Fluids in motion are subjected to various resistance forces, which are due to friction.  Friction may occur between the fluid and the pipe work, but friction also occurs within the fluid as sliding between adjacent layers of the fluid takes place.


The friction within the fluid is due to the fluid’s viscosity.  When fluids have a high viscosity, the speed of the flow tends to be low, and resistance to flow becomes almost totally dependent on the viscosity of the fluid. The condition is known as ‘Laminar flow’.


Fluids which have a low viscosity are usually moving at higher velocities.  The flow characteristics change, small eddy currents occur within the flow stream, and the friction between the pipe work and the fluid becomes a factor to be considered. This type of flow is known as ‘Turbulent flow’.