Shock waves from aircraft produce density gradients in the surrounding air that affect flight performance as well as generate sonic booms. These density gradients are mostly invisible to ordinary photography, but they bend light rays coming from distant background objects such as the sun. The resulting distortion in the background objects can be used to visualize shock waves as well as exhaust plumes using the schlieren technique. We have developed a novel system for obtaining schlieren images of aircraft in flight that uses a high-speed digital camera coupled to a telescope to obtain video of an aircraft flying in front of the sun or other background light edge. The video is digitally filtered and compiled to generate schlieren images.
Capability/Advantages over Other Technologies
The first schlieren images of aircraft in flight were obtained in the mid-1990's using the sun as a background edge. Images were recorded with moving film cameras, and then later on, electronic time delay and integration (TDI) cameras that were synchronized with the motion of the aircraft. The images recorded with these systems were impressive; however, the quality of the image was critically dependent upon the alignment between camera, telescope, cutoff mask, and the sun. Consequently, these systems were difficult to use, especially for air-to-air schlieren imaging. MetroLaser's high-speed camera system is far less sensitive to alignment, and the schlieren processing is carried out on a computer, giving the end-user far more control over the quality of the final image.
Flight-testing is often used as a final critical check of aerodynamic designs developed by computational and wind tunnel methods because the information obtainable in wind tunnels is subject to interference. Outdoor schlieren systems make it possible to examine shock waves and other phenomena from aircraft in flight. Applications exist in all forms of research and development associated with turbulent flow fields, including aero optics, flow control, drag, boundary layer transition, and flow separation.
Schlieren image of passenger jet in flight showing exhaust plume.