Hi, I've been reading over Transport Phenomena by Bird, Stewart, and Lightfoot and I've been going over the friction factor ##f##. I've gone through the whole development leading to the observation that, for time-averaged turbulent flow, when we neglect entry effects, ##f = f(Re; k/D)##, where ##k## is the (average?) height of pipe protuberances for rough pipes. I'm assuming that the reason why rough pipes introduce additional losses is because of the introduction of some minor form drag along the pipe wall whereas smooth pipes only have frictional drag. I'm not really understanding why this doesn't seem to be an issue for laminar flow though. Why is pipe roughness seemingly not a factor in frictional losses for laminar flow? The Moody diagram only has one curve for the laminar region corresponding to the Hagen-Poiseuille equation.