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| Simulated image using isosurfaces to visualize the angular velocity over a double cone. |
Breaking the Symmetry: Revolutionary Findings in Hypersonic Flow Behavior
By Science Correspondent
March 31, 2025
A groundbreaking study published earlier this month in Physical Review Fluids has challenged long-held assumptions about hypersonic flows, potentially revolutionizing how engineers design vehicles capable of traveling at speeds exceeding five times the speed of sound.
Researchers from the University of Illinois Urbana-Champaign have discovered that hypersonic flows over conical shapes—the basic geometry for many hypersonic vehicles—don't behave as symmetrically as previously thought, a finding with significant implications for future aerospace design.
"We've been designing hypersonic vehicles based on an assumption that may not reflect reality," explains Dr. Deborah Levin, who led the research alongside Ph.D. student Irmak Taylan Karpuzcu. "Our 3D simulations show unexpected breaks in flow patterns that simply weren't visible in earlier studies."
Unveiling the Unexpected
The research team utilized advanced supercomputing resources and specialized software to conduct fully three-dimensional simulations of hypersonic flows around conical shapes. What they found stunned the aerospace community: at Mach 16, the flow exhibited distinct non-axisymmetric patterns, contradicting the traditionally accepted model of concentric, uniform flow.
"Normally, you would expect the flow around the cone to be concentric ribbons," Karpuzcu noted, "but we noticed breaks in the flow within shock layers both in the single and double cone shapes."
These breaks were particularly prominent near the cone tip, where shock waves bring air molecules closer together. The researchers observed a "180-degree periodicity" in the flow pattern, dividing it into "two big chunks" around the cone—a phenomenon never before documented at these speeds.
Speed Matters
The research also revealed that this asymmetric behavior is speed-dependent. When simulations were run at Mach 6, the flow breaks were absent, suggesting a threshold above which traditional axisymmetric assumptions no longer apply.
"As you increase the Mach number, the shock gets closer to the surface and promotes these instabilities," explained Karpuzcu.
From Theory to Practice
To validate their observations, the team combined direct simulation Monte Carlo methods with triple-deck theory and linear stability analysis. The results confirmed that non-axisymmetric disturbances were being amplified through linear mechanisms, with the strongest amplification occurring for an azimuthal wave number of n=1.
For double-cone configurations, the implications were even more pronounced. Three-dimensional simulations resulted in smaller separation bubbles with weaker shocks compared to traditional axisymmetric models, and surface parameters varied significantly in the azimuthal direction.
Implications for Aerospace Design
These findings have immediate relevance for hypersonic vehicle development programs worldwide. The conical geometry used in the study represents a simplified version of many hypersonic vehicles, and understanding how the flow affects surface properties can help engineers develop more effective designs.
"This is a case where more realistic modeling reveals phenomena that could affect vehicle performance and safety," said Dr. Levin. "It's essential that we incorporate these insights into future designs."
The discovery comes at a critical time, as nations and private companies race to develop hypersonic capabilities for both civilian and military applications.
Computational Breakthrough
The research was made possible by access to Frontera, a leadership-class supercomputer at the Texas Advanced Computing Center, along with specialized software developed by Professor Levin's former graduate students.
"Running the 3D direct simulation Monte Carlo simulation is hard," acknowledged Karpuzcu, noting that the team tracked billions of particles to ensure accurate results. "It's more extensive than classical computational fluid dynamics methods... This makes sure there are enough particles within the flow field and collisions are captured properly."
As aerospace engineers digest these findings, the study serves as a reminder that even well-established assumptions deserve re-examination with advanced computational tools—and that such scrutiny may reveal surprising new physics that could reshape our approach to extreme-speed flight.
References
- Karpuzcu, I. T., & Levin, D. A. (2025). Loss of axial symmetry in hypersonic flows over conical shapes. Physical Review Fluids, 10, 033901. https://doi.org/10.1103/PhysRevFluids.10.033901
- University of Illinois Urbana-Champaign. (2025, March). Hypersonic simulation in 3D exposes new disturbances. Aerospace Engineering Illinois. https://aerospace.illinois.edu/news/74245
- SciTechDaily. (2025, March 28). Mach 16 Mayhem: Supercomputer Uncovers Chaos in Hypersonic Flows. https://scitechdaily.com/mach-16-mayhem-supercomputer-uncovers-chaos-in-hypersonic-flows/
- Karpuzcu, I. T., & Levin, D. A. (2024, July 9). Loss of Axial Symmetry in Hypersonic Flows over Conical Shapes. arXiv:2407.07137. https://arxiv.org/abs/2407.07137
- The Debrief. (2025, March 29). New Hypersonic Flight Simulations Just Revealed Something "Shocking" that Researchers Didn't Expect. https://thedebrief.org/new-hypersonic-flight-simulations-just-revealed-something-shocking-that-researchers-didnt-expect/
Mach 16 Mayhem: Supercomputer Uncovers Chaos in Hypersonic Flows
Researchers at the University of Illinois Urbana-Champaign have unlocked new insights into the turbulent behavior of hypersonic flows by using advanced 3D simulations.
