This paper develops the principle of global skin friction diagnostics based on temperature sensitive paint (TSP) measurements in flows. The asymptotic form of the energy equation at a wall is derived, and its inverse solution is sought by using the variational method to extract a relative or normalized skin friction field from TSP-measured surface temperature fields. The selection of the relevant parameters (the Lagrange multiplier and the standard deviation of a Gaussian filter) and the intrinsic limitations of this method are discussed in the light of uncertainty analysis. Experimental implementation of this method is described, and the feasibility of TSP-based global skin friction diagnostics is examined in impinging jet experiments. The normalized skin friction distributions extracted by using this method are in fairly good agreement with hot-film skin friction measurements and the theoretical solutions in the normal and oblique impinging jets.
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