Note that even for smooth pipes the friction factor is not zero. That is, there is a head loss in any
pipe, no matter how smooth the surface is made. This is a result of the no-slip boundary condition
that requires any fluid to stick to any solid surface it flows over. There is always some microscopic
surface roughness that produces the no-slip behavior (and thus f 6= 0) on the molecular level, even
when the roughness is considerably less than the viscous sublayer thickness. Such pipes are called
hydraulically smooth. Various investigators have attempted to obtain an analytical expression for
f = j(Re,e /D). Note that the Moody chart covers an extremely wide range in flow parameters.
The non-laminar region covers more than four orders of magnitude in Reynolds number from
Re = 4 · 103 to Re = 108. Obviously, for a given pipe and fluid, typical values of the average
velocity do not cover this range. However, because of the large variety in pipes D, fluids (r, and m
and and velocities (v), such a wide range in Re is needed to accommodate nearly all applications
of pipe flow.