Basic science.

I was sitting in the taxicab on my way to work this morning when a thought came to mind. Why does the cloths flutter in the wind? Why does paper flutter when you blow on it?

Why flutter at all when by common sense, it should've been pushed all the way back as the wind pushes. Look at this. And common sense tells us that it will stay that way as long as the wind blows.But it didn't happen this way becouse of a phenomenon called back pressure or rather reverse flow. It is a reality of fluid flow to induce turbulance when it came upon an obstuction. In this case, the backflow creates enough force to push the paper back against the prevailing wind. However the back flow occurs as circular flow as shown below and the strength decreases when the "circle" was blown away, thus the paper are now being pushed again by the prevailing wind. BUT, as the wind picks up, another "circle" forms and the backflow happens again. This happens many times a minute, even seconds and this accounts for the fluttering or vibrations you see when paper or cloths were blown by the wind.