Validation and uncertainty analysis of global luminescent oil-film skin-friction field measurement

Results from a skin-friction-field measurement method using global luminescent oil-film images, in which the oil film covers a large portion of the surface, were examined by comparing them with those obtained from conventional hot-wire measurements. The measurement processes including calibrations, oil-film image acquisition, and image processing are explained, and error sources are analyzed. The experimental results show that the skin frictions measured by both methods in a turbulent boundary layer on a flat surface agree when the oil-film thickness is less than the height of the viscous sublayer (corresponding to an oil-film thickness of less than five wall units). The uncertainty of the measurements is presented. When the measurement conditions were optimum (illumination, exposure time, frame rate, oil height, etc), the uncertainty was approximately 3%–4%. (paper)