Life is complex. So is fluid dynamics. Actually, you’d think fluid motion isn’t all that complicated. Liquids flow downhill, moving around stationary objects, and generally going from point A to point B. But in reality fluid motion is incredibly complex, one of the most ridiculously overwrought fields in all of mathematical physics. Turbulence, oscillations, viscosity, boundary layers: They all add up to make fluid motion fiercely hard to understand and calculate. But in complexity there can be beauty. Commander Chris Hadfield, on board the International Space Station, took this picture (above) on Mar. 26, 2013, showing exactly that fluidic gorgeosity.
That is Isla Socorro, a volcanic island located a few hundred kilometers off the west coast of Mexico and the southern tip of Baja California. As wind blows past the island, lovely atmospheric swirls form on the downwind side. These swirls are called von Kármán vortices, and the long chain of them gets the name von Kármán vortex streets. The cloud pattern makes them visible to the eye.
What causes them? In detail the mathematics is quite fierce, but how it works isn’t all that hard to understand in principle. Imagine you have a cylinder (a pencil, or a bucket, or a concrete pylon) that you place in flowing water. It’s an obstacle, and the water will flow around it. However, near the cylinder’s surface the water slows, piling up a bit. The water farther from the cylinder is moving faster. This causes eddies (vortices) to form, curls in the water. This kind of motion is a bit unstable, and can cause a slight force, pushing the water perpendicular to the direction of flow. But the water all around the flow pushes back, causing a sort of oscillation, like a pendulum swinging. The result is a series of vortices forming and flowing downstream, one on each side of the obstruction, alternating in pattern. Source: Atmospheric vortices flow past an island seen from space. Image credit: NASA