Flow nature of a fluid

The flow regime can be either laminar or turbulent, depending, for a given fluid, on the cooling system geometry in contact with the fluid and on the fluid velocity. For a given cooling system, the flow will become turbulent beyond a certain velocity of the fluid and it will be more especially as velocity increases.
The flow is laminar when it is possible to identify streamlines in the fluid which will slip one on each others ( caution ! not without friction) ; for instance, by injecting dye in the flow, we would see it moving gently while becoming slightly deformed .
A flow is turbulent when it is no more laminar, that is to say we cannot identify these streamlines any more ; the dye is diffused, mixed, chaotic, nothing easily identifiable except the presence of vortices.
This latter is one of the turbulence characteristics, which shows a whole set of vortices having very different size. This characteristic practically results in one of the properties of turbulence which concerns us in heat exchange : the mixing, we will return again to this topic ...

The following picture shows the differences between laminar and turbulent flow, for the latter the dye is much more mixed and homogenized.

Photo from the book " An album of fluid motion, Van Dyke, The Parabolic Press "

Now that we have in mind how a fluid is characterized, it is still necessary to dig a little more to manage to understand how the convection coefficient h behaves. In what follows, we will see what happens to the fluid close to the solid's surface, as it is here where everything of interest will occur. This zone bears the sweet name of the boundary layer.