Mean velocity measurements in a commercial steel pipe in the smooth to fully rough regime*

Mean velocity profiles and friction factors were obtained in fully-developed flow in a commercial, extruded steel pipe. The surface roughness was as delivered by the supplier, with $k_{rms} = 5 \mu$m. These are believed to be the first results ever obtained in commercial steel pipe under laboratory conditions. It was found that commercial steel pipe has a friction factor behavior that lies between the Colebrook type roughness, and sand-grain type roughness, in that the departure from the smooth pipe is rather sudden, with little or no evidence for the “inflectional” behavior characteristic of sand- grain roughness (and honed surface roughness). In addition, the equivalent sand-grain roughness of commercial steel pipe is closer to 2.0$k_{rms}$, rather than the commonly accepted value of 3.0$k_{rms}$. The velocity profiles in the rough regime show the expected departure below the logarithmic law, and they also follow Townsend's outer-layer similarity.