Life in a Whirl

As hurricanes and tornadoes, vortices are best avoided; but for many organisms they're richly rewarding.

By Steven Vogel|Sunday, August 01, 1993
RELATED TAGS: NATURAL DISASTERS
A vortex is just some fluid that goes around and around, but the ubiquity of vortices is enough to make one’s head spin. They come in large size--hurricanes, typhoons, tornadoes. they come in medium size--dust devils, waterspouts, whirlpools. and they come in small size--most notably the vortex you see as water drains from sink or tub.

For most of us humans, large vortices mean danger, and small ones are amusing. The rest are simply invisible and ignored. But what kind of relationship do other creatures have with vortices? If we’ve learned anything about how organisms deal with their surroundings, it’s that nature manages to capitalize on just about every aspect of the physical world. If something happens in a place and on a scale where animals and plants occur, then some creature will be found opportunistically turning that phenomenon to its advantage. Vortices are certainly no exception.

The way vortices work is probably a little unfamiliar to you. Consider a rotating wheel: since it’s a solid with a distinct shape, all its parts must rotate together. But what if the wheel were made of a fluid, such as air or water? (A fluid is anything that flows, whether gas or liquid.) It could, of course, still rotate as a whole. But since fluids have no particular shapes, different parts of a wheel of fluid might rotate at different speeds. Recall how a rotating skater spins faster by bringing her arms closer to her torso; decreasing the radius of a rotating body automatically increases its speed of rotation. Similarly, in a rotating region of fluid--a vortex--a bit of fluid speeds up if it moves in toward the center.

That’s what happens in the vortex around the drain: any initial rotation gets amplified into a major spin as the water moves toward the center. And that’s what you feel as a hurricane moves toward you--the wind gets increasingly violent as the center of the vortex gets closer. Now, you might logically assume that if speed increases as radius decreases, then at the center of a vortex the speed ought to be infinite--an alarming prospect. Fortunately, that doesn’t happen. Fluids have a kind of internal friction or stickiness, called viscosity--their natural resistance to flow. Of course, most fluids seem to flow quite easily; an individual bit of water or air slides over and around another with no trouble at all. But this easy movement becomes more difficult when the size of those bits gets small enough. As it turns out, very small bits of fluid prefer to march communally rather than dance individually. Increasing speed near the center of a vortex requires that neighboring bits of fluid go at different speeds, something viscosity opposes. So the middle of a vortex rotates as if it were solid, with the highest speed a little way out from it. As a result, the hurricane has a startlingly placid eye, however intense the surrounding winds. This curious relationship between speed and location has been found in vortices ranging from swirls in bowls to, in one case, a tornado. The late meteorologist Lewis Richardson summed it up nicely in this doggerel verse:

Big whirIs have IittIe whirIs

which feed on their veIocity,

whiIe IittIe whirIs have Iesser whirIs,

and so on to viscosity.

Yet another peculiarity of vortices is that they don’t easily form ends. With tornadoes, for example, one end is (at least in part) the surface of the earth, while the other is a gradual fade-out well aloft. Quite commonly, ends are completely missing--or, to put the matter another way, the ends are indistinguishably fused and the vortex forms a doughnutlike ring, or torus. Here the axis of the vortex is bent into a circle, with fluid moving in one direction (upward or downward) on the inside of the ring, and in the other direction on the outside.

A jet of fluid can generate such a toroidal vortex; viscous friction between the jet and the surrounding fluid makes the former roll up into one or more rings. A smoke ring is just this kind of vortex, and one that’s easy to see. But you can make a visible toroidal vortex just as well in liquid as in air. Lower the tip of an eyedropper of milk beneath the surface of a glass of water that has stood still for a few minutes. Then give a very slight but sudden squeeze--with a steady hand and a little practice, you can make what looks like a moving jellyfish. This is a routine game for an ink-squirting cuttlefish or octopus. A writhing blob is a fine target to offer a predator, especially as the internal motion of the vortex ring of ink will make it move one way, while jet recoil will make the squirter move in the other.

Upward and downward flows can also be created by temperature differences in a fluid. And these toroidal thermal vortices are as easy to make as your faux jellyfish. Just mix some pearlescent shampoo or dishwashing detergent with water (about a teaspoon per pint) and let the mixture settle in a pan (a black pot or frying pan is ideal). Brief application of a match to a point on the pan’s bottom produces both an upward and a downward flow--in effect, a thermal vortex.

