Thin-oil-film interferometric skin-friction measurements in short-duration supersonic flow

A direct experimental comparison was performed on two thin-oil-film skin-friction measurement techniques applied in short duration, supersonic flow facilities. While both techniques use laser interferometry to measure the time rate of thinning of a thin-oil-film, one technique acquires data with a dual-laser-beam (DLB) configuration and the other gathers information with an expanded-laser beam (ELB) configuration. The experimental investigation, as well as a theoretical uncertainty analysis, showed that when the data is limited due to short run-times, larger uncertainties or insufficient data for the determination of the skin-friction occurred with the DLB technique. Unlike the DLB method, the ELB technique can resolve the oil-film profile at a given instant, with two such measurements separated by a short time interval giving sufficient data to determine skin-friction. Thus the ELB technique is more suitable for short-duration facilities. (Author)