It is not easy to determine the functional dependence of the friction factor on the Reynolds
number and relative roughness (e /D). Much of this information is a result of experiments conducted
by J. Nikuradse in 1933 and amplified by many others since then. One difficulty lies in
the determination of the roughness of the pipe. Nikuradse used artificially roughened pipes produced
by gluing sand grains of known size onto pipe walls to produce pipes with sandpaper-type
surfaces. In commercially available pipes the roughness is not as uniform and well defined as in
the artificially roughened pipes used by Nikuradse. However, it is possible to obtain a measure of
the effective relative roughness of typical pipes and thus to obtain the friction factor.