Hypersonic aircraft may not require a different design approach

Hypersonic jetliners may be closer to reality thanks to Stevens Institute of Technology’s Professor Nicholaus Parziale, whose research focuses on hypersonic flight. Standing in the way of ultra-fast planes becoming a reality is the turbulence and heat they generate as they fly.

There’s a difference in how the air behaves around the aircraft at low speeds versus high speeds. In incompressible flow, occurring at speeds below 225mph, air density remains nearly constant, simplifying airplane design. However, near or above the speed of sound, it switches to compressible flow where air density changes significantly due to variations in pressure and temperature. “Compressibility affects how the airflow goes around the [aircraft’s] body and that can change lift, drag, and thrust required to take off or stay airborne,” Parziale says.

Aerospace engineers must understand how airflow works at 5x the speed of sound, and that remains an enigma, except for Morkovin’s hypothesis.

Formulated by Mark Morkovin in the mid-20th century, the hypothesis proposes that when air moves at 5x or 6x the speed of sound, the turbulence behavior doesn’t change much from slower speeds – the basic choppy motion of turbulence stays mostly the same.

Parziale says, “If the hypothesis is correct, it means we don’t need a whole new way to understand turbulence at higher speeds. We can use the same concepts we use for the slower flows.” Hypersonic aircraft traveling near Mach 6 don’t need a significantly different design approach.

So far no one has provided sufficient experimental evidence to support Morkovin’s hypothesis until it became the subject of Parziale’s new study, published in Nature Communications.

Parziale’s team used lasers to ionize krypton gas put into the air flowing inside a wind tunnel. That temporarily made krypton atoms form an initially straight, glowing line. Then researchers used ultra high-resolution cameras to take pictures of how that fluorescent krypton line moves, bends, and twists through the wind tunnel’s air. “As that line moves with the gas, you can see crinkles and structure in the flow, and from that, we can learn a lot about turbulence,” Parziale says. “We found at Mach 6, the turbulence behavior is pretty close to the incompressible flow.”

Although the hypothesis isn’t fully confirmed yet, the study suggests planes don’t need an entirely new design to fly at hypersonic speeds.

Using the computational resources to design a plane that will fly at Mach 6 – simulating all the tiny, fine details – would be impossible. “The Morkovin’s hypothesis allows us to make simplifying assumptions so the computational demands to design hypersonic vehicles can become more doable,” Parziale says.

Stevens Institute of Technology
https://www.stevens.edu