Feasibility of Skin-Friction Measurement and Critical-Line Identification in Transonic, High Reynolds Number Flow via GLOF and TSP

Abstract

This talk presents joint efforts in the development and application of surface-based, optical measurement techniques for the skin-friction determination and critical-line identification in transonic flows at high Reynolds numbers. The two considered experimental techniques are the Global Luminescent Oil Film (GLOF) for the skin-friction estimation, and the Temperature-Sensitive Paint (TSP) for the skin-friction based identification of separation and reattachment lines. The measurements were performed on the upper surface of a VA-2 supercritical airfoil model in the Transonic Wind Tunnel Göttingen (DNW-TWG) for a Mach number M = 0.72, a chord Reynolds number Re = 9.7 million, and Angles of Attack from AoA = -0.4° to 2.0°. Conventional surface pressure measurements and laminar boundary-layer computations were also conducted to support the analysis of the GLOF and TSP results. This presentation focuses on the results obtained at AoA = 1.5°, a condition at which flow separation, transition and reattachment occurred within a shock-wave/boundary-layer interaction. The analysis of temporal sequences of GLOF images using a linear least-squares method (GLOFSFE) allowed the determination of the skin friction in the laminar flow region with a reasonable confidence level, as confirmed by the agreement with the numerical reference data. The separation and reattachment lines identified via two unrelated TSP-based approaches (the OF approach, which relies on the energy equation at the wall, and the TH approach, which relies on the relationship between the friction velocity and the propagation celerity of temperature perturbations) were shown to be in mutual agreement, and were also found to agree with the reference numerical and experimental data. This contribution concludes reporting the current limitations and the recommended improvements for the skin-friction determination via GLOF and TSP in transonic flow experiments at high Reynolds numbers, with particular regard to turbulent flow regions.