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.

Figure optionsDownload as PowerPoint slideHighlights
► Problem of flow in the hydraulically smooth regime of turbulence is investigated.
► Perfectly smooth pipes actually do not exist.
► Hydraulically smooth regime does not occur only in the absence of roughness.
► Roughness effect can be minor, but with slightly increased value of the Reynolds number it will be inevitable appear.
► Related problems from hydraulics is analyzed in the spotlight of the recently developed equations.