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An Air Force Test Pilot School T-38C passes in front of the sun at a supersonic speed, creating shockwaves that are caught photographically for research.
NASA is using a modern version of a 150-year-old German photography technique -- schlieren imagery -- to visualize supersonic flow phenomena with full-scale aircraft in flight. The results will help engineers to design a quiet supersonic transport. Although current regulations prohibit unrestricted overland supersonic flight in the United States, a clear understanding of the location and relative strength of shock waves is essential for designing future high-speed commercial aircraft.
Photographic Shockwave Research Reaches New Heights with BOSCO Flights
Principal investigator Mike Hill (left) awaits data along with subproject manager Brett Pauer (right) as the Air Force Test Pilot School T-38 creates visible shockwaves in front of the sun via supersonic flight in March 2016.
Credits: NASA Photo / Ken Ulbrich
NASA’s goal of developing a quiet supersonic aircraft is another step closer following a pair of successful first flights in a series demonstrating patent-pending Background Oriented Schlieren using Celestial Objects (BOSCO) technology, effectively using the sun as a background in capturing unique, measurable images of shockwaves.
Improved image-processing technology makes it possible to capture hundreds of observations with each shockwave, benefiting engineers in their efforts to develop a supersonic aircraft that will produce a soft “thump” in place of a disruptive sonic boom.
The tests, flown from NASA’s Armstrong Flight Research Center in Edwards, California, build on other recent NASA tests to further the art of schlieren photography. Schlieren is a technique that can make important invisible flow features visible. Although schlieren has been in use for over a century, recent research by NASA has enabled its application in flight and greatly enhanced the detail of the images that can be obtained. In this case, NASA improved schlieren captured the visual data of shockwaves produced by a U.S. Air Force Test Pilot School's T-38 aircraft traveling at supersonic speeds. The tests used a camera lens filter commonly used when photographing the sun The filter, known as a hydrogen-alpha solar filter was installed in one of three modified high-speed cameras positioned strategically on the ground, and allowed visually fine details of the sun to be seen.
As a result of the research, the supersonic aircraft and its shockwaves are seen with distinct clarity in front of the solar background. Observing air density changes makes the details clearer, explained BOSCO principal investigator Mike Hill.
“The hydrogen alpha filter basically looks at light coming off of certain hydrogen atoms on the sun’s surface,” Hill said. “By looking at one specific wavelength of light it brings texture out on the image of the sun. That texture is what we use to process the raw images into schlieren images.
“As the light rays come through the flow around the airplane, the different air density caused by the flow bends the light, which allows us to see the texture of the sun’s surface move on the digital image. We can calculate how far each “speckle” on the sun moved, and that gives us the schlieren image.”
An Air Force Test Pilot School T-38 passes in front of the sun at supersonic speed, creating shockwaves that are captured using schlieren photography to visualize supersonic flow.
Credits: NASA Photo / Ken Ulbrich
This concept is similar to seeing heat waves that are coming off of a hot surface in the summer. The blur of the objects in the distance is visible because the hot air over the surface is a different density than the air around it. When light travels through that density change, it bends, causing objects in the background to appear blurry to the eye.
The BOSCO technique is one of two exciting new NASA developments in the field of schlieren. The second involves using cameras on one airplane to photograph another. Both techniques are capable of producing images of greatly improved quality, and each has unique features.
“One advantage for BOSCO is that we’re flying one airplane,” Hill explained. “We can have our cameras on the ground, and we can use consumer-grade telescopes and non-flight rated equipment. We don’t need to put any imaging equipment on an airplane, so there are obvious savings in operational costs.”
This method can be applied to imaging wing vortex locations and relative strengths important for NASA Armstrong’s research into improved efficiency for subsonic aircraft.
Background Oriented Schlieren (BOS), which is a useful method for capturing clear, accurate images of shockwaves, distorts background patterns, allowing the location of the waves to be analyzed, tracked and compared in a series of photographs captured by the high-speed cameras.
BOSCO continues the work of Calcium-K Eclipse Background Oriented Schlieren, or CaKEBOS, which initially validated the concept of using the sun as a background in BOS photography. The hope with the new imaging system used in BOSCO is to capture a more detailed picture of the flow field around the aircraft.
Commercial Supersonic Technology subproject manager Brett Pauer says the first flights for BOSCO met high expectations. Pauer was involved in NASA’s photographic research of shockwaves through AirBOS, CaKEBOS, and Ground-to-Air Schlieren Photography System (GASPS).
Shockwaves produced by a U.S. Air Force Test Pilot School T-38 banking at Mach 1.05 are captured by a new ground-operated camera and filter to study flow patterns and provide NASA engineers with methods of furthering research toward developing a soft “thump” in place of heavy sonic boom.
Credits: NASA Photo
“I am very happy with these flights,” Pauer stated. “Our Air Force pilots were spot on, our ground operators performed very well, and we captured some spectacular images. These have been our most successful ground-based schlieren flights yet.”
The data collected from the flights will help engineers determine the most sufficient method of designing and executing further tests in NASA’s research of shockwaves created by supersonic flight. The overall goal of the schlieren imaging research is to develop a system to image the shock waves propagating from the bottom of the aircraft to the ground. This necessitates imaging a side view of the aircraft in near level flight.
Visualizing these complex flow patterns of shockwaves produced by a supersonic vehicle will allow NASA researchers to validate design tools used to develop the proposed Quiet Supersonic Technology (QueSST) research aircraft. QueSST will be the first ever aircraft to demonstrate supersonic flight with the soft sonic “thump”, and could unlock the future to commercial supersonic flight over land.
Matt Kamlet, Public Affairs NASA Armstrong Flight Research Center