 Another flow characterization we make is the distinction between developing flow and developed flow. To consider the difference, let's look at uniform flow entering a pipe. The presence of friction along the pipe walls causes slower fluid movement along the sides, a region that we call the boundary layer. The further the flow gets into the pipe, the bigger the boundary region gets, until it encompasses the entire pipe. At that point, the flow has a fully developed velocity profile, and we refer to the flow as being fully developed. The length into the pipe at which the boundary layers intersect is referred to as the entrance length, and the flow in that region is described as developing. The pressure drops relatively linearly in the fully developed region, but the developing region has a larger associated pressure difference. The difference between the pressure decrease along the developing region and how much the pressure would decrease if it were fully developed in that region is called the entrance pressure drop. Treating the entire pipe as being fully developed requires us to either account for the entrance pressure drop or neglect its effect. The empirical correlations we use to predict entrance region length look like this. For laminar flow, the dimensionless proportion of entrance length to diameter is approximately 0.06 times the Reynolds number in the fully developed region. For most of the turbulent flow range, that proportion is approximately 1.6 times the Reynolds number in the fully developed region raised to the 1 fourth power. I will also point out while we're here that in many cases, the pipe length is so long relative to the entrance region that we neglect the entrance effect and assume the whole pipe has fully developed fluid flow. We should not get so comfortable with that simplification that we forget it's an assumption we make. Do not get so comfortable with the entrance region not existing that you forget it can exist at all.