Evaluating the applicability of a phase-averaged processing of skin-friction field measurement using an optical flow met
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R E G UL A R P A P E R
Chungil Lee
•
Taekjin Lee • Taku Nonomura • Keisuke Asai
Evaluating the applicability of a phase-averaged processing of skin-friction field measurement using an optical flow method Received: 8 November 2019 / Revised: 2 April 2020 / Accepted: 8 April 2020 Ó The Visualization Society of Japan 2020
Abstract The phase-averaged skin-friction analysis based on global luminescent oil film (GLOF) was conducted for periodically fluctuating unsteady phenomena at the frequency of approximately 150 Hz which is estimated based on Karman vortex shedding. An unsteady pressure transducer and a camera were synchronized, and the time-averaged and phase-averaged skin-friction fields were investigated. The timeseries image pair data obtained by the camera were decomposed into eight intervals of a phase angle of p=4 with synchronizing the signal of the unsteady pressure. The phase-averaged result shows the periodical pattern corresponding to the vortices structure generated from the edge of the test model which was not resolved by the time-averaged result. The phase-averaged processing was successfully applied to the GLOF measurement, and the results showed the detail information of skin friction at each phase. Keywords GLOF Skin-friction measurement Phase-averaged method Luminescent oil flow
1 Introduction Skin friction is one of the most important surface quantities for the comprehension of the behavior of the flow field, such as boundary layer, flow separation and reattachment. Therefore, various techniques of skinfriction measurement have been developed for a long time (Winter 1977; Hanratty and Campbell 1996; Naughton and Sheplak 2002; Plesniak and Peterson 2004). The Preston tube is one of the most famous techniques of skin-friction measurement and can measure skin friction at a limited location. However, it is difficult to measure the skin friction with the Preston tube over the sharp geometry, such as a thin airfoil. The other techniques of skin-friction measurement are micro-electro-mechanical system (MEMS) (Meloy 2011), an image-based interferometer for oil-film measurement (Naughton and Brown 1996) and shear sensitive liquid crystal (SSLC) (Redia and Wilder 2001). These techniques can be considered as direct or semi-direct methods because the detected quantity responds to the strength of the skin friction. MEMS is a point measurement sensor of skin friction with a high-temporal responsibility. The spatial resolution of MEMS depends on sensor density. The image-based interferometer for oil-film measurement is a method of the skin-friction measurement by computing the duration of thickness change in an oil film. The SSLC technique has been developed by Reda of the NASA Ames Research Center. This measurement method gives us the quantitative skin friction from the transient change in color that is caused by the change in the cholesteric structure of SSLC due to change in shear stress. Liu et al. (2008) explained the global luminescent oil film (GLOF) which is a method applying optical flow to
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