Abstract: This chapter highlights flow visualization methods; special emphasis is devoted to optical methods that can be used when density changes are present due to changes in temperature and/or composition of the fluid or to high Mach numbers of the stream (compressible flow).
Key words: hydrogen bubble, interferometer, Schlieren,
6.1 Objectives of the visualization
Flow visualization provides information on the whole flow field immediately understandable without the need for data processing. Since air and water are transparent, the flow field can be made visible only indirectly and therefore one of the following techniques is needed:
■ light scattering by gaseous, solid or liquid particles seeded in the stream;
■ behavior of materials deposited on the surface of the body immersed in the flow;
■ changes of the refractive index produced by changes in density (optical methods).
Pressure – or temperature-sensitive paints, liquid crystals, infrared thermography and particle image velocimetry cannot be considered as visualization methods since they are true measurement techniques.