A Laser Tags the Air's Own Molecules to Measure Hypersonic Wind Speed

A team led by the University of Texas at Arlington has measured the speed of air moving through a hypersonic ground-test facility by tagging the air's own nitrogen molecules with a laser pulse and timing how far they travel. The work was published Aug. 31 in Communications Engineering, an open-access Springer Nature journal.

The method is called Femtosecond Laser Electronic Excitation Tagging, or FLEET. A femtosecond laser, one whose pulses last quadrillionths of a second, excites nitrogen already present in the airstream, and tracking those tagged molecules over time yields the flow velocity, the authors report. Because the tracer is the air itself, nothing is seeded into the flow and no probe is placed in it. That is what the paper means by a direct and nonintrusive measurement, and the authors write that it offers "high resolution while preserving the integrity of the flow."

The measurements were made inside an arc-heated test facility, or arc jet: a rig in which gas is heated on the ground to recreate, as the paper puts it, "the natural conditions of high altitude and thermally demanding flights." The paper reports velocities in two places: the undisturbed hypersonic flow, and the air close to the surface of an object placed in that flow. The demonstration is in that ground facility, not on a vehicle in flight.

The authors state that instrumentation currently used to measure flow parameters in such facilities "lacks the capability for direct, nonintrusive measurements, largely due to its limited applicability in such hostile environments." Their own assessment of where the work leads is that the results "pave the way towards the next generation of measurement and testing instrumentation for hypersonic aerothermal test facilities."

Seven authors are listed, affiliated with UT Arlington's Aerodynamics Research Center, the Department of Aerospace Engineering at Texas A&M University, SpeckoDyne Corporation and Hy-SET LLC.