Skin-Friction Measurements by Oil-Film Interferometry and Critical Evaluation of Two Optical-Flow Methods for Skin-Friction Estimation

Abstract

The work presented here was conducted under a DNW research and development support titled "Demonstration of the feasibility of skin-friction measurements by Oil-Film Interferometry on a transonic aerofoil model in the DNW-TWG". The newly developed three-color oil-film interferometry (3C-OFI) method for wall shear stress measurements was subjected to a practical test on a large test model in an industry-relevant transonic wind tunnel DNW TWG in Göttingen. By using three-color interferometry, it has been possible to achieve a particularly robust and reliable computer-aided detection of the temporal and spatial deformation of the oil-film thickness as well as the wall streamline pattern on the test surface. The measurements were performed on the suction side of the supercritical aerofoil model VA-2 with a chord length of 1 m at a Mach number of 0.72 and took place in the test section with adaptive walls, which offers only limited optical access due to its design. Two high-resolution monochrome cameras and two RGB LED spotlights were used simultaneously to record the wall shear stress distribution along the centreline in a 0.25 m wide and 0.75 m long strip. The study demonstrated the capabilities of 3C-OFI method to detect flow topology and measure wall shear stress under specific operating conditions of a transonic wind tunnel. The obtained experimental data and the accompanying numerical simulations validated with it were used to critically evaluate two methods for the global estimation of skin friction based on Liu's optical flow (OF) concept. Based on the very clear results, it was determined that the two methods investigated for estimating skin friction, GLOF-SFE and TSP-SFE, are fundamentally unsuitable for scientific applications.