The feasibility of remotely acquiring infrared images (thermograms) of aircraft surfaces in flight to locate flow-transition boundaries has been investigated. As used here, "remotely" means that the infrared instrumentation is mounted aboard an observing aircraft that flies along with an observed aircraft, the surface flow on which one seeks to analyze.
Infrared thermograms are, in effect, maps of surface temperatures. Because the rate of mixing in a turbulent boundary layer is greater than in a laminar boundary layer, the turbulent boundary layer transfers heat between the freestream and a surface at a rate greater than that of the laminar boundary layer. Therefore, a surface that is initially warmer than the freestream exhibits a higher temperature in the presence of a laminar than in the presence of a turbulent boundary layer. Similarly, a surface that is cooler than the freestream exhibits a lower temperature in the presence of a laminar than in the presence of a turbulent boundary layer. Therefore, further, one can utilize a thermogram that shows adjacent surface regions with different temperatures to locate the transition between turbulent and laminar boundary layers on the surface.
In-flight thermograms have been acquired from a camera located in or on the aircraft of interest. This approach entailed a number of limitations and disadvantages, including a small field of view and the time and cost of instrumenting each aircraft that one seeks to observe.
In initial tests of the present remote-observation approach, the observing aircraft was an F-18 airplane equipped with a remotely actuated infrared camera and tracking system, and the observed aircraft was a T-34C airplane (see figure). Surface areas of interest were treated by covering them with thin black vinyl contact sheets to minimize reflections, to reduce thermal conductance into the structure, and to raise surface temperature through solar heating.
It was determined from the results of these tests that the desired thermograms can be acquired remotely, and that transition locations and patterns can be extracted from the thermograms. It was also determined that with optimal geometry between the observed and observing aircraft, spatial resolution as low as 0.1 in. (2.5 mm) can be realized. The fields of view obtained in the tests were significantly wider than those in similar images obtained with an on-board system. The images obtained were comparable in quality to those obtained with an on-board system.
Plans for research to be performed in the near future call for obtaining images from a business jet and a large transport airplane, attempting to depict local shock waves and flow separation in addition to laminar-to-turbulent flow transition, and obtaining images without vinyl surface treatment.
While the remote-observation approach has been found to overcome most of the disadvantages of the previous on-board approach, it entails limitations of its own. These limitations include distortion caused by relative motion between the airplanes during image frames and by changes between observational geometries in successive image frames. Research has been performed to develop a capability to process image data to correct for such distortions and to effect general enhancement of images (e.g., to increase signal-to-noise ratios and optimize contrasts).
This work was done by Daniel W. Banks of Dryden Flight Research Center and C.P. van Dam and Henry J. Shiu of U.C. Davis. DRC-98-73