Flow Visualization using the Shadowgraph and Schlieren Optical Methods

Abstract The use of experiments is one of the most efficient ways to teach physics, since students become more motivated when fundamental theoretical concepts are associated with technological applications. The development of cars, aircraft and rockets, for example, requires detailed knowledge of the flow of fluids around and inside these vehicles. This article describes the fundamentals of geometric optics in heterogeneous media and presents the shadowgraph and schlieren optical techniques that are widely used in research centers, universities and companies in the study of aerodynamics and flows in general. These optical methods are based on the deflection of the light rays caused by variations in the flow density. The schlieren-Toepler method is discussed in detail, deriving equations for contrast, sensitivity and optical alignment. An implementation of the direct shadowgraph method for classroom use is demonstrated and, throughout the article, several images obtained by the described optical methods are presented.

摘要 实验教学是物理学教学最为高效的途径之一，这是因为当基础理论概念与技术应用相结合时，学生的学习动机将显著增强。以汽车、航空器与火箭的研发为例，其过程需要精准掌握此类运载工具内外的流体流动规律。本文阐述了非均匀介质中的几何光学（geometric optics）基础，并介绍了阴影摄影法（shadowgraph）与纹影法（schlieren）这两类被广泛应用于科研机构、高校及企业，用于空气动力学与各类流体流动研究的光学技术。此类光学方法基于流动密度变化引发的光线偏折效应。本文详细讨论了纹影-托普勒法（schlieren-Toepler method），推导了其对比度、灵敏度与光学对准相关的计算公式。此外，本文还演示了一种适用于课堂教学的直接阴影摄影法实现方案，并在全文中展示了多幅通过所述光学方法获取的实验图像。