Aiaa-2000-0835 Comparison of Three Optical Methods for Measuring Model Deformation

The objective of this paper is to compare the current state-of-the-art of the following three optical techniques under study by NASA for measuring model deformation in wind tunnels: (1) video photogrammetry, (2) projection moiré interferometry, and (3) the commercially available Optotrak system. An objective comparison of these three techniques should enable the selection of the best technique for a particular test undertaken at various NASA facilities. As might be expected, no one technique is best for all applications. The techniques are also not necessarily mutually exclusive and in some cases can be complementary to one another. INTRODUCTION Model deformation may be defined as the changes in shape of a wind tunnel model (particularly the wings and control surfaces) under aerodynamic load. These changes in the design geometry can cause differences between the acquired and expected wind tunnel results if the expected results are based upon rigid body assumptions. Differences can also occur between acquired wind tunnel data and computational predictions based upon rigid body assumptions. These differences can lengthen and degrade the aircraft design process. The measurement of model deformation has thus been of interest for a number of years. The accurate prediction, as well as the measurement, of model static aeroelastic deformations is becoming increasingly important, especially for transonic transports. It is essential to accurately predict deformations in the wind tunnel in order to duplicate the desired CFD configuration. Deformations must often be taken into account when comparing CFD predictions to experimental measurements at off-design conditions. Increased reliance is being placed on high Reynolds number testing for configuration development ________________________ * Research Scientist, Senior member AIAA † Research Scientist Copyright © 2000 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. and CFD validation, making accurate predictions of static aeroelastic deformations very important, since deformations increase significantly at the higher dynamic pressures associated with high Reynolds number testing. Computational methods such as the finite element method (FEM) need to be calibrated and validated with wind tunnel model deformation measurements in order to ensure accurate predictions. Constraints on optical access, such as limited view ports, illumination, and targeting options often contribute to the requirement for custom, instead of commercial, measurement systems for large wind tunnels. Two custom techniques being developed and applied by NASA to measure model deformation are video model deformation (VMD) and projection moiré interferometry (PMI). The rapid development of relatively low cost electronic imaging, driven largely by the consumer video market, coupled with improvements in low cost computing have benefited the development of custom VMD and PMI techniques for the measurement of model deformation in NASA wind tunnels. VMD is based upon digital photogrammetry (or videogrammetry) using recorded and processed digitized video images from a CCD camera (figures 1, 2). Single-camera, single-view photogrammetry is then used to determine object plane coordinates and angles corresponding to targets placed at known semispan locations. A near real-time target-tracking version of VMD has been developed under NASA contract by the High Technology Corporation. Projection moiré interferometry (PMI) (figure 2) is a video-based global technique allowing spatially continuous model deformation measurements to be acquired over the entire camera field-of-view, without the use of targets. The topology of the deformed surface can be determined through off-line image processing. The third technique under study by NASA for both model attitude and model deformation measurements is the commercially available photogrammetric system, made by Northern Digital, Inc. known as the Optotrak RH-2020 (figure 3). This system has been successfully used for both aeroelastic and angle of attack measurements, in wind tunnel and flight tests. The Optotrak system, which utilizes photogrammetry of synchronized and discriminated infrared emitting