Determination of Plug Flow
For flows in pipes, if flow is turbulent then the laminar sublayer caused by the pipe wall is so thin that it is negligible. Plug flow will be achieved if the sublayer thickness is much less than the pipe diameter (< where f is the Darcy friction factor (from the above equation or the Moody Chart), is the sublayer thickness, D is the pipe diameter, is the density, u* is the friction velocity (not an actual velocity of the fluid), is the shear on the wall, and is the pressure loss down the length L of the pipe. is the relative roughness of the pipe. In this regime the pressure drop is a result of inertia-dominated turbulent shear stress rather than viscosity-dominated laminar shear stress.
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