ARNOLD AIR FORCE BASE, Tenn. —
Working alongside a local business and academic professionals at the University of Tennessee Space Institute, team members at the Arnold Engineering Development Complex at Arnold Air Force Base are studying optical measurement techniques to better assess the characteristics of turbine engine exhaust during ground testing.
“The US Air Force is developing new turbine engines that present special challenges during ground testing, such as heating and acoustics in the test cells,” said Capt. Brian Gatzke, Air Force Test Division. Propulsion, Test Division, AEDC. “If low-cost, non-intrusive measurements could be made to better understand the interaction of the exhaust plume with the test cell, test programs would benefit from more efficient testing and more comprehensive system analysis.”
The Propulsion Test Branch team contacted Non-Contact Technologies, LLC, a small business in Tullahoma, and UTSI to recommend appropriate measurement methods to use in test cells. In 2020, NCT and UTSI received a Phase I Small Business Technology Transfer, or STTR, through AFWERX to address the need in AEDC.
During Phase I, NCT and UTSI explored measurement techniques that would be ideal for the harsh environment of the test cells. Because the project produced favorable results, a follow-up Phase II project was awarded in 2021 to continue work on a larger scale for two different measurement techniques.
One of the non-intrusive techniques UTSI researchers use for optical diagnostics in their supersonic and hypersonic wind tunnels is focused laser differential interferometry, or FLDI. FLDI measures density perturbations in compressible fluxes through differential light patterns generated by configuring focused lasers to interfere with each other.
“The UTSI lab installed a series of lenses, prisms and filters to measure the turbulent and acoustic content of a jet generated on a test platform,” said Theron Price, a doctoral candidate at UTSI. “The high-velocity jet represents engine exhaust on a smaller scale.
“With an FLDI system, you can measure the amplitude and frequency of disturbances in or around a plume. By pointing a laser through the plume, it can capture fluctuations in density within the plume and can even estimate local turbulent velocity scales. By pointing the laser outside the plume, you can capture the acoustics of the outflow of the plume.”
The other measurement technique recommended by NCT and UTSI is the retroreflective shadow chart.
“This technique takes traditional graphic shadow imaging to a larger scale and with a simple setup that can be easily implemented in the field,” said Dr. Phil Kreth, an assistant professor at UTSI. “The retroreflective shadow graph system produces images formed by the shadow of an object or flow disturbances on a reflective surface.”
Image light is focused in such a way through the exhaust plume that the refracted light appears to a camera as light and dark regions, which vary depending on the density of the air in the plume.
“By using high-speed cameras with the system, we can image both the flow patterns in the plumes and the surrounding acoustic fields at very high rates,” Kreth said.
Shadowgraph demonstrations were done at UTSI and then on a larger scale at AEDC test cells.
“The use of optical flow visualization has been used for many years in wind tunnel tests, Gatze said. “Now we are looking at how we can use it for jet engine testing and research.”
According to Terry Hayes, chief technology officer at NCT, the goal of Phase I is to develop a technique that can benefit not only DoD research and test organizations, but also commercial industry.
“UTSI led the development of field-deployable, non-intrusive flow measurement techniques and associated data reduction methodology,” said Hayes. “In addition, our NCT team has the expertise on the dynamic data analytics side and can bring field-ready rugged equipment production and non-intrusive measurement expertise to the table. Working together, our goal is to create a solution that provides the AEDC and key Air Force stakeholders with a capability that meets their nozzle testing research requirements and also provides a deployable system that can be used in the field or an altitude test cell to meet the requirements of its customers.”
