Empirical equation, which relates the head loss, or pressure loss \displaystyle \frac{dp}{dl}, due to friction along a the pipe length to the average velocity u of the fluid flow for an incompressible fluid:
| (1) | \frac{dp}{dl} = - \frac{ f \, \rho \, u^2 \, }{2 d} |
where
f | Darcy friction factor |
\rho | fluid density |
d | pipe diameter |