Leveraging supercomputing power and custom-built software, they discovered unexpected instabilities and flow breaks around cone-shaped models at Mach 16, disturbances never seen before in previous 2D or experimental studies. These findings could significantly impact the design of future hypersonic vehicles by helping engineers understand how extreme speeds interact with surface geometries in new ways.
Hypersonic Flows and New Discoveries
At hypersonic speeds, air behaves in complex ways as it interacts with a vehicle’s surface, forming features like boundary layers and shock waves. For the first time, researchers in the Department of Aerospace Engineering at the Grainger College of Engineering, University of Illinois Urbana-Champaign, have observed new disturbances in these interactions using fully 3D simulations.
Running high-resolution 3D simulations at hypersonic speeds requires immense computational power, making such work costly and challenging. Two key resources made this study possible: access to Frontera, a leadership-class supercomputer funded by the National Science Foundation at the Texas Advanced Computing Center, and specialized software developed over the years by several of Professor Deborah Levin’s former graduate students. Levin led the study alongside her Ph.D. student, Irmak Taylan Karpuzcu.
A New Look at Flow Instabilities
“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. It wasn’t wrong. It was just all that was possible then,” said Karpuzcu. “We have those data to compare, but having the full picture now in 3D, it’s different. 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.”
Surprising Breaks at Mach 16
Karpuzcu said they observed the breaks near the tip of the cone, and with a shock wave near where the air molecules were closer together making them more viscous at Mach 16.
“As you increase the Mach number, the shock gets closer to the surface and promotes these instabilities. 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.”
Karpuzcu said the cone geometry represents a simplified version of many hypersonic vehicles and understanding how the flow affects surface properties can help lead to design considerations.
Unexpected Findings in 3D
“Our group’s in-house software made it efficient to run the simulation in parallel processors, so it’s much faster. 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.”
He said the most difficult part of the work for him was in analyzing why the break in the flow was happening.
“The flow should be going in all directions, but uniformly. We needed to justify what we were seeing. Our literature review indicated that a linear stability analysis based on triple-deck theory can be applied to this flow. After analyzing the complex formulations and connecting them to our case, we developed a code to numerically simulate the problem again. Running the 3D direct simulation Monte Carlo simulation is hard, but then we set up a second computer program to make sure everything works and is within the limits for our flow conditions. When we did that, we saw the break in two big chunks in 180-degree periodicity around the cone.”
The Power of Monte Carlo Simulations
Karpuzcu said the beauty of the direct simulation Monte Carlo is that it tracks each air molecule in the flow and captures the shocks.
“When you use other methods to calculate fluid dynamics, it’s all deterministic. When we introduce a particle to the flow field, there is a probability of that particle colliding with other particles or any solid surfaces that’s calculated on physics-based formulas, but the output is a roll of the dice. The Monte Carlo method does random, repetitive attempts. It’s more extensive than classical computational fluid dynamics methods and we’re tracking billions of particles. This makes sure there are enough particles within the flow field and collisions are captured properly.”
Reference: “Loss of axial symmetry in hypersonic flows over conical
shapes” by Irmak T. Karpuzcu and Deborah A. Levin, 7 March 2025, Physical Review Fluids.
DOI: 10.1103/PhysRevFluids.10.033901
Loss of axial symmetry in hypersonic flows over conical shapes
Phys. Rev. Fluids 10, 033901 – Published 7 March, 2025
Abstract
The assumption of axial symmetry for hypersonic flows over conically shaped geometries is ubiquitous in both experiments and numerical simulations. Yet depending on the free stream conditions, many of these flows are unsteady and their transition from laminar to turbulent is a three-dimensional phenomenon. Combining triple-deck theory/linear stability analysis with the kinetic direct simulation Monte Carlo method, we analyze the azimuthal eigenmodes of flows over single- and double-cone configurations. For Mach 16 flows, we find that the strongest amplification rate occurs for the non-axisymmetric azimuthal wave number of 𝑛=1. This occurs in regions quite close to the tip of the cone due to the proximity of the conical shock to the viscous shear layer where non-axisymmetric modes are amplified through linear mechanisms. Comparison of triple-deck linear stability predictions shows that in addition to the azimuthal wave number, both the temporal content and amplification rate of these non-axisymmetric disturbances agree well with the time-accurate DSMC flowfield. In addition to the loss of axial symmetry observed at the conical shock, the effect of axial symmetry assumptions on the more complex shock-shock and shock-boundary layer interactions of a flow over a double cone is- considered. The results for the separation region show that axisymmetric and three-dimensional simulations differ in almost all of the main flow structures. Three-dimensional flowfields result in a smaller separation bubble with weaker shocks and threedimensional effects were manifest in the variation in surface parameters in the azimuthal direction as well. Interestingly, the DSMC simulations show that the loss of axial symmetry in the separation region begins near the cone tip.
