Research into hypersonic flight has a new measuring tool, as engineers at The University of Texas at Arlington (UTA) use a laser technique to track extreme airflow in an arc-jet wind tunnel. The team from UTA’s Aerodynamics Research Center published the work in Communications Engineering, a Nature portfolio journal, and says the method could change how scientists measure the conditions spacecraft face when re-entering Earth’s atmosphere.
Aerospace engineering graduate students Aaron Gieder, Nikkhil Chander and Alyssa Velky, along with Professor Luca Maddalena, showed how the technique can characterize complex plasma flow in the shock layer. Coauthors include Texas A&M University, Hy-SET LLC and Speckodyne Inc.
Why hypersonic flight is hard to measure
Hypersonic flight is generally accepted to mean speeds of Mach 5 or greater. At these speeds, a shockwave forms ahead of a vehicle, and the thin region between that shockwave and the vehicle’s body, called the shock layer, fills with extremely hot plasma.
Measuring airflow inside the shock layer is difficult because the plasma emits intense light that can overwhelm scientific instruments. Yet this is exactly where engineers need data. When smooth airflow turns turbulent, it can raise the heat load on a vehicle by 10 times or more, and that extra heat affects how thick the thermal protection system on a spacecraft or hypersonic vehicle needs to be.
“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,” Maddalena said.
How the FLEET laser technique works
The technique is called femtosecond laser electronic excitation tagging (FLEET) velocimetry. It works in a few steps:
- Extremely short laser pulses, lasting about one quadrillionth of a second, split nitrogen molecules in the air into individual atoms.
- When the atoms recombine into molecules, they emit light, creating a thin glowing line that moves with the airflow.
- A high-speed camera tracks that line over time, which lets researchers calculate how the air is moving.
Unlike a physical probe, the laser does not disturb the flow. That allows scientists to observe how hypersonic air behaves and how a vehicle’s surface affects the air moving around it.
Using FLEET under realistic conditions is still challenging. Oxygen in the high-temperature air weakens the light emitted by the nitrogen molecules, which makes it harder to capture clear images.
Testing in tougher conditions than flight
Maddalena noted that ground tests are, in one sense, harsher than the real thing. “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,” he 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.”
That last point matters for the wider field. If researchers can measure and correct for the quirks of a given test facility, results from different wind tunnels become easier to compare and to apply to real vehicles.
UTA’s arc-jet wind tunnel
UTA’s arc-jet wind tunnel was the first to successfully record FLEET velocimetry measurements in an arc-jet plume. When it came online in 2019, it was the first university-based arc-jet wind tunnel of its kind, and it remains one of only a handful of arc-jet wind tunnels in the United States. Arc-jets use an electric arc to heat gas to extreme temperatures, recreating the plasma conditions of atmospheric re-entry on the ground.
As a writer, I find it fascinating that a pulse of light lasting a quadrillionth of a second can reveal what happens in the layer of plasma right next to a spacecraft’s surface. For more on space vehicles and propulsion, see our stories on plasma technology for a return trip from Mars and NASA’s SR-1 Freedom nuclear power module.
FAQ
What is FLEET velocimetry?
FLEET stands for femtosecond laser electronic excitation tagging. Ultra-short laser pulses split nitrogen molecules, and the glowing line formed when the atoms recombine is tracked by a high-speed camera to measure airflow.
Why does turbulence matter in hypersonic flight?
Turbulent airflow can increase the heat load on a vehicle by 10 times or more, which affects how thick its thermal protection system must be.
What speed counts as hypersonic?
Hypersonic flight is generally accepted as speeds of Mach 5 or greater.