In laminar flow the fluid moves in separate layers. The current lines are parallel to the pipe axis even when velocity remains unchanged and has low values. The flow is turbulent when there is intensive rearrangement of particles because of the existence of cross and some other vortices of particles. The difference between these two types of flow is the fluid velocity. In laminar flow the velocity of the fluid is comparatively low. It was found that the transition from laminar to turbulent motion is not a gradual process but occurs spontaneously and at a leap. The Reynolds number is used to determine the difference between the two regimes.

To calculate this number we have to know the pipe diameter and the kinematic viscosity of the fluid. This is extremely important when channels, that convey air from one point to another, have to be estimated as well as for pipe networks, used to carry water or another fluid for the radiator heating system, water radiant, convector heating, etc. The velocity at which one of these regimes changes into the other is called critical or the Reynolds number is critical.

It was determined experimentally that all real flows with Reynolds number smaller than 2320 are laminar and if Re is between 2320 and 13800 then the two types of flow are possible. When the velocity of the fluid is greater than 13800, the flows are only turbulent.