Scanning-Mode Shadowgraphy for Examining Shock Waves

Lewis Research Center, Cleveland, Ohio

Scanning-mode shadowgraphy has been proposed as an alternative optical technique for diagnosis of shock waves. Under suitable conditions, scanning-mode shadowgraphy could overcome the limitations of classical shadowgraphy in such a way as to make shock waves more visible and measurable.

In classical shadowgraphy, a collimated beam of light wide enough to cover the entire flow region of interest is aimed across the flow and onto a projection screen, photographic plate, array of photodetectors, or other imaging device. Any variation in the density of the flowing medium is accompanied by a variation in the refractive index and gives rise to a shadow, which alters the distribution of brightness of light striking the imaging device. Due to simplicity of this concept, shadowgraphs have been used with success in diagnosing shocks and other flow phenomena.

Figure 1. A Narrow Laser Beam Could Be Scanned along a flow region suspected of containing a shock wave.

One limitation of classical shadowgraphy involves power density: because the beam of light is spread over a fairly wide area, either the resulting illumination is dimmer than desired, or else it is necessary to use a high-power source of light. Another limitation is that the secondary, generally weaker, phenomena caused by light diffraction and scattering on flow inhomogeneities are not visible.

Figure 2. A Laser-Beam Image Becomes Smeared Out from its compact, round cross section when it crosses a shock wave.

In scanning-mode shadowgraphy, one would overcome the power-density limitation by collimating the light into a pencil-thin beam instead of a much wider beam. The beam could be scanned along the flow region by a translating mirror or by a rotating or acousto-optical scanning reflector placed at the focal point of a collimating lens (see Figure 1). Upon encountering a region with a strong gradient of density (e.g., a shock wave), the beam would become deformed or scattered, with consequent changes in the pattern of light on the imaging device.

Experiments were conducted to compare classical and scanning-mode shadowgraphy as applied to flows of air in converging/diverging nozzles at mach numbers of the order of 2. Each nozzle was equipped with side windows. A wide, uniform beam for classical shadowgraphy was generated by a 3-mW He/Ne laser and collimating optics. A narrow beam for scanning-mode shadowgraphy was generated by a 0.5-mW He/Ne laser. Both beams were aimed through the test section of the nozzle via the windows. Figure 2 shows the results obtained in one experiment. In general, the images obtained with the scanning narrow beam revealed shocks more effectively than did the images obtained with the wider beam. This finding seems to confirm the potential of pencil-beam scanning-mode shadowgraphy for development of relatively compact, low-power apparatuses for rendering shock waves visible.

This work was done by G. Adamovsky of Lewis Research Centerand D.K. Johnson of the University of Akron. For further information,access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 127on the TSP Order Card in this issue to receive a copy by mail ($5 charge). Inquiries concerning rights for the commercial use of this invention should be addressed to