Researchers from The University of Texas at Arlington's Aerodynamics Research Center have published a study in the journal Engineering Communications, a Nature portfolio journal. Their work details a laser-based technique applied to arc-jet flows that could transform how scientists measure hypersonic air flow, such as the extreme conditions encountered by spacecraft re-entering Earth's atmosphere.
Aerospace engineering graduate students Aaron Gieder, Nikkhil Chander and Alyssa Velky, along with Professor Luca Maddalena, presented findings showing how the technique can characterize complex plasma flow in the shock layer. These measurements could help scientists better understand turbulence in plasma flow and heat transfer during hypersonic flight, generally accepted as speeds of Mach 5 or greater.
"This method allows us to measure turbulence really close to the surface of the material in the shock layer between the shockwave and the body of the vehicle," Dr. Maddalena said.
Measuring airflow within the shock layer is difficult because the extremely hot plasma emits intense light that can overwhelm scientific instruments.
A technique called femtosecond laser electronic excitation tagging (FLEET) velocimetry uses extremely short pulses, lasting one quadrillionth of a second, to split nitrogen molecules in the air into individual atoms. When the atoms recombine into molecules, they emit light, creating a thin line that moves with the airflow. A high-speed camera tracks the line over time, allowing researchers to measure airflow.
Unlike a physical probe, the laser does not disturb the air flow, allowing scientists to observe how hypersonic air behaves and how a vehicle's surface affects the air moving around it. When smooth airflow becomes turbulent, it can increase the heat load on a vehicle by 10 times or more. The increased heat seen experimentally can affect the thickness used in the design of the thermal protection systems needed for spacecraft and hypersonic vehicles.
Using the FLEET technique under realistic flight conditions is challenging because oxygen in the high-temperature air weakens the light emitted by the nitrogen molecules, making it difficult to capture clear images of the airflow.
"We're testing in worse conditions than what occurs in actual hypersonic flight because the plasma flow we generate in ground testing facilities has higher turbulence than what is experienced in flight," Maddelena said. "Now that we've proven we can make these measurements, we can use science to explain real conditions and correct for ground testing effect, no matter which wind tunnel is used."
The findings demonstrate a new approach to measuring airflow under extreme hypersonic conditions, potentially improving how researchers test and evaluate hypersonic vehicles.
Coauthors of the study include Texas A&M University, Hy-SET LLC and Speckodyne Inc.
UTA's Aerodynamics Research Center features an arc-jet wind tunnel that was the first to successfully record FLEET velocimetry measurements in an arc-jet plume. It was the first university-based arc-jet wind tunnel of its kind when it was brought online in 2019, and it is one of only a handful of arc-jet wind tunnels in the United States.
— Jeremy Agor, College of Engineering
About The University of Texas at Arlington (UTA)
The University of Texas at Arlington is a growing public research university in the heart of Dallas-Fort Worth. With a student body of over 42,700, UTA is the second-largest institution in the University of Texas System, offering more than 190 undergraduate and graduate degree programs. Recognized as a Carnegie R-1 university, UTA stands among the nation's top 5% of institutions for research activity. UTA and its 300,000 alumni generate an annual economic impact of $28.8 billion for the state. The University has received the Innovation and Economic Prosperity designation from the Association of Public and Land Grant Universities and has earned recognition for its focus on student access and success, considered key drivers to economic growth and social progress for North Texas and beyond.
