Laminar, Transitional and Turbulent Flow
Heat transfer, pressure and head loss in a fluid varies with laminar, transitional or turbulent flow.
There are in general three types of fluid flow in pipes
 laminar
 turbulent
 transient
Laminar flow
Laminar flow generally happens when dealing with small pipes and low flow velocities. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe isn't moving at all.
Shear stress in a laminar flow depends almost only on viscosity  μ  and is independent of density  ρ.
Turbulent flow
In turbulent flow vortices, eddies and wakes make the flow unpredictable. Turbulent flow happens in general at high flow rates and with larger pipes.
Shear stress in a turbulent flow is a function of density  ρ.
Transitional flow
Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges. Each of these flows behave in different manners in terms of their frictional energy loss while flowing and have different equations that predict their behavior.
Turbulent or laminar flow is determined by the dimensionless Reynolds Number.
Reynolds Number
The Reynolds number is important in analyzing any type of flow when there is substantial velocity gradient (i.e. shear.) It indicates the relative significance of the viscous effect compared to the inertia effect. The Reynolds number is proportional to inertial force divided by viscous force.
The flow is
 laminar when Re < 2300
 transient when 2300 < Re < 4000
 turbulent when 4000 < Re
Related Topics

Fluid Mechanics
The study of fluids  liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
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