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Photograph of a wind tunnel model using a schlieren system along
 with a schematic explaining the operation of the system

Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model. In some wind tunnel tests, the aerodynamic forces on the model are measured. In some wind tunnel tests, the model is instrumented to provide diagnostic information about the flow of air around the model. In some wind tunnel tests, flow visualization techniques are used to provide diagnostic information about the flow around the model.

This page describes an older flow visualization technique called schlieren photography. Schlieren photography is similar to the shadowgraph technique and relies on the the fact that light rays are bent whenever they encounter changes in density of a fluid. Schlieren systems are used to visualize the flow away from the surface of an object. The schlieren system shown in this figure uses two concave mirrors on either side of the test section of the wind tunnel. A mercury vapor lamp or a spark gap system is used as a bright source of light. The light is passed through a slit which is placed such that the reflected light from the mirror forms parallel rays that pass through the test section. On the other side of the tunnel, the parallel rays are collected by another mirror and focused to a point at the knife edge. The rays continue on to a recording device like a video camera.

Now if the parallel rays of light encounter a density gradient in the test section, the light is bent, or refracted. In our schematic, a shock wave has been generated by a model placed in the supersonic flow through the tunnel test section. Shock waves are thin regions of high gradients in pressure, temperature and density. A ray of light passing through the shock wave is bent as shown by the dashed line in the figure. This ray of light does not pass through the focal point, but is stopped by the knife edge. The resulting image recorded by the camera has darkened lines that occur where the density gradients are present. The model completely blocks the passing of the light rays, so we see a black image of the model. But more important, the shock waves generated by the model are now seen as darkened lines on the image. We have a way to visualize shock waves.

The earliest schlieren photographs of shock waves were black and white images. The image shown here is a color schlieren image produced by putting a prism near the slit and breaking the white light into different colors. Notice that the resulting image is two dimensional while, in reality, shock waves are three dimensional. So the schlieren photographs provides some valuable information about the location and strength of the shock waves, but it requires some experience to properly interpret the results of the process.

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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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