Modern Optical Techniques in Fluid Mechanics

Abstract

Optical techniques are widely used in fluid mechanics to observe and measure properties of flow fields such as velocities or densities. Many of these techniques are qualitative but of great value in guiding intuition for further research by quantitative means. Beautiful examples can be seen in the Album of Fluid Motion (Van Dyke 1982). Optical techniques are usually known for their largely nonintrusive properties as compared with methods like the Pitot tube or the hot-wire technique. The last few years, however, have seen some examples where light has been used not only to probe fluid flows but to generate them (Lauterborn 1980). This gives rise to a new classification of optical techniques in fluid mechanics (see Figure 1). Flow­ visualization techniques use light as an information carrier where the information is impressed on the light beam by the fluid flow. Flow­ generation techniques use light as an energy carrier to initiate fluid flow by radiation pressure, heating, or optical breakdown. Flow-visualization techniques may be coarsely subdivided into two categories: those that make use of light scattered by tiny particles in the fluid and those that make use of variations in refractive index. Among the methods that rely on scattered light, laser Doppler anemometry is now a standard means of obtaining fluid velocities. This method and its various refinements are well documented (Durst et al. 1976, Durrani & Greated 1977, Drain 1980, Schulz-DuBois 1983) and are not discussed here. In laser Doppler anemometry, the fluid velocity can be measured with high accuracy as a function of time but only at a single point in the fluid at any given time. The ultimate aim, of course, is the simultaneous determination of fluid velocities in a whole volume of a fluid. First steps in this direction