Self-Nulling Schlieren Imaging for Aircraft in Flight

Schlieren imaging is an especially useful tool for studying shock waves created by aircraft, because shock waves create strong refractive index gradients that can be visualized. Until recently, the large and delicate instrumentation required for schlieren photography mostly restricted the technique's use to ground test facilities. Previous work has shown that full-scale schlieren images of an aircraft in flight can be synthesized by analyzing high-speed video of the aircraft flying across the sun. The solar schlieren method is especially useful for air-to-air schlieren photography, because an airborne observation platform provides a unique perspective view of the second aircraft. We propose to advance this technique via a new self-nulling schlieren imaging method which has the potential to substantially increase the sensitivity of the technique. This technology could be also used to study large-scale aerodynamic problems where conventional laboratory schlieren imaging is impossible.

Flight tests are often used as a final critical validation of aerodynamic designs developed by computational and wind tunnel methods because the information obtainable in wind tunnels is subject to interference. Outdoor schlieren systems using the sun and moon as background edges make it possible to visualize shock waves, intense noise features, jet plumes, and other phenomena from full-sized aircraft in flight. The proposed software makes it possible to obtain schlieren images using relatively simple camera hardware that may be deployed aboard research aircraft. 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. A possible future application is characterizing wingtip vortices from aircraft. The proposed development of schlieren imaging software will be extremely important in flight tests, where few aero-optical instruments can visualize airflows around full-sized aircraft in a flight environment.

The commercial aviation industry is pursuing new designs for small supersonic jets that have low boom-shape profiles. A successful design will stimulate a market for supersonic air travel, and new aircraft are expected to roll out within the next decade. Flight tests will be necessary as a final critical check of aircraft performance, and schlieren imaging is very effective at visualizing the shock waves generated by aircraft. The proposed software provides a means to obtain schlieren images using image data collected by camera systems that are already in use aboard research aircraft. These developments will provide aircraft manufacturers with an inexpensive means of visualizing aircraft boom shape profiles in full-scale flight tests.

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