Measurement of Wall Shearing Stress in the Boundary Layer by Means of an Evaporating Liquid Film

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Abstract
A mass transfer method for measurement of local wall shear stress in laminar and turbulent flow is presented. A thin film of a suitable fluid is coated on the model surface and evaporation rate determined from film thickness measurements using light interference fringes. Application of the method to wall stress measurements in flat plate flow is presented and shows good correlation with theory. In the present state of development, the technique may be used for determining the shear stress distribution over a surface relative to the value at some reference station. The absolute magnitude of the shear stress at the reference station must be found by another method. In this respect the mass transfer method is similar to the interferometric method of measuring density distributions in high-speed flows.

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