Improvement of extraction method of the Fizeau fringes spacing in the oil film interferometry and measurement of wall shear stress on an airfoil surface

Abstract

We aimed to measure the two-dimensional wall shear stress using oil film interferometry (OFI) for the flow around the suction surface of a two-dimensional airfoil. In a previous study, we measured the direction of wall shear stress by applying particle image velocimetry analysis method to Fizeau fringes images. In this study, we developed and compared oil application methods and OFI analysis methods to obtain the wall shear stress magnitude around the reattachment point. When we used the existing oil application methods, the fast Fourier transform (FFT)-based method could not perform the measurement due to the low quality of the Fizeau fringes around the reattachment point. However, narrowing the oil application width improved the quality of the Fizeau fringes. Next, we used an image-analysis-based algorithm to calculate the magnitude of wall shear stress. The old and new analysis methods showed qualitatively similar results for x/c = 0.08 to 0.9. We found that the image-analysis-based algorithm outperformed the FFT-based algorithm in terms of the measurement success rate. Finally, the local skin friction coefficient distribution could be measured using an image-analysis-based method and images of both the new and the old oil application methods around the reattachment point. However, the local skin friction coefficient around the reattachment point was higher for the 1 mm wide oil application than for the 15 mm wide oil application. Therefore, we consider that accounting for the upstream oil spread is important for OFI measurements around the reattachment point of a two-dimensional airfoil.

Keywords

• oil film interferometry
• image analysis
• airfoil flow
• separated flow
• wall shear stress