ARNOLD AIR FORCE BASE, Tenn. —

Advanced laser diagnostics are paving the way for testing at unprecedented speeds at the Arnold Engineering Development Complex Hypervelocity Tunnel 9 in White Oak, Maryland.

The new system was developed and demonstrated to better understand the Tunnel 9 flow field thermodynamic properties and inform the Computational Fluid Dynamics (CFD) engineers’ development of a new Mach 18 capability at Tunnel 9.

Originally conceived to test at Mach 20, Tunnel 9 has thus far achieved Mach 14, the highest speed in any AEDC wind tunnel.

“The higher Mach number is necessary in order to meet the requirements of our test customers,” said Tunnel 9 advanced diagnostician and physicist Michael Smith. “The advanced diagnostics, including Coherent anti-Stokes Raman Spectroscopy (CARS) and velocity measuring techniques, operate at a kHz data rate, permitting much lower uncertainty in the data due to the higher sampling rate. This is very important for hypersonic facilities, which typically have short run times.”

Laser diagnostics are optically-based measurements applied to the wind tunnel flow field to measure fundamental properties of the test gas such as temperature, density, pressure and velocity.

“Laser diagnostics are different from conventional diagnostics in that they use a beam of laser light to make the measurement instead of a physical probe,” Smith said. “Conventional diagnostics tend to disturb the wind tunnel flow, whereas laser diagnostics do not because they utilize light and are thus deemed non-intrusive diagnostics.”

The upgraded laser diagnostics are used with the CARS process to gather information about the gas molecule to which it is applied.

“It is a technique in which laser beams are focused to a common focal spot resulting in another laser, the CARS beam,” Smith said. “The CARS beam contains information about the gas. The CARS technique permits one to measure the internal energy modes of the gas molecule, which is important in order to determine the correct Mach number of the flow field.”

The technology used to measure sample speed and capture flow physics has improved since CARS measurements were first applied to the Tunnel 9 Mach 14 flow field in 2004.

“As we improve our diagnostic technologies, we can learn much more about the quality of our flows,” Smith said. “Precisely measuring temperature, velocity or other qualities improves our understanding of the physics and helps our customers get accurate results.

“These improvements open up new understandings that have paved the way to give facility designers the information they need to design a capability to test as high as Mach 18.”

Recently, through the Small Business Innovative Research (SBIR) program, a laser system provided the first non-intrusive, direct velocity measurement capability, which was then converted to a CARS system with additional optics and hardware.

This produced an additional gain in signal of a factor of approximately 1,000, Smith said.

“The result was a data rate of 1kHz and a much higher signal-to-noise ratio and, thus, a much lower uncertainty of the measurement,” he said.

The 1 kHz laser diagnostic project began around three years ago and was to supply only a velocity capability, but with the successful demonstration of the 1 kHz CARS system, the SBIR effort is ongoing to provide Tunnel 9 the velocity measuring system as well as the 1 kHz CARS system.

Smith said that effort should be finished in March of 2021.

“The new laser diagnostic systems will provide for unprecedented measurement capabilities,” he said. “The flow velocity and temperature can now be measured at a high rate. Additionally, the state of the gas is important in the determination of flow Mach number and quality of the data.”

With the advanced laser diagnostics ability, the Mach 18 capability, which began five years ago, is moving closer to the finish line.

Last year, the Tunnel 9 Mach 18 nozzle design was successfully verified using CFD codes. The nozzle is needed to extend the range of desired conditions from the Mach 7, 8, 10 and 14 currently available at the facility.

“This capability will provide for additional test customers, as it provides them with a badly needed higher Mach number test condition,” Smith said.