11. Flow Visualization

Most fluids, gaseous or liquid, are transparent media, and their motion remains invisible to the human eye during direct observation. Techniques allowing visualization of the flow, usually referred to as flow visualization, are discussed in this chapter. A great variety of such methods is known that enable one to make fluid flows visible, in the fluid mechanical laboratory, in industrial environments, and for field experiments. These methods rely mostly on the addition of a tracer material to the flowing fluid, e.g. dye or smoke, and what is then observable is merely the motion of the tracer. Differences between the motion of the tracer and that of the fluid are aimed at being minimal. Presented in this chapter are surveys of available tracer materials, techniques of introducing the tracer to the flow, techniques of proper illumination of the flow scene, methods of providing optical access to the flow, and recording of the observable information. Some of the flow visualization methods provide qualitative information on particular flow patterns, others allow to measure the flow velocity quantitatively.