Central to the concept of forced internal flow is the idea of fully developed flow. Fluid at the entrance of a pipe has the same velocity at all cross section locations. The fluid velocity at the wall, however, has zero velocity - the no slip condition. As we travel with the fluid down the pipe, we find that the effect of the wall is felt more and more within the core of the pipe. In other words, a boundary layer is forming on the walls.

Unlike external flow where boundary layers are free to grow forever, in internal flow the boundary layer eventually will run into itself in the center of the duct. The region in which this boundary layer growth takes place is the developing region. After this point, known as the developing length, the velocity profile no longer changes with the flow direction and the flow is said to be fully developed.