Burning questions: Researcher works to advance hypersonic technologies
Liselle Joseph has been awarded a Young Investigators Program award from the Air Force Office of Scientific Research to study fluid-ablation interactions in hypersonic flight.
When a vehicle flies at hypersonic speeds, or five times the speed of sound, its surface temperature can pass 3,200 degrees Fahrenheit — almost twice as hot as lava.
To protect them in flight, they're built with a sacrificial layer of material that shields structural components and systems from this extreme heat. But as that protective material intentionally ablates, or erodes away, it affects the airflow and how the vehicle flies, creating a complex loop of cause and effect.
To better understand this process, known as fluid-ablation interactions in hypersonic flight, Assistant Professor Liselle Joseph has been awarded a prestigious Young Investigators Award of $450,000 from the Air Force Office of Scientific Research. Joseph, who holds joint appointments in the Kevin T. Crofton Department of Aerospace and Ocean Engineering and the Virginia Tech National Security Institute, will pursue novel experimental research that replicates hypersonic flight conditions in Virginia Tech’s hypersonic wind tunnel over the next three years.
“We are trying to accurately represent the hypersonic environment and study how the thermal protection material ablates into the flow field,” said Joseph. “Ablation affects cooling, shape, weight, and surface topography. At the same time, the lost material is entering the flow field. How does that change the behavior of the air flow? Does it increase certain turbulence parameters or have a significant effect on drag? These are the questions we’re trying to answer.”
Her research could enable the design of faster, more survivable, and more maneuverable hypersonic vehicles for the Air Force and Department of Defense.
Blazing trails in academic research
Researching hypersonic flight comes with a host of technological challenges. Variables such as aerodynamics, turbulence, heat transfer, and material chemistry all come into play when that blistering-hot airflow meets the ablative material.
To date, the research has focused on either the materials or the aerodynamics.
But Joseph is curious about the conditions at the intersection between the ablative material and the airflow. While previous research has been mostly theoretical, based on simulations and computational data, the field lacks validation against realistic flight conditions. Her novel approach includes heating the vehicle model to better simulate the extreme temperatures in hypersonic flight — the only research of its kind being conducted outside of an Air Force laboratory.
Joseph will conduct experiments in Virginia Tech’s hypersonic wind tunnel, a Mach 7 high-speed blowdown tunnel at the Advanced Propulsion and Power Laboratory. Her research group will be collecting high-quality measurements, such as:
- Wall shear stress, or how hard the airflow “scrapes” the surface
- Pressure fluctuations in the turbulent boundary layer
- Surface temperature distribution
The experimental data will be compared with the material ablation properties, or how fast the surface burns away, as well as mass loss and ablation patterns.
“Hypersonics is a relatively unexplored field, and there are still a lot of open questions,” said Joseph. “At hypersonic speeds, things behave differently. The heat changes everything, and the coupling between variables is much stronger than in low-speed flows. That complexity is exciting to me, and there’s a real need for research in this area right now.”
