Consider the 2-D unsteady, incompressible viscous flow past an airfoil at a sufficiently high Reynolds number (, as shown below.
 
 
 
 

We find that the boundary layer over both the upper and lower surfaces may be divided into three regions. They are (1) laminar region, (2) transitional region, and (3) turbbulent region.
 
 

The laminar region is the region where "Blasius" like velocity profiles exist. If we monitor quantities such as u,v or p at any point in the laminar region, we find that these properties remain remarkably steady. Indeed, if we introduce disturbances into this flow at any time, either deliberately or due to external noise sources, we find that these disturbances get damped out at later time levels. Furthermore, we find that an intially 2-D flow remains "nominally" 2-D, with very little w- compoent in the flow. In laminar flows, the primary physical mechanisms for the trnasport of mass, momentum and energy are convection by the mean flow, and molecular diffusion (called "viscous effects").
 
 

The turbulent region has remarkably different characteristics, compared to laminar flow. We find that the flow is highly unsteady, although the fluctuations are small, and occur about some "steady" mean flow levels. The flow has small, but measurable oscillations in the w- component of velocity. Finally, the velocity profile is fuler, and the skin friction is higher than in laminar flow. As we will see later, the primary mechanisms for transport of mass, momentum and energy in turbulent flows are convection by the mean flow, and convection by "turbulent" eddies. Molecular diffusion is a relatively slow mechanism, and takes a backseat to the mean flow and eddies, except near the wall.
 
 

The buffer region that separates the laminar and turbulent flow regions is called the transitional zone, and is the topic of our present study.
 
 

The phenomenon of transition thus corresponds to a dramatic and abrupt change in the flow characteristics, within a small region. Transition is not limited to boundary layers over solids, but also occur in other flows. Some of these flows are described below.
 
 

Flow within a Pipe: Reynolds studied flow through circular pipes with smooth walls, made of glass. Wtaer was the working medium. He injected a dye along the pipe center line, and studied the subsequent behavior of the dye. The parameter he varied from study to study was the mass flow rate Q, given by Q=rUA , where r is the density, A is the cross section area of the pipe, and U is the "mean" velocity. Since U changed, the Reynolds number rUD/m was also chaged, where D is the pipe diameter.
 
 

For Reynolds number 2000, the flow pattern was smooth, and steady. For the higher Reynolds numbers, the dye pattern was initially steady. It developed oscillations soon, howvever, and eventually became chaotic. He caled the Reynolds number Re = 2000, the transition Reynolds number. Subsequent studies showed that transition Reynolds number coule be raised if the lab set up was very steady, if the walls of the pipe were very smooth, and if the inflow entering the pipe was very steady. In practice, none of these conditions exist, and the flow becomes turbulent once Re exceeds 2000.