Can pipes be actually really that smooth?

In some recent papers a few approximations to the implicit Nikuradse–Prandtl–Karman equation were shown. The Nikuradse–Prandtl–Karman equation for calculation of the hydraulic friction factor is valid for the hydraulically smooth regime of turbulence. Accuracy of these approximations for the friction factor in so called smooth pipes is checked and related problems from the hydraulics are analyzed in the spotlight of the recently developed equations. It can be concluded that pipes can be treated as smooth below certain value of the Reynolds number but after that even new polished pipes with a minor roughness follow the transitional and subsequently the rough law of flow at a higher values of the Reynolds number.

Review of explicit approximations to the Colebrook relation for flow friction

Because of Moody's chart has demonstrated applicability of the Colebrook equation over a very wide range of Reynolds number and relative roughness values, this equation becomes the accepted standard of accuracy for calculated hydraulic friction factor. Colebrook equation suffers from being implicit in unknown friction factor and thus requires an iterative solution where convergence to 0.01% typically requires less than 7 iterations. Implicit Colebrook equation cannot be rearranged to derive friction factor directly in one step. Iterative calculus can cause a problem in simulation of flow in a pipe system in which it may be necessary to evaluate friction factor hundreds or thousands of times. This is the main reason for attempting to develop a relationship that is a reasonable approximation for the Colebrook equation but which is explicit in friction factor. A review of existing explicit approximation of the implicit Colebrook equation with estimated accuracy is shown in this paper. Estimated accuracy compared with iterative solution of implicit Colebrook equation is shown for the entire range of turbulence where Moody diagram should be used as the reference. Finally, it can be concluded that most of the available approximations of the Colebrook equation, with a few exceptions, are very accurate with deviations of no more than few percentages.