5. Similarity

Since Newton, Fourier, Ampère and many others, the question of the dimensions of entities in the physical world has been hotly debated: does an entity have an intrinsic dimension? We now know that it is the nature of the entity that is intrinsic, and that the dimension varies according to the system of units chosen as a reference to describe it. International convention has established the SI international system of units, but to deal with similarity, it is occasionally necessary to depart from this obligation.

Similarity took on great importance when, in 1883, O. Reynolds found experimentally that a flow of water in a pipe changes structure when a dimensionless grouping of physical quantities, since called the Reynolds number, takes on a certain critical value. And in the early 20th century, the German school of fluid mechanics developed the representation of phenomena...