The Colebrook equation
1/[square root of (f)]=-2 log[([epsilon]/3.7D)+2.51/[R.sub.e][square root of (f)])] (1)
has long been used for calculating the friction factor, f, for incompressible and some compressible flows in uniform pipes, ducts, and conduits. It is asymptotic to both the accepted smooth-surface and rough-surface pipe equations.
Although widely used, the Colebrook equation is iterative because the unknown friction factor appears on both sides of the equation. To solve for this unknown, one must start by somehow estimating the f value on the right side of the equation, solve for the new f on the left, enter the new value back on the right side, and ...