2. Seeing the invisible: flow, phase object or scattering object

Most flows of interest to human activity are transparent to visible light, although a few important examples contradict this assertion: blood flows, lava flows in volcanology or molten metals in the iron and steel industry. The development of a visualization method therefore generally requires the choice of a physical phenomenon to overcome the problem of fluid transparency. The dichotomy observed throughout history remains valid today: the two main families of methods exploit either light scattering by tracers, or variations in the refractive index of the fluid itself.

• Flow, "diffusing object

  Leonardo da Vinci's initial idea of following the movements of tracers placed within the fluid has been taken up and improved in a wide variety of forms: liquid or gaseous flows seeded with dyes, fumes or solid or liquid microparticles,...