Aerospace Research Provides New Understanding Of Atmospheric Turbulence

His data will help engineers improve the computer codes they use to develop propulsion systems for future high-speed aircraft. Freund will explain his experiments and present his data during the American Institute of Aeronautics and Astronautics (AIAA) Joint Propulsion Conference July 6-9 in Seattle.

The "boom" is a large amplitude "acoustic disturbance" in aerospace terms and is used to simulate severe atmospheric turbulence in the laboratory. The difficulty in setting up the experiment was not only generating the boom, but generating it quickly enough to simulate real-world conditions. Freund solved the problem with a novel device nicknamed the "bump." The details are described on Freund's World Wide Web home page at http://www.ase.uc.edu/~dfreund/research.htm.

Freund records the "acoustic reflection coefficients," a measure of how much of the boom is reflected back from the engine. Freund is developing a huge database, showing what happens under various engine operating conditions. Current computer codes rely on approximations to simulate the reflection process, but Freund discovered that these approximations are not even close to simulating what really happens.

"We found that the reflection comes from a number of stages of the compressor, drastically different from what people were assuming in the past. The amplitude of the reflection is considerably less than what they predict, and it's considerably longer than what they would predict."

The information also solves a long-standing problem in high-performance aircraft design. Armed with Freund's data, designers now can modify their codes to predict more accurately how a system will respond to severe atmospheric turbulence.

"Once you get the information, it becomes the glue that allows designers to take their inlet code and a compressor code that are normally operated by different people and different companies and make them talk to each other," said Miklos Sajben, Freund's adviser and Ohio Eminent Scholar of Aerospace Engineering. "It's like finding an interpreter for two people who can't speak each other's language." Sajben also believes the findings will have a lasting impact on how such computations are performed in aerospace engineering practice.

The work has attracted the attention of aerospace companies and of NASA. Sajben has been awarded a multi-year grant by NASA to use a very similar experimental rig (that uses much of Freund's system) to study another type of atmospheric disturbance involving temperature gradients. "The next project is to see what happens if the engine swallows a big hunk of hot air," said Sajben. "That can create similar problems, and it is a common event."

Freund's unique experimental setup and research findings earned him the 1997 Gordon C. Oates Air Breathing Propulsion Graduate Award from the AIAA Foundation. Freund will be presented with his award and a $5,000 check during the AIAA conference. He will present his research results as part of the general meeting and again during a special invited talk as part of the award presentation.

Freund is a native of Macon, Georgia and a graduate of Centerville High School near Dayton Ohio.

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