Engineers develop 360° surface temperature recording for wind tunnels

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Windward side view of the Dream Chaser model obtained with the new viewing system

The Arnold Engineering Development Complex in the USA is developing equipment for one of its wind tunnels that can provide a 360° capability for aerospace tests that use temperature-sensitive paint.

Engineers at the center, at the Arnold Air Force Base in White Oak, Maryland, have so far built a prototype system mounted in the plenum of Wind Tunnel 9 (T9) to allow for data collection from the bottom of the test cell – the most challenging view from an engineering perspective.

The capability to completely map temperature-sensitive paint (TSP) on models means the Arnold Engineering Development Complex (AEDC) is one step closer to painting the entire picture on wind tunnel models.

Dream Chaser

This plenum view system prototype, which has taken about a year to complete, has been successfully deployed for testing of the Sierra Nevada Corporation’s Dream Chaser.

The Dream Chaser spacecraft is designed to carry out commercial space transportation. It will initially transport cargo to and from the International Space Station between 2019 and 2024.

“So far all the data we’ve been getting looks really good and agrees very well with the discrete sensors as well as computational fluid dynamic predictions,” said Tunnel 9 project engineer Inna Kurits. “We feel it’s a great accomplishment given the short amount of time we had to put the prototype together to meet the customer objectives.”

TSP consists of a special paint, an ultra-violet illumination source and a scientific charge-coupled device camera to obtain surface temperature data. The paint is applied to the model in two layers – an undercoat and the TSP layer.

The undercoat provides a uniform reflective surface for the TSP. The illumination source excites the TSP layer, which results in fluorescent light emissions with its intensity inversely proportional to the surface temperature on the model.

“The paint gets darker as it heats up and gets brighter as it cools off,” Kurits said. “A unique relationship between the emission intensity and paint temperature can be established through a calibration process and allows the paint to be used as a global surface temperature sensor. The measured surface temperature time histories can be used to compute surface heat transfer.”

Windward side

Models tested at Tunnel 9 are completely coated in TSP. Equipment used to collect data from the TSP, such as the lights and cameras, is typically mounted on the top of the test cell and on one side allowing engineers to see half of the model, providing them with a 180° view.

The windward, or bottom, side of a test article is typically of the most interest to test customers, so the models are usually mounted in an inverted fashion to allow viewing of the bottom of a model from the top of the test cell.

However, this conventional method would not work with the Dream Chaser model, because the test could not be conducted with the model inverted due to the angle-of-attack range of interest. The team therefore developed a way to get around the optical access limitation.

“We needed to be able to image the windward side of the test article from the plenum of the tunnel as opposed to the top of the test cell, which is currently the standard mode of operation for TSP,” Kurits said.

“We had an idea of how to get this done, but no working prototype, so we had to put one together in short order specifically for this test program.”

Prototype development

The Tunnel 9 team developed the prototype plenum view system in several months. Challenges include that the equipment has to operate under a vacuum and the limited space within the plenum.

The equipment could also not be allowed to interfere with the tunnel pitch mechanism or the ability to open and close the test cell. Adjusting the equipment inside the plenum also proved difficult, because there is no physical access to the test cell once it is closed.

“We were able to identify cameras that could operate in a vacuum with water cooling as opposed to our standard cameras, which use fans,” Kurits said. “The other piece of the puzzle was finding the UV LED lights that are powerful enough and small enough to allow effective illumination of the model from the small space available in the plenum.”

Tunnel 9 Optical Technician AJ Spicer performed much of the laboratory work necessary to figure out how to cool the LEDs so they could be operated at full power in a vacuum without overheating.

“He also figured out how to package all the components in the small space available to us,” Kurits said.

Another challenge that presented itself to the team, and one it is continuing to work through, is data reduction.

“Going from the current system we have to a 360° system would double the amount of data we collect every run,” Kurits said. “The data processing starts taking a very long time and the files become very large.”

Kurits commended the work of Tunnel 9 project engineer John Juliano, who she said has done much work over the past year to streamline the Tunnel 9 data reduction processes and improve code efficiency. This will ultimately allow the Tunnel 9 team to process the large data volume associated with the 360° capability in the same amount of time as before.

Test deployment

Kurits said the team is not planning to deploy the 360° capability on the Dream Chaser test just yet. The first step will be to provide a simultaneous top and bottom view and a view of one side during the next test, providing engineers with a roughly 270° view.

To achieve a full 360° data collection capability, lights and cameras would be needed on all four sides of the test cell. Kurits said this is the ultimate goal, and more equipment must be procured to reach that point.

“The advancement that will allow 360° TSP capability is putting cameras and lights on the bottom of the test cell, specifically in the plenum of the tunnel,” she said.

Once the 360° system is deployed, Tunnel 9 will be able to provide customers with a more complete data set, such as quantitative heat transfer over the entire surface of a model. This system would be among the other innovations at Tunnel 9 implemented to enhance efficiency and customer satisfaction.

“The technology is constantly improving,” Kurits said. “We are trying to keep our system class-leading by keeping up with the improvements in camera and LED technologies.

“Newer cameras are smaller, faster, have higher resolution, and are cheaper. New LEDs have higher light output for the same power input. Newer computers can process larger volumes of data faster.

“All of these technologies result in improved data quality and efficiency, and hence, directly benefit the customer.”

January 17, 2018

This article was originally published on the Arnold Air Force Base website here.

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Ben has worked all of his career as a journalist and now editor, covering almost all aspects of technology, engineering and industry. In the last 16 years he has written on subjects from nuclear submarines and autonomous cars to future design and manufacturing technologies and commercial aviation. Latterly editor of a leading engineering magazine, he brings an eye for a great story and lots of experience to the team.

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