Photogrammetry in Oil-Film Interferometry

Abstract

Theapplication of photogrammetry to the measurement of skin friction using oil-film interferometry isdiscussed. Oil-film interferometry and photogrammetry are first described to justify the need for photogrammetry, and then the method is applied to two flows. The results indicate that when model curvature is high or the camera-to-model distanceisshort,largeuncertainties(oftheorderof10%)inskinfrictioncanbeintroducedifphotogrammetryisnot applied. For most applications demanding high-accuracy skin-friction measurements, approaches such as those described in this article should be considered. HEmeasurementofwallshearstressinaerodynamictestinghas increased significantly in the last decade. The increased use is primarily a result of the growth of image-based oil-film interferometry, which provides measurements of the mean wall shear stress that can be obtained relatively quickly and with good accuracy. The technique has been applied to simple laboratory flows as well as in large wind tunnels. Accompanying the increased use of oil-film interferometry has been a demand for higher accuracy and less time-intensive analysis. One way that these two issues can be addressedisthroughtheuseofphotogrammetry.Althoughthisisthe case, photogrammetry has only been applied to oil-film interferometry in a few cases (e.g., Zilliac [1]), and an in-depth discussion of its importance has not been presented. The use of photogrammetry to facilitate oil-film interferometry measurement of skin friction is the focus of this paper. The theory behind photogrammetry is presented, and specific issues related to oil-film interferometry are discussed. Specific examples of oil-film interferometry measurements that include photogrammetry are then provided. From these example cases, it is clear that photogrammetry reduces and quantifies geometric/imaging bias errors, and it substantially simplifies the handling of geometry in the analysis process. With uncertainty analyses of oil-film interferometry indicatingaccuraciesof2–3%whenperformedwithcare[2,3],these geometric/imaging bias errors become increasingly important if that level of accuracy is to be maintained in complex geometries. Althoughaparticularanalysismethodfordeterminingthewallshear stress from the fringe patterns obtained using oil-film interferometry isusedheretodemonstratetheapplicationofphotogrammetry,itwas simply chosen for convenience. The photogrammetry method discussed here can be used by other oil-film interferogram-analysis approaches to improve the geometry handling.