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

Abstract

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%.

Feasibility of skin-friction field measurements in a transonic wind tunnel using a global luminescent oil film

“…In the present work, only general information is reported about this measurement technique and the related data analysis. For a detailed explanation, discussion, and validation in a low-speed flow, the reader is referred to earlier work, and in particular to Liu et al (2008); and Lee et al (2020c).…”

“…2.2 and 4.2); and h * is the unit oil-film thickness, which is a calibration coefficient. The calibration information is obtained by following Lee et al (2020c); the calibration procedure will be described in Sect. 4.2.…”

“…In the present study, the LLS-based method (Lee et al , 2020c was used for the GLOF image analysis. As discussed in detail in those publications, this method enables the extraction of the skin-friction field from a sequence of GLOF images with a sufficient number of oil-film image pairs.…”

“…As will be discussed in Subsection 2.3.3, this assumption is not mandatory for the application of the GLOFSFE, but it allows a simplification of the present description. In this case, the LLS method provides ̂ as (Björck 1996;Lee et al , 2020c:…”

“…A further GLOF image analysis method based on the linear least-squares (LLS) method was presented in , where it was applied to extract relative skinfriction fields in subsonic and supersonic flows. The LLSbased method was validated for quantitative skin-friction measurements through an experiment on a flat-plate model in a low-speed flow (Lee et al 2020c). This method has been also extended to obtain relative skin-friction distributions in a low-speed, unsteady flow field (Lee et al 2020a, b).…”