Oil Film Interferometry
One of the most accurate and reliable way to measure the shear stress is oil film interferometry (OFI) technique. In this technique, oil is applied to the surface of the wind tunnel model. When fluid/air flows over the model surface, the shear stress that develops on the model surface causes the oil to create a thin film (thickness of this film is in the order of microns). When this film is illuminated by the quasi-monochromatic light, it produce interference patterns. If the image of the interference pattern is captured sometime after the flow has started, the spacing between fringes can be related to the shear stress acting on the oil film. Several oil drops or lines can be applied to the model to simultaneously determine shear stress at many locations on the model.
List of Experiments