The Darcy friction factor was first described by Julius Weisbach, a German professor who in 1850 published the first modern thextbook on hydrodynamics. It is named after Henry Darcy, a French engieer whose experiments in 1857 first established the effect of roughness on pipe resistance ## 1. Darcy Weisbach equation $\frac{\Delta p}{L}=f_D\frac{\rho}{2}\frac{v^2}{D}$ where, $f_D$ is the Darcy frirction factor $\Delta p$ is the pressure drop $L$ is the length of the pipe $\rho$ is the density $v$ is velocity $D$ is diameter or interms of pressure head pressure head is $h_f = f_D\frac{L}{D}\frac{v^2}{2g}$ for Laminar flow $f_D=\frac{64}{Re}$ ## 2. Fanning Friction factor Fanning friction factor $f=\frac{\tau}{\rho\frac{v^2}{2}}$ where, $f$ is fanning friction factor $\tau$ is shear stress Its related to Darcy friction factor by $f_D=4f$ See [[Methods for calculating friction factor]] Source: *Fluid Mechanics*, Frank White