chapter  6
Characteristics of laminar and turbulent flow
Pages 21

Fluid viscosity is the measure of the internal resistance sustained in a fluid being transported in a pipe or duct as one layer moves in relation to adjacent layers. At ambient temperature, heavy fuel oils, for example, possess a high viscosity while the lighter oils possess a low viscosity. The walls of the pipe or duct provide the solid boundaries for the fluid flowing and because of the friction generated between the boundary and the fluid interface, which has a drag effect, and fluid viscosity, the velocity of flow varies across the enclosing boundaries to produce a velocity gradient. In a straight pipe or duct, maximumfluid velocitywould be expected to occur along the centreline and zero velocity at the boundary surfaces.