Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. Its role is changing due to the increasing use ofcomputational fluid dynamics, which reduces the need for all such experimental fluid flow measurement techniques.
The av-week article features interesting Schlieren photographs of modern (again actual) supersonic military aircraft:
From the article:
Optical schlieren uses a collimated light source that shines on the target from behind. Variations in the density of air as it flows over the object, and is compressed and expanded by shockwaves, change its refractive index and distort the beam, causing changes in the light intensity that can be visualised using a shadowgraph.
The Ground-to-Air Schlieren Photography System (GASPS), developed by MetroLaser and tested by NASA Dryden FLight Research Center, uses two telescopes, digital cameras – and the Sun as the collimated light source. The test subjects were a NASA F-15 and F/A-18 flying supersonically over Edwards AFB in California.
Transonic shock wave visualization will enable researchers to optimize the airframe shape of a future civilian supersonic transports. Controlling the shockwave around the airframe, most notably around the engine air intakes, would help to reduce the acoustic noise “foot print” when traveling over populated areas. Perhaps leading to overland commerical supersonic flights.
Aside from the scientific benefits the pictures are quite visually stunning as they were taken as the aircraft flew in front of the Sun.