The flow of a fluid in a pipe may be laminar or turbulent. The distinction between laminar and turbulent pipe flow was first pointed out by Osborne Reynolds (1883). The actual transition or turbulent flow may take place at various Reynolds numbers, depending on how much the flow is disturbed by vibrations of the pipe, roughness of the entrance region, etc.
Fig. 3. Fluid Flow in a Pipe
The region of flow near where the fluid enters the pipe is the entrance region. A boundary layer is produced along the pipe wall so that the initial velocity profile changes with distance along the pipe, until the fluid reaches the end of the entrance length. The flow in long, straight constant diameter sections of a pipe becomes fully developed, i.e. the velocity profile is the same at any cross section of the pipe. The details of velocity profile are different for laminar and turbulent flows. The nature of the pipe flow is strongly dependent on whether the flow is laminar or turbulent. This is a direct consequence of the differences in the nature of shear stress in laminar and turbulent flows. The shear stress in laminar flow is a direct result of momentum transfer among the randomly moving molecules. The shear stress in turbulent flow is largely a result of momentum transfer among the randomly moving molecules, finite-sized bundles of fluid particles.
Fig.4. Entrance region, developing flow, and fully developed flow in a pipe system
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