6. Density-Based Techniques

The methods dealt with in this section are based on changes of fluid density; hence its index of refraction. As a result of these changes, optical phase and, coupled with it, direction of propagation of a light wave transmitted through the flow are altered in comparison to the properties of the incident light. The available signal can be presented in planar form, i.e., as a flow picture, and the methods are often referred to as optical flow visualisation, because the changes in index of refraction are detected and measured by optical techniques. The obtainable information is integrated along the whole path of the light in the fluid field (line-of-sight methods) and, in a three-dimensional (3D) object field, special techniques for interpreting the signal pattern are necessary (tomography) in order to provide local data values of the quantity to be determined, e.g., density. Four major groups of experimental methods can be distinguished: shadowgraphy, schlieren technique, moiré techniques, and interferometry. The fluid mechanical problem areas to which these optical measuring techniques can be applied are compressible flow, convective heat transfer, mixing and mass transfer, combustion, and flows with density stratification.