6. Laminar and turbulent flows

A few months after Reynolds had discovered the sudden appearance of a change in the structure of the permanent flow in a tube when the velocity was increased, Couette made a similar observation for a fluid contained between two concentric cylinders, one of which rotates at constant speed. Subsequently, the existence of two regimes was observed in all flow conditions. The regime at moderate speed is called laminar, while the other is called turbulent, to mark the fact of a mixing of the fluid which is superimposed on the main movement. This can be observed on a meteorological scale: a weather vane marks a direction that is more or less constant over time, with oscillations around the mean. Wind intensity is itself variable over time, with a constant average speed modulus. Last but not least, mixing is itself observable through the complex movement of clouds.

In the...