When fluid first enters a pipe its flow is not fully developed. Instead the fluid has to travel a certain distance undisturbed before it becomes fully developed. This is also true when a fluid goes around a curve in the pipe system. The curve in the pipe will disrupt the velocity profile of the fluid, and it will need to travel a certain distance in a straight pipe to have a fully developed flow again. Refer to equation 1 to calculate the entrance length for laminar flow, and equation 2 to calculate the entrance length for turbulent flow.

L = Entrance Length

D = Pipe Diameter

Re = Reynolds Number

Notice from the diagram that a boundary layer starts to form as the flow becomes developed. The boundary layer represents where the viscous effects are produced along the pipe wall to create the velocity profile. Also notice while the flow is developing there is a region where there is no viscous effects; this is called the inviscid core.

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