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9.4 Boundary Layer Transition
Transition is the zone where the laminar boundary layer changes into a turbulent boundary layer as shown in
Fig. 9.4a. As the laminar boundary layer continues to grow, the viscous stresses are less capable of damping
disturbances in the flow. A point is then reached where disturbances occurring in the flow are amplified, leading
to turbulence. The critical point occurs at a Reynolds number of about 105(Recr 105) based on the distance
from the leading edge. Vortices created near the wall grow and mutually interact, ultimately leading to a fully
turbulent boundary layer at the transition point, which nominally occurs at a Reynolds number of 3 × 106(Retr
3 × 106). For purposes of simplicity in this text, it will be assumed that the boundary layer changes from
laminar to turbulent flow at a Reynolds number 500,000. The details of the transition region can be found in
White 3.
Transition to a turbulent boundary layer can be influenced by several other flow conditions, such as free-stream
turbulence, pressure gradient, wall roughness, wall heating, and wall cooling. With appropriate roughness
elements at the leading edge, the boundary layer can become turbulent at the very beginning of the plate. In this
case it is said that the boundary layer is  tripped at the leading edge.
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