Skin Friction from Surface Optical Flow

This chapter describes extraction of a skin friction field from the surface optical flow (SOF) that is defined as a convection velocity of a surface scalar quantity based on the evolution equations of the surface temperature, scalar concentration, and enstrophy in which the SOF is proportional to skin friction. These evolution equations are written as a generic form of the optical flow equation. Therefore, the SOF can be determined by solving the optical flow problem from a time sequence of fields of a surface quantity (temperature, scalar concentration, or enstrophy). This SOF method is applied to extraction of skin friction fields from the surface enstrophy fields obtained in direct numerical simulation (DNS) in a turbulent channel flow and surface temperature fields obtained in time-resolved temperature- sensitive paint (TSP) measurements in the NACA0015 airfoil flow and impinging jets.