What defines laminar, turbulent or transitional flow?
Laminar flow occurs when the forced movement of a liquid through a conduit can be described as a telescopic sliding of adjacent concentric layers of liquid without transverse mixing. Each layer tends to maintain its own identity. The resistance to motion is mainly due to molecular interactions within the liquid. In a long straight conduit, the liquid velocity is zero at the wall and gradually increases to its maximum value at the conduit centerline. Laminar flow conditions are usually met when the Reynolds number of the flow (Re = VD/ ) is less than 2,000.(4) The laminar velocity profile within a circular conduit is parabolic.Turbulent flow occurs when the forced movement of a liquid through a conduit causes erratic mixing. Individual fluid particles no longer follow predictable paths and the resistance to flow is no longer entirely due to the molecular forces within the liquid. The turbulent flow velocity profile is almost flat (in a circular conduit) due to the transverse mixing caused by the random motion of the individual fluid particles. This condition is usually met when the Reynolds number Re = VD/ is greater than 10,000.(4)Transitional flow is the gray zone in which the flow begins to lose its laminar characteristics to become turbulent due to an increase in velocity. The core becomes turbulent while the external ring remains laminar due to wall effects. Thus, in transition a fluid has characteristics of laminar and of turbulent flow. Making accurate predictions about fluid behavior in the transition zone is difficult. The velocity profile becomes a modified parabola flattened at its center. As the flow velocity increases, the turbulent core grows while the laminar ring shrinks. This condition is usually met when the Reynolds number Re = rVD/m ranges between 2,000 and 10,000.(4)