Purdue Wind Tunnel Key For 'Hypersonic Vehicles,' Future Space Planes

By using the only wind tunnel capable of running quietly at "hypersonic" speeds, Purdue University engineers have conducted experiments to yield critical data for designing an advanced aircraft called the X-51A, powered by engines called scramjets.

The X-51A test vehicle is expected to evolve into missiles capable of flying at Mach 6 -- or six times the speed of sound -- enabling them to hit mobile "time-critical" targets.

Scramjets also may propel future military and civilian space planes.

The quiet wind tunnel operation is critical for collecting data to show precisely how air flows over a vehicle's surface in flight. No other wind tunnel runs quietly while conducting experiments in airstreams traveling at Mach 6, says Steven Schneider, an aerospace engineer and professor in Purdue's School of Aeronautics and Astronautics.

"A quiet wind tunnel yields more accurate data because it more closely simulates flight," he says.

Specifically, engineers need detailed information about how airflow changes from "laminar," or smooth, to turbulent as it speeds over an aircraft's surfaces. The information is essential to properly design vehicles that fly at hypersonic speeds, or faster than Mach 5, nearly 4,000 mph, Schneider says.

The X-51 project is led by the Air Force Research Laboratory and the Defense Advanced Research Projects Agency, and the vehicle is being built by Pratt & Whitney and the Boeing Co. Purdue engineers are part of a national team of researchers from government, academia and industry handling different aspects of the vehicle.

The Purdue research focuses on the forebody, or front portion of the craft, using a foot-long model for wind-tunnel testing. Research findings are providing information in two vital areas: maintaining the turbulent flow of air into the engine's combustor to keep the scramjet running properly, and increasing the amount of smooth airflow over the vehicle's upper surface to reduce friction and heat that could damage or destroy the vehicle. The higher the Mach number, the greater the friction and heat generated in flight.

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Tiny Sensors Could Aid Environmental Protection, Pharmaceuticals

By Emil Venere

Researchers at Purdue University have shown how to create a new class of tiny sensors for applications ranging from environmental protection to pharmaceutical preservation.

Although similar "gas-flow sensors" are currently being used for a variety of industries, the new sensor is the first that works on the scale of micrometers and nanometers, or millionths and billionths of a meter, respectively, said Steven Wereley, an associate professor of mechanical engineering.

Gas-flow sensors currently used, including those in residential gas meters to determine how much to charge customers, operate on a principle known for at least 100 years. According to that principle, as gas flows over a surface, such as the wall of a pipe or an object flying through the air, molecules of gas nearest the surface remain stationary. The molecules farther away from the surface move progressively faster.

"That model works really well in many situations, including aerodynamics and applications where the scale of the flow is large compared to the size of the molecules," Wereley says.

This principle, however, does not apply to gas flowing through channels on the scale of micrometers or nanometers, meaning ordinary designs will not work for sensors needed for applications on those scales. In such applications, gas molecules immediately adjacent to the wall of a tube do flow and are said to "slip."

"This exception to the model carries important design implications," Wereley said.

Findings will be detailed in a research paper to be published in the February issue of the Journal of Micromechanics and Microengineering. The paper describes how the sensor is designed.

Gas-flow sensors that operate on the scale of micrometers and nanometers could have applications in environmental protection, particularly to measure the leakage of hydrocarbon fumes from fuel tanks in new cars on the manufacturing line. Federal environmental guidelines specify how much leakage is allowable.

Automakers currently test empty fuel tanks by pressurizing them with a gas, such as helium, and then measuring whether the pressure drops, indicating leakage. The test is limited because, while it can determine whether a tank is leaking, it cannot reveal how severe the leak is. Using a sensor capable of measuring gas flow on small scales would make it possible to yield more accurate data.

An accurate test also could be applied to the pharmaceutical industry, which preserves drugs in packages filled with a gas free of the molds and impurities of ambient air. Pharmaceuticals are shipped and stored in the packaging, and the industry tests packages for leakage, but gas-flow sensors could be used to test them more accurately.

The Purdue researchers worked with industry to develop the sensors, which currently are too costly to be manufactured profitably. The research is associated with the Microfluidics Laboratory at the Birck Nanotechnology Center in Purdue's Discovery Park.


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