Researchers at Nasa's Armstrong Flight Research Center in California have developed a new technique for capturing visual images of supersonic shockwaves. While the shockwaves are invisible to the naked eye, they can be captured by framing the supersonic aircraft's movement against a bright light source or speckled background.
Nasa's patent-pending Background-Oriented Schlieren using Celestial Objects (BOSCO) technique shows the distortions caused by the aerodynamic flow of the shockwave passing between the camera and the background. These are made more visible using special mathematical image processing algorithms. This method requires only simple optics and a suitably textured background, such as the crater-pocked surface or the Moon or the granulated appearance of the Sun and its sunspots as viewed through certain filters.
Traditional schlieren photography techniques required complex and carefully aligned optics to capture refracted light rays that revealed the air density gradients around supersonic objects, often with the camera positioned in another plane and set up to use the desert floor as textured backdrop to show the passage of shockwaves. Previous iterations of ground-based schlieren systems relied on the Sun's edge as a light source, meaning that only two observations of the shockwave could be made as the aircraft crossed in front of the Sun.
However, Armstrong engineer Edward Haering discovered that using a calcium-K (CaK) filter to reveal the granulated texture of the Sun's chromosphere made it easy to capture the distortions caused by supersonic shockwaves. This allowed the team to "make hundreds of observations of each shockwave, greatly increasing the acuity of the camera system."
In combination with modern image processing methods, this means that shockwaves can be captured using only simple camera equipment. "Using a celestial object like the Sun for a background has a lot of advantages when photographing a flying aircraft," said Armstong principal investigator Michael Hill. "With the imaging system on the ground, the target aircraft can be at any altitude as long as it is far enough away to be in focus."
The new system eliminates the need for an airborne camera platform, reduces the complexity of flight coordination and uses consumer grade astronomy equipment. The team estimates that a similar system could be built for as little as $3,000 (£2,000). The resulting footage and photos of supersonic waves are spectacular and the system could also be used to show other kinds of air density changes, from wing vortices to engine plumes, on anything from subsonic aircraft to trucks on a highway.
Haering said the team now plans to combine different schlieren photography techniques to "visualise the complex flow patterns on future aircraft that will allow quiet supersonic overland flight for all."