Advances in high-speed optical diagnostics help characterize airflows from subsonic to hypersonic speeds

By Tom Jenkins|December 2024

The Aerodynamic Measurement Technology Technical Committee advances measurement technology for ground facilities and aircraft in flight.

In January, engineers at Sandia National Laboratories in New Mexico characterized the flow of their free-piston Hypersonic Shock Tunnel via novel laser diagnostics. The tunnel, which was first operated in 2021, uses noncontact shock heating to generate high-temperature gas supplied to a nozzle. This process dissociates the oxygen and nitrogen molecules in the air to form other atomic and molecular compounds, including nitric oxide. Rapid expansion through the nozzle produces flow in thermochemical nonequilibrium through processes that are not well understood. The engineers developed a nitrogen and oxygen temperature measurement technique based on coherent anti-Stokes Raman scattering that can operate at a 100-kilohertz rate. They measured the flow velocity simultaneously via nitric oxide molecular tagging velocimetry. Finally, they measured nitric oxide temperature via laser absorption spectroscopy, a technique developed in conjunction with Purdue University of Indiana. The measurements captured the temporal evolution of the flow and revealed pronounced thermal nonequilibrium, with each molecular species having a higher vibrational temperature than rotational temperature. This was the most extensive characterization of shock tunnel flow ever performed, and researchers are referencing the measurements in their creation of models in hypersonic computational fluid dynamics codes.

Also in January, researchers at the Boeing/Air Force Office of Scientific Research Mach-6 Quiet Tunnel at Purdue demonstrated two-photon laser-induced fluorescence measurements of carbon monoxide at rates faster than 1 kHz. They pushed the state-of-the-art two orders of magnitude to 100 kHz with a burst-mode laser and optical parametric oscillator at a wavelength of 230.1 nanometers, suitable for use in hypersonic wind tunnels. The two-photon carbon monoxide technique allowed for advances in coupled measurements of ablation products and gas mixing in boundary layers and wakes.

In July, researchers at the University of Toronto used an array of micro-electromechanical-systems-based synthetic jet actuators to reattach separated flow on a stalled wing, based on a NACA0025 airfoil at a Reynolds number of 100,000 and 10-degree angle of attack. Smoke visualizations highlighted the spanwise control effects on the shear layer at the trailing edge. With the high-speed, multiple-view visualization, they identified that lower duty-cycle synthetic flow control was capable of keeping the flow attached. An optimum voltage and duty cycle for the actuators led to full flow reattachment, demonstrated by a thin, steady shear layer and a stable smoke pattern centered about the midspan. The visualizations illustrated the effects of using low-duty cycles for power-efficient synthetic jet flow control over wing surfaces.

In May, California-based MetroLaser Inc. delivered several high-speed- and ultrahigh-speed-focusing schlieren systems for the Von Karman Gas Dynamics Facility Wind Tunnels at the Arnold Engineering Development Complex in Tennessee. In June, these turnkey systems delivered high-quality images at a focal plane and quasi-planar images at framing rates of 10 kHz, 20 kHz and 100 kHz. Further, these systems used short-pulsed light sources with pulse durations of 50 nanoseconds to 2 microseconds, thereby yielding images that can be considered frozen based on Taylor’s Hypothesis and are therefore representative of supersonic and hypersonic flow regimes. For time-resolved images of relevance to hypersonic dynamics, similar 1 MHz to 5 MHz systems are also available. In August, MetroLaser operated several of its ultrahigh-speed holography systems at 100-kHz, 1-MHz and 5-MHz measurement rates, partnering with researchers at Texas A&M University to interrogate several hypervelocity impact situations pertaining to projectile-material impacts and projectile-hydrometeor impacts in Texas A&M’s two-stage light gas gun. Also in August, MetroLaser teamed with the University of Michigan to develop and build a state-of-the-art, ultrahigh-speed sensor using two-color heterodyne interferometry, which is planned for future megahertz-rate electron number density measurements in the Electric Arc Shock Tube Facility at NASA’s Ames Research Center in California.

Contributors: Pierre Sullivan and Justin Wagner