ABSTRACT
The ability to visualize flows of liquids and gases by density-based methods is based on light refraction and on the fact that changes in the density of a fluid lead to changes in its refractive index. The exact relation between density and refractive index may be expressed in several ways. One of the most well-known variants describing the dependence of gas refractive index on its density is the Gladstone–Dale equation. Shadowgraph is one of the earliest and simplest techniques used for visualizing fluid flows. Lasers provide intense and spatially coherent light, but intense diffraction and coherent artifact noise degrade the resulting image. The schlieren method is a technique widely employed nowadays for qualitative and quantitative analysis of fluid flows. The essence of the background-oriented schlieren (BOS) method is the comparison of two images of the same background, taken with and without the investigated transparent object between the camera and the background.
