles, missiles, supersonic passenger jets such as the Concorde, and the space shuttle during re-entry. Their high-speed research may be applied in interpreting the test results of the X-33, a subscale prototype of a full-scale reusable launch vehicle. The full-scale craft is slated for development after the turn of the century. (Information on the X-33 is available at http://stp.msfc.nasa.gov/stpweb/x33/x33home.html)

Schneider's research group will present results from its latest study Jan. 12-15 in Reno, Nev., at a meeting of the American Institute of Aeronautics and Astronautics.

Understanding hypersonic airflow, air moving faster than Mach 5, is critical in designing high-speed aircraft. Airflow determines how hot a craft gets and how much drag it experiences from friction as it travels through the atmosphere. Drag increases the operating costs of such vehicles by reducing fuel efficiency, and if a vehicle gets too hot, it can be damaged to the point of failure. That's why the space shuttle has tens of thousands of protective, heat-resistant tiles on its underside, each of which is numbered and must be inspected between flights.

Drag and heat buildup on a surface are at their worst when the supersonic airflow is turbulent. When the airflow is smooth, or laminar, drag is reduced and less heat is transferred to the surface. How, why and when the airflow changes from smooth to turbulent is still poorly understood, Schneider says, and it's one of the major barriers hindering the development of new hypersonic vehicles, including a reusable re-entry vehicle to replace the aging space shuttle.

"Currently designers are considering a new re-entry vehicle with a metal skin," Schneider says. "This would eliminate the tile system used on the space shuttle, which is expensive to maintain.

"However, a metal skin will be feasible only if the laminar-turbulent transition happens later in the vehicle's re-entry than it sometimes does for the space shuttle. If the flow becomes unsteady too early, the metal skin would heat up and possibly melt. X-33 flight tests may show that the metal skin as currently designed just won't work. Research like ours is needed to determine what kinds of designs will be successful."

The most sophisticated wind tunnels are at NASA's Langley Research Center in Virginia, which had a Mach 6 quiet-flow tunnel and is experimenting with a Mach 8 tunnel, Schneider says. The Langley facility also has conventional wind tunnels that go up to Mach 20.

Purdue's existing quiet-flow wind tunnel produces a Mach 4 airstream, traveling at four times the speed of sound. The 68-foot-long, 12-inch-diameter tube runs nearly the length of Purdue's Aerospace Sciences Laboratory along the ceiling. The experimental chamber is a narrower, 4-inch-diameter part of the tube where researchers place high-tech models that simulate the shapes of aircraft surfaces, such as the lower surface of the X-33.

The models are connected to sophisticated electronics that measure and record airflow data. For example, Collicott has developed highly sensitive optical devices to detect, measure and track instabilities in the airflow. The air flows over a test piece at Mach 4 for only about 3.5 seconds, but Schneider says that's plenty of time for the instruments to collect data.

Purdue's wind tunnel uses a pipe, closed on one end, to supply special dust-free air. The other end is connected to a huge tank sitting just outside Schneider's laboratory. Between the tank and the test area is a special diaphragm. Pumps remove the air from the tank, creating a vacuum between the tank and the diaphragm. When the diaphragm is broken, air flows at Mach 4 through the pipe and the test area and into the tank.

"The short run-time we have in our tunnel is advantageous because in order to get a longer time, you would need a larger vacuum tank and more powerful pumps, which adds considerably more expense," Schneider says. "This setup allows us to get great data at relatively little cost."

The new Mach 6 tunnel, to be built in the Aerospace Science Lab, also will rely on this type of design, called a Ludwieg tube. The tunnel will be 140 feet long with a 9-inch-diameter test area, which will allow researchers to use larger-scale test models.

The major advantage of the new tunnel is that it will be 10 to 100 times quieter than other tunnels, which is critical in studying noise-sensitive phenomena.

"The data currently being collected from experiments designed to study laminar-turbulent transitions are ambiguous because of the high level of noise in conventional supersonic wind tunnels," Schneider says. "Ironically, the noise comes from turbulent air -- the same phenomenon we're trying to study."

As the air flows through the experimental area of a wind tunnel, it flows across the tunnel's interior wall, where it also experiences laminar-turbulent transition, Schneider explains. Consequently, as the tunnel size or pressure inside the tunnel increase, it becomes increasingly difficult to maintain a quiet flow.

"The higher pressure we can keep the tunnel quiet at, the better data we can get," he says. "So, to make the wall as smooth as possible, it is made of stainless steel and polished to a high gloss."

Purdue's Mach 6 wind tunnel is being funded in part by The Boeing Co. and the Air Force Office of Scientific Research.

Sources: Steven Schneider, (765) 494-3343; e-mail, steves@ecn.purdue.edu
Steven Collicott, (765) 494-2339; e-mail, collicot@purdue.edu
Writer: Amanda Siegfried, (765) 494-4709; e-mail, amanda_siegfried@purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu

Photo caption
Steven Schneider, associate professor of aeronautics and astronautics at Purdue, places a test object inside a Mach 4 wind tunnel. His studies on supersonic airflow may lead to a new generation of reusable space vehicles. (Purdue photo by Richard Myers-Walls, Purdue Center for Instructional Services)
Color photo, electronic transmission, and Web and ftp download available. Photo ID: Schneider.tunnel
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