Oh, wow. Aerodynamics research has never looked as pretty as it does with this new variation of an old technique for imaging supersonic shockwaves.
Image of a supersonic T-38C using a new technique of background-oriented Schlieren imaging backlit by celestial objects. Image credit: NASA
Supersonic shockwaves are invisible to the naked eye, but we can photograph the change in refractive index resulting from the gradient in air density. The basics of schlieren imaging techniques are over a century old, but the need for complex optics and bright light sources have mostly kept its utility in aerodynamics research contained to just scale models in wind tunnels.
A supersonic jet over the Mojave Desert, processed by removing the background and combining multiple frames to capture the shockwaves. Image credit: NASA
But we’re in an era where background oriented schlieren is opening up a whole new vista of opportunity to capture real aircraft in flight. The first step of taking air-to-air schlieren images involved taking photographs from above against the speckled backdrop of the Mojave Desert. But the latest evolution of taking these images backlit by celestial objects is producing absolutely gorgeous works of scientific art.
Previous ground-based schlieren systems of photographing shockwaves backlit by the sun permitted only two observations, once as the spacecraft entered the leading edge of the sun and again as it departed the trailing edge. But sunspots make the sun a naturally-speckled surface if you look in the right wavelength, and shockwaves distort those spots. By using a calcium-K (CaK) optical filter, the granulated texture of the sun’s chromosphere is enough that engineers can capture the distorted shockwave as jets fly across the entire disc of the sun.
Shockwaves of a backlit supersonic T-38C are visible by using a calcium-K optical filter as it passes in front of the sun. Image credit: NASA
Along with being gorgeous, this also makes studying shockwaves easier. Principle investigator for the Calcium-K Eclipse Background Oriented Schlieren Michael Hill explains:
Using a celestial object like the sun for a background has a lot of advantages when photographing a flying aircraft. 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 technique is also cheaper, made from repurposing existing equipment:
The CaKEBOS imaging system was very simple, consisting of consumer grade astronomy equipment we had from previous tests,. Someone could probably build a system that would get similar results for around $3000.
The team is optimistic that this will help progress aerodynamics research into supersonic flight, possibly even finding a way to design quieter aircraft.
Schileren image of a supersonic T-38C backlit by the sun’s edge and processed with NASA code. Image credit: NASA
With a bit of luck, we’ll be using this technique a lot more in the future, and not just for jet aircraft. Hall muses, “We could potentially perform schlieren photography on anything we could get between our camera and the sun.”