A surface-imaging skin-friction instrument provides measurement of the wall shear-stress vector over a large surface region during a single wind tunnel test using an oil-film interference method. During a test, a camera captures images of the fringe pattern produced by illuminating an oil film with quasi-monochromatic light. By using a visual tracer in the oil, the surface streamlines can also be determined. Analysis of the fringe images using a Hilbert-transform-based technique determines the oil thickness distribution in the region where fringes are visible. A combination of the oil thickness and surface direction is then used to calculate the surface shear-stress distribution by numerically solving the thin-oil film equation: where tau(sub x) and tau(sub z) are the x- and z-components of the surface shear stress, and mu is the dynamic viscosity of the oil. All quantities in this equation are known from the height measurement except tau(sub x) and tau(sub z). However, an additional piece of information is needed to solve this equation. If the surface streamline direction gamma is obtained from visual tracers or from a streamline image, then tau(sub x) = tau cos(gamma) and tau(sub z) = tau sin(gamma), and the equation may be solved for tau. To test the technique in a demanding situation, it was applied to a three-dimensional (3-D) flow in which a cylinder is mounted normal to a flat plate. An incoming turbulent boundary layer encounters the vertically mounted cylinder and a highly complex 3-D flow results.

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