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Which Way Does the Air Flow at Supersonic Speeds
April 22, 2019

In the quest for ever faster air travel, one formidable challenge has been the issue of deafening noise, a sonic boom, created when aircraft exceed the speed of sound, approximately 767 mph. Now, according to information provide by the University at Buffalo, an aerospace engineer is working to solve problems associated with exceeding the sound barrier.

The image above is a 3D computer simulation of air flowing over a hill creating turbulence at transonic speed. The ring-like features are eddies of air. Credit: James Chen / University at Buffalo.

“Imagine flying from New York City to Los Angeles in an hour. Imagine incredibly fast unmanned aerial vehicles providing more updated and nuanced information about Earth’s atmosphere, which could help us better predict deadly storms,” remarked James Chen, PhD, an aerospace engineer and assistant professor in the Department of Mechanical and Aerospace Engineering at the University at Buffalo’s School of Engineering and Applied Sciences.

As the corresponding author of a study, “First-order approximation to the Boltzmann–Curtiss equation for flows with local spin,” published in the Journal of Engineering Mathematics, Dr. Chen’s work pertains to Austrian physicist Ludwig Boltzmann’s classical kinetic theory, which uses the motion of gas molecules to explain everyday phenomena, such as temperature and pressure.

Funded by the U.S. Air Force’s Young Investigator Program, which supports engineers and scientists who show exceptional ability and promise for conducting basic research, Dr. Chen’s research extends classical kinetic theory into high-speed aerodynamics, including hypersonic speed, which begins at 3,836 mph or about five times the speed of sound. The new study and others by Dr. Chen attempt to solve long-standing problems associated with high-speed aerodynamics.

“Reduction of the notorious sonic boom is a just a start. In supersonic flight, we must now answer the last unresolved problem in classical physics: turbulence,” noted Dr. Chen. “There is so much we don’t know about the airflow when you reach hypersonic speeds. For example, eddies form around the aircraft creating turbulence that affect how aircraft maneuver through the atmosphere,” he explained.

In an effort to solve these complex problems, researchers have historically used wind tunnels. While effective, the inhibiting factors has been that these labs can be expensive to operate and maintain. Consequently, many researchers, including Dr. Chen, have pivoted toward direct numerical simulations (DNS).

“DNS with high-performance computing can help resolve turbulence problems. But the equations we have used, based upon the work of Navier [Claude-Louis Navier] and Stokes [George Gabriel Stokes], are essentially invalid at supersonic and hypersonic speeds,” noted Dr. Chen.

As detailed in his Journal of Engineering Mathematics paper, Dr. Chen’s research centers on morphing continuum theory (MCT), which is based on the fields of mechanics and kinetic theory. MCT aims to provide researchers with computationally friendly equations and a theory to address problems with hypersonic turbulence.

Ultimately, the work could lead to advancements into how supersonic and hypersonic aircraft are designed, everything from the vehicle’s shape to what materials it is made of. According to Dr. Chen, the goal is a new class of aircraft which are faster, quieter, safer, and less expensive to operate.

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