Skin Friction from Global Luminescent Oil-Film Visualizations

This chapter describes the global luminescent oil-film (GLOF) skin friction meter that is simply called GLOF. The thin oil-film equation is discussed to elucidate the evolution of an oil-film on a surface in an aerodynamic flow. The thin oil-film equation is recast to a form related to the luminescent intensity of an optically thin luminescent oil-film, and it is further projected onto the image plane. The projected thin oil-film equation in the image plane has the same mathematical form of the physics-based optical flow equation, where the equivalent skin friction is represented as the optical flow. Therefore, the variational method used for the optical flow problem is applied to extraction of relative skin friction fields from GLOF images. An error analysis for GLOF is given, and averaging of snapshot solutions is proposed for reconstruction of a skin friction field. One example of the application of GLOF is a low aspect ratio wing at a high angle of attack (AoA), focusing on the complex skin friction topology on the upper surface and the Poincare-Bendixson index formula as a topological constraint in global flow diagnostics. Another example is a wing-body junction where the complex skin friction topology is reconstructed from GLOF images obtained from different views