Abstract From a unified perspective, this review describes global skin friction diagnostics based on surface flow visualizations. The foundations of the developed image-based methods are relations between skin friction and other measurable surface quantities (e.g. oil-film thickness, temperature, scalar density and pressure), which are derived from the relevant governing equations in fluid mechanics. Interestingly, these relations can be re-cast into a generic form of the optical flow equation in the image plane, and therefore skin friction fields can be extracted from surface visualization images by using a variational method as an optical flow problem. Global skin friction diagnostics in typical experiments are described, including experimental setups, image processing, and topological interpretation of extracted skin friction fields.

中文翻译：

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全球皮肤摩擦测量和解释

摘要 从统一的角度，这篇综述描述了基于表面流动可视化的全局皮肤摩擦诊断。已开发的基于图像的方法的基础是皮肤摩擦与其他可测量的表面量（例如油膜厚度、温度、标量密度和压力）之间的关系，这些量来自流体力学中的相关控制方程。有趣的是，这些关系可以重新转换为图像平面中光流方程的通用形式，因此可以通过使用变分方法作为光流问题从表面可视化图像中提取皮肤摩擦场。描述了典型实验中的全局皮肤摩擦诊断，包括实验设置、图像处理和提取的皮肤摩擦场的拓扑解释。