 Friction loss is a significant economic concern wherever fluids are made to flow, whether entirely enclosed in a pipe or duct, or with a surface open to the air.In the following discussion, we define volumetric flow rate V̇ (i.e.volume of fluid flowing) 
V̇ = πr2v

where
In long pipes, the loss in pressure (assuming the pipe is level) is proportional to the length of pipe involved.