Turbulent and laminar flow in pipes

The flow in conventional control valve installations is almost always turbulent. Laminar flow can occur with very viscous fluids such as lube oil and with very small flow velocities.
Laminar flow can be explained as a microscopic viscous interaction between several layers of fluid. In laminar flow the fluid particles move in parallel paths or streamlines. A particle has only axial velocity along a streamline. Fluid layers slide relative to each other, and the streamline of an individual particle can be predicted . If the velocity is increased above a certain limit the laminar flow field is disturbed and
fluid particles start to move in erratic paths. When this happens, the flow becomes turbulent. Transition from laminar to turbulent flow can be predicted by a single parameter,the Reynolds number (Re), as defined in equation :
Re = flow velocity * nominal diameter / kinematic viscosity
Above a critical value of the Reynolds number the flow goes through a transition to become turbulent. The critical value of Reynolds number is around 3000 for flow in a straight pipe. When the flow is turbulent, part of the flow energy is used to create eddies, which cause increasing pressure losses. Turbulent flow can be described as irregular secondary motion of fluid particles. Secondary motion does not correspond to
the principal direction of the flow. The flow path of a single flow particle is irregular.