A simplified Moody diagram for circular pipe flow. Only three transport curves are shown: the laminar case, turbulence in a pipe with smooth walls, and one choice of rough wall turbulence. For the rough wall curve, the scale height of the roughness is 5% of the pipe diameter. Below the minimum critical Reynolds number, a perturbed flow will always return to the laminar state. A transition to the turbulent state is marked by a discontinuity in the transport level. The minimum magnitude of the discontinuity in this case is approximately 2.