Space Shuttle Reentry: Laminar to Turbulent Air Flow

[Q_Goest]

I would have never guessed that during reentry, and at very high mach number, the flow of air over the Shuttle’s wing is laminar! To make it even stranger, as velocity decreases, there is a transition to turbulent flow across the wing (I suspect this is also caused by an increase in air density as the Shuttle decends into the atmosphere).

The shuttle's transition from laminar to turbulent flow occurs naturally as the spacecraft slows down, typically at velocities around mach 8, or eight times the speed of sound, about 20 minutes after atmospheric entry begins.

Also very interesting is that this laminar flow actually provides some measure of insulation.

The goal of the research is to gain a better understanding of how smooth, laminar airflow, which provides a thin layer of insulation during peak heating, can change to the disturbed, turbulent flow that can cause downstream temperatures to climb, possibly affecting aerodynamics and causing damage.

And the affect is quite dramatic, causing up to a 500 degree F reduction in temperature for the laminar flow.

For Discovery's boundary layer transition - BLT - experiment, a single tile on the bottom of the shuttle's left wing, about 10 feet behind the leading edge, featured a 0.25-inch-high ridge that was expected to cause the insulating boundary layer downstream to go turbulent between mach 12 and 14. That, in turn, was expected to raise the temperatures of downstream tiles by 500 to 600 degrees Fahrenheit.

The tiles in that region of the wing normally experience maximum temperatures of 1,500 to 1,600 degrees when the re-entry airflow across the wing is smooth. Preliminary reports indicated the maximum temperature experienced by the "protuberance tile" was around 2,000 degrees, somewhat less than expected.

Ref: http://spaceflightnow.com/shuttle/sts119/090329blt/
Interesting article, well worth reading!