Local surface shear stress measurements from oil streaks thinning rate

Abstract

This paper presents an approach to evaluate the time-averaged local shear stresses based on the analysis of video images of oil streaks. The time-averaged deformation rate of thin oil streaks, deposited on a model exposed to an air stream, increases pro rata with the skin friction, while it is inversely proportional to the viscosity. Hence, the oil viscosity represents an important parameter in the method to be carefully characterized and selected for the level of aerodynamic skin friction, flow velocity, and time interval to be resolved. Post-processing image techniques were applied to reconstruct the rate of change in the oil thickness from oil streak length monitored by a high-speed camera. The present technique was demonstrated on a model exposed to subsonic and supersonic flows, and results were compared with theoretical calculations. The present measurement procedure offers ancillary information to the conventional qualitative oil dot visualizations, providing the wall shear stresses at a reduced cost and complexity.