9.4-Boundary Layer Transition.pdf

Boundary Layer Transition

http://edugen.wiley.com/edugen/courses/crs2436/crowe9771/crowe9771...

9.4 Boundary Layer Transition

TRANSITION is the zone where the laminar boundary layer changes into a turbulent boundary layer as shown in

Fig. 9.4 A . 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 10 5 (Re cr 10 5 ) 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 ċ 10 6 (Re tr

3 ċ 10 6 ). 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 ÐtrippedÑ at the leading edge.

1 of 1

1/15/2009 12:55 AM