Oil, with r = 950 kg/m3 and 6== 2E −5 m2/s, flows through a 30−cm-diameter
pipe 100 m long with a head loss of 8 m. The roughness ratio is e /d = 0.0002. Find the average
velocity and flow rate.
Iterative Solution. To start we need to guess f . A good first guess is the ”fully rough” value
(wholly turbulent) for e /d =0.0002 from Moody chart. It is f ≈0.015. Now from Darcy-Weisbach
formula (4) we have