The molecular interaction between flowing fluid and pipeline walls which resists the free fluid flow.
This effect can be modelled through a friction body force acting on all fluid volume against the fluid velocity \bf u direction:
| (1) | \frac{\delta F}{\delta V} = \frac{\delta p \cdot A}{\delta l \cdot A} = \frac{d p}{ dl} |
which is basically representing a pressure loss along pipeline.
The most popular mathematical model is given by Darcy–Weisbach equation.
See also
Physics / Fluid Dynamics / Pipe Flow Dynamics
[ Darcy friction factor ] [ Darcy–Weisbach equation ]
[ Pipe Flow Friction Losses @model ][ Darcy friction factor in water producing/injecting wells @model ]