A simplified photogrammetry procedure in oil-film interferometry for accurate skin-friction measurement over arbitrary geometries

Abstract: An accurate and straightforward skin-friction measurement procedure over a three-dimensional (3D) bump geometry is proposed. The procedure employs oil-film interferometry (OFI) measurements coupled with photogrammetry techniques. Photogrammetry is vital for accurate quantitative skin friction measurements over curved surfaces and at different oil flow imaging settings. However, it requires information on surface geometry obtained from multiple reference points on the surface and placing the camera in the exact location and orientation before and after the wind-tunnel run, which complicates the application of OFI. A new simplified method reported here is based on camera calibration algorithms and a flexible small checkerboard. The proposed procedure removes the elaborate steps typically performed in photogrammetry while retaining the same measurement accuracy. In particular, it allows obtaining OFI data without a priori knowledge of model geometry and only taking images after experiments at arbitrary undocumented camera locations. The simplified method was first validated with the Clauser method over a flat surface in a canonical turbulent boundary layer. Then the method was applied to obtain the skin-friction distribution for turbulent flow over 3D Gaussian bump geometry, which exhibits smooth body flow separation. The obtained high-quality benchmark data over the bump geometry are aimed to serve as validation data for numerical simulations for the challenging task of predicting flow separation. Graphical abstract: [Figure not available: see fulltext.].