Preprint Article Version 1 This version is not peer-reviewed

# Advanced Iterative Procedures for Solving the Implicit Colebrook Equation for Fluid Flow Friction

Version 1 : Received: 26 July 2018 / Approved: 26 July 2018 / Online: 26 July 2018 (04:47:16 CEST)

A peer-reviewed article of this Preprint also exists.

Pavel Praks and Dejan Brkić (2018). Advanced iterative procedures for solving the implicit Colebrook equation for fluid flow friction. Advances in Civil Engineering vol. 2018, article 5451034 Pavel Praks and Dejan Brkić (2018). Advanced iterative procedures for solving the implicit Colebrook equation for fluid flow friction. Advances in Civil Engineering vol. 2018, article 5451034

Journal reference: Advances in Civil Engineering 2018, 2018, 5451034
DOI: 10.1155/2018/5451034

## Abstract

Empirical Colebrook equation from 1939 is still accepted as an informal standard to calculate friction factor during the turbulent flow (4000 < Re < 108) through pipes from smooth with almost negligible relative roughness (ε/D→0) to the very rough (up to ε/D = 0.05) inner surface. The Colebrook equation contains flow friction factor λ in implicit logarithmic form where it is, aside of itself; λ, a function of the Reynolds number Re and the relative roughness of inner pipe surface ε/D; λ = f (λ, Re, ε/D). To evaluate the error introduced by many available explicit approximations to the Colebrook equation, λ ≈ f(Re, ε/D), it is necessary to determinate value of the friction factor λ from the Colebrook equation as accurate as possible. The most accurate way to achieve that is using some kind of iterative methods. Usually classical approach also known as simple fixed point method requires up to 8 iterations to achieve the high level of accuracy, but does not require derivatives of the Colebrook function as here presented accelerated Householder’s approach (3rd order, 2nd order: Halley’s and Schröder’s method and 1st order: Newton-Raphson) which needs only 3 to 7 iteration and three-point iterative methods which needs only 1 to 4 iteration to achieve the same high level of accuracy. Strategies how to find derivatives of the Colebrook function in symbolic form, how to avoid use of the derivatives (Secant method) and how to choose optimal starting point for the iterative procedure are shown. Householder’s approach to the Colebrook’s equations expressed through the Lambert W-function is also analyzed. One approximation to the Colebrook equation based on the analysis from the paper with the error of no more than 0.0617% is shown.

## Subject Areas

Colebrook equation; Colebrook-White; iterative methods; three-point methods; turbulent flow; hydraulic resistances; pipes; explicit approximations; Newton-Rapson; Household’s methods