Computer-Graphics-Based Optical Tracking for Hypersonic Free-Flight Experiments

Abstract

A novel optical-tracking technique is presented for aerodynamic measurements on free-flying models in hypersonic wind tunnels. This technique is a generalization of the edge-fitting concept developed for simple geometries in earlier work; it tracks a given model by matching its outline, captured in high-speed backlit imaging, to a computer-generated silhouette. Computer-graphics algorithms enable the generation of a mesh-resolution silhouette for any geometry in arbitrary orientation; for models without rotational symmetry, the method can thus follow at least five-degree-of-freedom motion from a single view (with six degrees of freedom possible with nonparallel illumination). In calibrated bench-top experiments, submicron positional accuracy is shown to be achievable. In Mach 6 wind-tunnel experiments in the University of Maryland HyperTERP shock tunnel, the drag of a sphere is derived with less than 1% uncertainty contribution from the position measurements, whereas the corresponding lift and drag of a capsule geometry could be obtained with less than 4% and 1% uncertainty. These accuracies are comparable to or better than existing short-duration force measurement techniques and are achieved without significant pre-experiment calibration. The technique is also highly flexible, in that the same software can be applied to measurements on a variety of test geometries with few modifications.