Plug flow in a tube is an ideal-flow assumption in which the fluid is well mixed
in the radial and angular directions
The fluid velocity is assumed to be a function of only the axial position in the
tube
Plug flow is often used to approximate fluid flow in tubes at high Reynolds
number. The turbulent flow mixes the fluid in the radial and angular directions
Also in turbulent flow, the velocity profile is expected to be reasonably flat in
the radial direction except near the tube wall.