Schlieren photography
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Image:Schlierenfoto Mach 1-2 gerader Flügel - NASA.jpg
Schlieren photography is a visual process that is used to photograph the flow of air (or other compressible fluids) around objects. Used by physicist Ernst Mach to study supersonic motion, it is widely used in aeronautical engineering. Its role is changing due to the increasing use of computational fluid dynamics, where the same principle is used to display the computed results as flow images.
The basic system uses light from a single collimated source shining on, or behind, a target object. If the fluid flow is uniform the image will be steady, but any turbulence will cause scintillation, the shimmering effect that can be seen on hot surfaces on a sunny day.
In order to make the effect more pronounced, a knife-edge is placed in front of the camera, positioned to block off about half of the light. In uniform flow this will simply make the photograph half as bright. However in turbulent flow the light's path will be bent, due to changes in the index of refraction caused by density gradients. In this case some of the light that would otherwise be seen in the camera will instead hit the knife-edge, and some that would normally be blocked will become visible. The result is a set of lighter and darker patches corresponding to different densities in the fluid.
When a knife-edge is used, the system is generally referred to as a schlieren system, which measures the first derivative of density in the direction of the knife-edge. If a knife-edge is not used, the system is generally referred to as a shadowgraph system, which measures the second derivative of density.
A simple schlieren system can be built with a single mirror with a long focal length; telescope mirrors can be used for this purpose. The camera and light source are placed close together on either side of the focus, aimed at the mirror. Any straight-filament light bulb will serve; those used in a refrigerator are fine. The target is then placed between the camera and the mirror, typically as close to the mirror as possible to reduce the effective angle between the light source and camera. Light from the filament travels to the mirror where it is collimated, then across the target and to the camera.
With high-speed cameras, high-magnification lenses and very bright lights, schlieren photography can make the details of airflow visible. This makes it invaluable for studying streamlining and aerodynamics. Replacement of the knife-edge by a colored "bullseye" target by Wally Howes resulted in Rainbow Schlieren which makes these changes even more obvious.
The adaptive optics pyramid wavefront sensor is a modified form of schlieren (having two perpendicular knife edges formed by the vertices of a refracting square pyramid).
Unfortunately few if any complete schlieren optical systems are commercially available today, making it necessary for the experimenter to assemble the system from optical parts. The theory and practice of schlieren imaging, including instructions for setting up a simple "science fair" schlieren optical system as well as professional equipment, are given for example in Ref. 1. During the Soviet era, however, many fine schlieren systems were produced. These were designated "IAB-451" and were based on the Maksutov telescope principle. Many of these instruments still survive in the former Soviet Union and China.
[edit] References
1Settles, G. S., Schlieren and shadowgraph techniques: Visualizing phenomena in transparent media, Berlin:Springer-Verlag, 2001.