Outside the kitchen, this kind of vortex is commonly found in air, and it’s exploited by a variety of creatures for getting around without much expense. Only a few kinds of animals--bats, birds, and insects--are capable of powered flight, and it costs a powerful lot of metabolic energy. Gliding is a lot cheaper, but a glider continuously descends. The trick, then, for hawk or sailplane pilot is to find air that’s ascending.

That’s what eventually results when the morning sun heats the ground. Air near the ground becomes more buoyant than air above, and bubbles of warm air periodically break loose and ascend. Air on the sides of a rising bubble is retarded by the surrounding still air (viscous friction again), so air in the middle rises faster, and the bubble soon becomes a doughnut. When that happens, air on the inside of the ring moves upward faster than the ring as a whole. So a bird or a sailplane just has to glide in a circle within the inner region of the ring. Both do such thermal soaring, but birds, smaller and slower, can glide in tighter circles and use smaller vortices. In a heavily vegetated region, a soaring hawk often indicates the location of a plowed field or a road, a place where solar heating of the ground is especially effective and thus where bubbles often break loose. Serendipitously, a field or road is also where prey find the least cover--a good place to check out for dinner. Indeed, if motorists cooperate, a bird need not even hunt living prey. Presumably a hawk or vulture might patrol a road, ascending in one thermal and then gliding in a gentle descent along the road until it either meets another thermal or finds a kill.

Only for takeoff does the soaring hawk have to flap its wings. But if a creature is small enough, an unpowered start is possible. Newly hatched spiderlings of many species climb to the tops of bushes and trees and spin out silk strands; the silk apparently acts as a wind direction indicator. When a strand is drawn upward, the spiderling lets go of whatever it’s hanging onto and is carried aloft. The filament’s flexibility is such that it is drawn into the center of the eddy, where it appears to serve as a drag-increasing device to slow the spiderling’s descent.

As a method of locomotion this scheme is certainly effective: its benefit is great while the cost is small. Charles Darwin, while on the Beagle, noted a mass descent of spiderlings (thoroughly terrestrial creatures) fully 60 miles off the South American coast. Yet the silk strand represents less than one percent of a spiderling’s total mass, so the creature is getting a very big bang for its metabolic buck. A few moths also practice this kind of ballooning, most notably the smallest caterpillars of the notorious gypsy moth that’s responsible for major episodes of defoliation in eastern North America.

In a different medium and on a smaller scale, we find arrays of similar vortices often forming in shallow pools of liquids. Anything that makes the water on the bottom less dense than that near the top can generate these vortices, whether it’s heating at the bottom (which causes molecules at the base to move apart), evaporation at the top (making the top cooler, and thus more dense, than the bottom), or the activities of light-powered algae, whose chemical changes to water near the top also increase the water’s density. These effects often appear in laboratory culture dishes and in natural puddles, where the phenomenon is called bioconvection. Substantial benefits accrue from this self-stirring--in particular it prevents local depletion of nutrients and accumulation of wastes.

Vortices result as well from flows around solid objects. They’re almost always present just behind unstreamlined objects such as cylinders. If you run either without a shirt or with a wet one, you may feel coldest not in front but behind, especially between your shoulder blades. You hear more rushing noise when facing directly into a wind than when turned even a little to left or right. In both cases the airflow doesn’t stream smoothly around you. Instead it separates from the surface at your body’s widest points, leaving turbulent vortices farther downwind--that’s what chills your back and makes noise in ears located behind cheekbones. Move a downward-pointed finger through soapy water and a pattern of vortices forms behind it. If you arrange a piece of blackboard chalk so it sticks up from the bottom in a stream of water, you’ll notice that the chalk erodes more on the downstream than the upstream side--back-side vortices have been at work.

