Some research started by Stephen Koehler and Maarten Rutgers in the wind tunnel of Joseph Haritonidis at the Ohio State university. This quick study was precipitated after some discussions on what flapping flags look like. Typically you only see things like in the first image. We wanted to explore things a bit more systematically with different fabrics, wind speeds, and flag sizes.
A relatively thin and light fabric, first a slow speed, and then at high speed.
A thinner, denser, and more flexible fabric than above. First at slow speed, then at high speed.
Finally a gauze like fabric In this case pressure differences on either side of the flag can equalize, and no waves tend to travel downstream. Slow waves do travel vertically and make for an interesting display seen end on.
Photos Copyright (2000) Maarten Rutgers, Stephan Koehler, and Brent Daniel.