Making fluid flows visible

Two principles of making a fluid flow visible are discussed in this paper. First, the internal edges or boundaries of the primary fluid can be made visible by introducing into it a secondary substance of a different phase. What actually can be seen is the movement of this foreign substance, which is assumed to have the same motion as the primary fluid; this requires that the particles of the foreign substance be as small as possible while still visible. The second principle applies to fluids having optical properties that change while the fluid is flowing. If the relationship between the state of the flow and the change in an optical parameter is known, optical methods could be applied to measure certain properties in the flow field without disturbing the fluid by introducing a measuring probe or a foreign material into it. Since the optical index of refraction of a gas is a function of a gas density, variations in the refractive index can be measured by means of optical interferometry.