Stand beside a mountain stream and observe the water for a while and you’ll see that most of it moves in a continuous flow. Look a little closer, however, and you’ll see that the bed of the stream and objects in and along the stream — logs, boulders, the legs of a bear — all have an effect on the flow due to friction. Look closer still — say, at a rock near the edge of the stream — and you’ll find that the flow may slow to a halt if not actually move in the opposite direction, creating what is commonly called a whirlpool, but more accurately described as an eddy. Peer at the boundary between the flowing current and the eddy and you’ll see smaller eddies form and detach again and again, dissipating as they flow downstream because they are no longer powered by the object in the stream that created them.
Known as detached eddies in the science of fluid dynamics, these disconnected but still churning whorls can also be spawned in the atmosphere, as bodies of air move over the landscape or interact with each other. Heat water to the right temperature and move a low pressure area over that water and you may spawn a monstrous hurricane that lasts for days and travels thousands of miles. Move that same hurricane over dry land, however, detaching it from its power source, and it will slowly dissipate, even as it may still wipe entire communities off the face of the earth.
The key component of an eddy, and what distinguishes an eddy from a vortex, is that in the middle of an eddy there is a void — a place of calm that experiences none of the rotational effects of the moving fluid that defines the eddy itself. Drop a leaf in the center of an eddy caused by even the most ferocious mountain stream and it will float exactly where you dropped it. In a vortex there is no void, but vortices can also detach like eddies. This is known, sensibly enough, as vortex shedding — a phenomenon that has led to practical applications in the real world such as winglets on airplanes. [ Read more ]