Hypersonic vehicles’ complex interactions with gases in the air are being revealed for the first time, thanks to a new 3D simulation by researchers in the Department of Aerospace Engineering at the Grainger College of Engineering, University of Illinois Urbana-Champaign.
The shockwaves and boundary layers of these high-speed interactions displayed in the simulations provide new information on how flow impacts surface properties in vehicles traveling at Mach speeds. Engineers can apply this information to produce the next generation of hypersonic vehicles.
Simulating Top Speeds With Top Tech
The Grainger College researchers, Dr. Deborah Levin and her PhD student Irmak Taylan Karpuzcu, were fortunate enough to spend time on the highly advanced Frontera computer system at the Texas Advanced Computing Center and access software developed by earlier graduate students working under Levin. Built with support from the National Science Foundation, Frontera provided the pair with the technological capabilities that previous studies in this area lacked.
“Transitioning flows are 3D and unsteady in nature, regardless of the flow geometry. Experiments were conducted in 3D in the early 2000s didn’t provide enough data to determine any 3D effects or unsteadiness because there weren’t enough sensors all around the cone-shaped model,” explained Karpuzcu. “It wasn’t wrong. It was just all that was possible then.”
“We have those data to compare, but having the full picture now in 3D, it’s different,” he added. “Normally, you would expect the flow around the cone to be concentric ribbons, but we noticed breaks in the flow within shock layers both in the single and double cone shapes.”
“Shocking” Hypersonic Findings
In their simulations, the researchers ran a conically shaped object at Mach speeds against air molecules. The Grainger pair chose the cone shape as a simplified universal representation of most hypersonic vehicles. They found breaks close to the cone’s tip, and where the molecules were closer together, a shockwave made them more viscous.
“As you increase the Mach number, the shock gets closer to the surface and promotes these instabilities,” Karpuzcu said. “It would be too expensive to run the simulation at every speed, but we did run it at Mach 6 and did not see the break in the flow.”
The Grainger researchers used the Monte Carlo sampling method to run repeated randomized samples. While physics-based formulas can accurately determine the probability of individual particles colliding, the randomized method provides more complete results for the billions of particles the team tracked compared to classical computational fluid dynamics.
Software Advantage
Discovering why the breaks occurred was the team’s biggest challenge. The team expected the flow to be uniformly moving in all directions, leaving them to search the existing literature for an explanation. However, the pair discovered that they could apply a triple-deck theory linear stability analysis to the unexpected flow.
Levin and Karpuzcu combed through the complex formulations to discern how to use them before writing code to run another simulation. Although running their 3D simulation turned out to be more difficult than expected, the pair successfully offset the work with a second computer program to ensure the simulation worked.
“Our group’s in-house software made it efficient to run the simulation in parallel processors, so it’s much faster,” Karpuzcu explained.
“There were already data from experiments under high-speed conditions so we had some intuition about how the simulations would look, but in 3D we found breaks that we didn’t expect to see.”
By capturing the first full 3D picture of hypersonic flow behavior, the Grainger team’s groundbreaking simulation opens the door to designing faster, more efficient vehicles that can withstand the extreme conditions of Mach-speed travel.
The paper “Loss of Axial Symmetry in Hypersonic Flows over Conical Shapes” appeared on March 7, 2025 in Physical Review Fluids.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.