Significantly, not only do vortices form on the chalk’s downstream side, but the fluid making up these vortices flows lengthwise through them, moving upward from the bottom. Therein lies an opportunity capitalized on by several otherwise dissimilar animals. Quite a few creatures arrange themselves essentially as protruding cylinders in flows-- sometimes in tubes they construct, sometimes just by attaching at one end to sand, stone, or soil. Most often they have feeding structures on the opposite end--tentacles, filters, and so forth--with which they catch edible particles coming by in the flow. These animals are engaged in a version of what’s generally called suspension feeding, a term that describes how many animals obtain their food by separating out tiny bits of it suspended in a large volume of water. Another set of animals specializes in detritus feeding: they pick up edible material that has settled out of the water into a loose coating on the bottom. Ascending flow in downstream vortices, though, affords our cylindrical creatures a chance to use suspension-feeding equipment to feed on detritus as well. Not only can the vortices bring detritus up to tentacles and filters, but they preferentially raise the more nutritive stuff--edible organic material may be more dense than water, but it’s less dense than inedible inorganic grit. Simply by virtue of its shape and posture in a flow, then, an upright cylinder is a sorting machine.

Who uses this particular opportunity? Investigators have found two shoreline marine worms, members of very different lineages, taking advantage of their tiny whirlpools. They’ve also found a blackfly larva that appears to have the best of both worlds. The larva lives in rapid streams, where it attaches itself to a rock at its hind end and sticks up as a tapering cylinder topped with a pair of identical filtering fans. But the larva twists lengthwise and bends its torso so the fans end up not on either side of the body, but above and below each other. The upper fan captures suspended material in the passing water while the lower one captures detritus in water ascending from the bottom in one of the back- side vortices.

Back-side vortices aren’t the only vortices a creature might induce in the air or water rushing past it. Another kind of vortex occurs just above the soil or streambed as a result of one of the most counterintuitive but universal features of flowing fluids. However rapid the flow of wind or stream, at the actual surface of any solid exposed to the flow, the speed of the fluid is zero. That’s why one swipe of a dishcloth is as effective as a long exposure to strong detergent and rapid flow, and why dust accumulates on fan blades. It helps to think of the moving fluid near the surface of a solid as a stack of many very thin layers. Right at the surface of that solid, viscous friction is going to drag the first layer of fluid to a complete halt. Viscosity will slow the next layer above that, but not totally. The layer above that will be slowed even less, and so on.

In short, within every moving fluid next to every solid surface, the speed of flow varies from zero at the surface to full mainstream speed a short distance away. As a result, any object in a flow near a surface will be subjected to different speeds of flow on its upper and lower sides. And therefore an object will be rotated as well as carried downwind or downstream; even without touching the bottom, it will roll along.

The consequences can be quite curious, especially when the moving object is just a part of the fluid rather than a suspended solid. If the wind blows in one direction during a snowstorm, one might expect the snow to accumulate on the upwind sides of trees. Instead quite the opposite happens--snow is ordinarily scooped out just upwind of trees and deposited on the downwind side. Air and snowflakes rotate just ahead of the tree, moving downward like a trench-digging machine near the tree’s upwind surface. It looks, and the look is no illusion, as if a vortex has been caught by the tree and bent into a horseshoe, as unable to break and reform as a skier with a ski sticking downhill on each side.

Such a vortex can not only move material around on the surface, it can actually dig into the surface itself. Once more, investigators have found several animals that take advantage of the phenomenon. Among them are two kinds of mayfly larva that live in the surface layer of sandy sediment on river bottoms. Facing upstream, one larva uses its forelegs to create a vortex that excavates a shallow pit in front of its head. The vortex persists in the pit, aided by the postures of head, antennae, and forelegs. Material arriving in the flow lingers, rotating in the vortex, and detritus moving slowly along the bottom is drawn into the vortex. Both effects bring edible particles to the filtering hairs on the insect’s front legs.

Another mayfly larva feeds on small fly larvae living in the sand. The mayfly doesn’t dig them out--instead it arches its body and lowers its head to create the right conditions for a vortex to form and dig an upstream pit. Meanwhile the larva grabs any prey exposed in the process and slowly moves backward--downstream--so new sand is constantly excavated. Why dig when the current can be persuaded to do the work?

