There are two dimensionless groups given the name Darey number.
![](/content/5107/eqn025.gif)
where Δp is the pressure drop, D the diameter, ρ fluid density, u fluid velocity, and L the length over which the pressure drop is measured. f is the Fanning friction factor (see Friction Factors).
![](/content/5107/eqn026.gif)
where D' is the permeability coefficient of a porous medium with units m2/s (see Darcy's Law).