Flow in a commercial steel pipe

Mean flow measurements are obtained in a commercial steel pipe with krms/D= 1/26000, where k(rms) is the roughness height and D the pipe diameter, covering the smooth, transitionally rough, and fully rough regimes. The results indicate a transition from smooth to rough flow that is much more abrupt than the Colebrook transitional roughness function suggests. The equivalent sandgrain roughness was found to be 1.6 times the r.m.s. roughness height, in sharp contrast to the value of 3.0 to 5.0 that is commonly used. The difference amounts to a reduction in pressure drop for a given flow rate of at least 13 % in the fully rough regime. The mean velocity profiles support Townsend's similarity hypothesis for flow over rough surfaces.