This paper describes a global luminescent oil-film skin-friction meter that is particularly useful in global skin-friction diagnostics for complex flows. This method is developed based on the relationship between the oil-film thickness and luminescent intensity of an oil film doped with luminescent molecules. The projected thin oil-film equation is given in the image plane, which relates skin friction with the normalized luminescent intensity. A variational formulation with a smoothness constraint on skin friction is given, and the corresponding Euler-Langrage equations are solved to obtain a snapshot solution for a relative skin-friction field. Successive snapshot solutions are superposed to reconstruct a complete relative skin-friction field, and the corresponding absolute field can be further determined by in situ calibration. This method is examined through numerical simulation and experiments.
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