Probably no animal large enough to see with the naked eye flies or swims without creating vortices. And these vortices are essential to the animal’s fluid movement. Consider how a wing produces lift. It’s shaped and positioned so fluid must travel more rapidly across its upper surface than its lower. As you may remember from an early physics class, more-rapid flow is associated with lower pressure, and lower pressure on the top than the bottom means an upward force--lift. Lift production, though, can be explained at a more basic level, taking a more powerful but less intuitive viewpoint. In a sense, a lift-producing wing is enveloped by an odd sort of vortex, a bound vortex, in which air goes forward (upwind) beneath the wing, upward in front, rearward (downwind) above it, and downward behind it. The greater the difference between the speeds of flow above and below the wing--the greater the lift--the stronger is this bound vortex. Now, air doesn’t really move upwind beneath the wing, so this bound vortex looks at first glance like a polite fiction or a mathematical device. The situation is much like that of a baseball thrown with some spin to make it curve. Because of the spin, one side of the ball seems to be going away from the batter. But really all that’s happening is that one side of the spinning ball is approaching the batter faster than the other--the ball as a whole continues on its forward flight, and no point on the surface of the ball is ever really moving backward in space toward the pitcher.

At any rate, the bound vortex around a wing is real enough to be part of a proper vortex ring, much like, and yet the opposite of, a thermal vortex.

There’s more to keeping a plane aloft. In fact, the ways in which flying machines--and creatures--create vortices quickly get quite complex. Essentially what happens is that when an aircraft takes off, a starting vortex of equal intensity and opposite spin to the wing’s bound vortex is left behind at the airport (where it is occasionally a nuisance for subsequent fliers). Bound and starting vortices are then connected by tip vortices extending back from the ends of the wings--again, nature seems to prefer that her vortices not have ends. Together, all these vortices complete one large vortex ring. The direction of spin in this vortex ring, however, is opposite that of a thermal vortex. The fluid in a thermal vortex ascends in the middle and descends peripherally; by contrast, the aircraft’s vortex system descends centrally and ascends peripherally (remember, flow in the bound vortex goes downward just behind the wing). So the vortex system as a whole moves downward. That’s just what it has to do- -the craft must make passing air move downward to keep itself up; it can only produce an upward force, lift, by throwing air downward.

A bird that’s flying slowly gets its lift mainly during the downstroke of its wings. With no lift on the upstroke (it’s hard to push air downward by moving a paddle upward), air isn’t moving faster above than below the wing, so no bound vortex forms--the old bound vortex slips off the wing at the end of each downstroke to make a complete and free vortex ring, which moves earthward in the wake of the bird. Indeed, the peculiar wing structure and rather bouncy flight of most large moths and butterflies makes sense when the animals are treated as devices that with each wing stroke fling vortex rings downward. But when a bird flies a little faster, it no longer sheds a sequence of individual vortices--instead it produces a ladderlike vortex system, with tip vortices trailing behind and zigzagging a little up and down with each upstroke and downstroke of the wings. The tip vortices are connected at each extreme of their zigzags by crosswise rungs--the bound vortices shed by the wings. At still higher speeds, the rungs disappear and only the tip vortices are left, zigzagging up and down as before, and now a little inward and outward as well--inward on the upstroke, when less lift is produced, and outward on the downstroke, with more lift.

Again nature takes advantage of the unavoidable aspects of physical reality. Recall that tip vortices turn so that air on the outside- -away from the bird--is rising. Upwardly mobile air is something to be treasured by flying creatures some 800 times denser than their surroundings--that’s what thermal soaring and spider ballooning are all about. How about flying just enough behind and aside another bird to get a little lift from this area of upward air movement? During migration and other steady flights, many large birds fly in a V formation, with each bird behind and to the side of its immediate predecessor. A symmetrical V has no special advantage, so you usually see more birds on one side of the leader. But you don’t see a branched V, or Y--that would put a bird in the bad downward air directly behind another. Incidentally, while the leader may have to work a little harder than the followers, it doesn’t have to do extra work on their account--without followers, the upward motion of the outer part of its tip vortices would simply be discarded.

While we know of many other instances in which creatures exploit vortices, surely we don’t know all that exist. For better or worse, vortices don’t ordinarily impose themselves on us. That, however, is merely an accident of the transparency of both air and water, and of our practice of walking and running on solid surfaces rather than swimming or flying through continuous media. We’re thereby denied easy familiarity with vortices, the familiarity that would help us guess what various living creatures might be doing with them. That’s too bad, for such guesses are (put more grandly) hypotheses that we can test for coincidence with reality. We can be assured, though, that since natural laws demand that vortices be ubiquitous, there must be many unrecognized instances of organisms using vortices for their own good purposes--situations just waiting for a little creative guesswork on our part. Nature may be all in a spin, but she’s certainly not spinning out of control!
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