The Wind Tunnel (Page 3)

Reading the smoke patterns from the wind tunnel test is a little like a doctor interpreting an X-ray. Let's look at some results!

Look at a video of the tennis ball and the bowling ball. In particular note the size of the wake (the flow between the streamlines on the top and bottom) on the right side of the video. Compare the size of the wake on the tennis ball and on the smooth bowling ball. The wake of the bowling ball is much smaller; this is a sign that there is less drag.

Look for the flow separation in both videos. On the tennis ball you can see the point smoke no longer follows the curvature of the ball. For the bowling ball you see that that point is much farther towards the right side of the ball. Do you see that on the smooth ball the flow stayed "attached" (followed the curvature of the ball) longer (farther down the ball) than it did on the tennis ball.

On an airplane wing it is generally critical that the flow stay attached. If you pitch the nose of an airplane up, it changes the "angle of attack" (the angle the air meets the vehicle) of the wing. If the angle of attack of the wing is too steep, the flow separates early (towards the front or leading edge of the wing) and there is a loss of lift. In extreme cases the airplane can no longer fly and the wing "stalls". (Stalls in aircraft are associated with sudden drops in altitude, and if the pilot does not recover from the stall an accident can occur.)