From: Steven P. Schneider [steves@ecn.purdue.edu]
Sent: Tuesday, February 11, 2003 8:34 AM
Subject: laminar-turbulent transition, shuttle

We have seen several stories on "laminar-turbulent
transition" in the "boundary layer" on the shuttle.

It seemed like the concept of the boundary layer was
a little mixed up with the concept of laminar-turbulent
transition in the boundary layer, in some of these stories.  Perhaps if I try
to clarify these ideas this might be of some interest.

To try to clarify, I refer to a couple of pictures
from a (copyrighted) book, "An Album of Fluid Motion",
by Milton Van Dyke, Parabolic Press, 1982.  
See http://www.stanford.edu/~vandyke/publications.htm
to obtain a copy, $15 in paperback.
I can email a scanned version of the figures if necessary.

Fig. 29 from the first scanned page shows flow
at low speeds past a flat plate.  Most of the flow
is unaffected by the plate, only the flow very near
the surface is affected by friction with the plate.
Fig. 30 from the first page details what happens very
near the surface.  If the plate is fixed, and the water
flows past it, the speed of the water right near the surface
is the same as the plate, and there is a thin "boundary layer"
in which the flow speed goes from zero near the plate to
the freestream speed away from the plate.  You can see
this variation of speed in Fig. 30.  This is a very low speed
case with a very thick boundary layer, so a picture could be taken.
However, the same sort of thing happens on the shuttle.  

Fig. 30 shows a smooth steady laminar boundary layer.  Fig. 157 on the next
page shows a turbulent boundary layer.    A flat plate is just below
the lower surface of the image.  The air speed is higher than
in Fig. 30.   A smoke wire is placed vertically,
upstream of the picture, to the left.  Little drops of oil
form on the wire and are vaporized by heating, forming continuous
streaks of smoke.  The horizontal streaks
in the upper part of the image is laminar flow, in the region away from the plate.
However, in the boundary layer near the lower part of the image, near
the wall, the flow is unsteady and irregular.  This is a turbulent
boundary layer.

Steven P. Schneider
Assoc. Prof., Purdue AAE
Aerospace Sciences Lab/Purdue Univ. Airport
1375 Aviation Drive 
West Lafayette, IN 47907-2015 USA
tel 765-494-3343 fax 765-496-3321
email steves@ecn.purdue.edu