The Soviets have the fastest subs in the world. Very possibly they have already begun to exploit new and sometimes bizarre ways of coaxing extra speed out of submarines by reducing the skin-drag of their submarine hulls. Such techniques are also being sought by the U.S. Navy through u.s. universities, and industrial and Navy research laboratories. Examples of such methods of drag reduction: hulls that pump out a mucus-like secretion or release clouds of microscopic bubbles; hulls that are heated from stem to stern; hulls covered with fine grooves; even hulls with soft skins that subtly change shape — a trick some scientists think dolphins may have developed. The object of the latest research is to reduce skin-drag turbule a factor that contributes nearly half of the overall drag a submarine’s engines must overcome in driving the vessel forward.
As Michael Reischman of the Office of Naval Research explains it, skin drag results from tiny turbulent eddies that swirl chaotically within a “boundary layer” of water, only a fraction of an inch thick, as it moves along a hull. Ordinarily, a boundary layer of water (or air, in the case of airplanes) is invisible — but it is real enough to pose some of the thorniest problems in physics. In explaining the behavior of boundary layers, scientists say that the molecules of a fluid that come into direct contact with a moving solid surface tend to adhere to it and get pulled along at nearly the same speed as that of the moving surface. These molecules drag along neighboring molecules, but farther away from the skin the pull on the fluid is less, so it moves more slowly. At the outer edge of the boundary layer, fluid molecules are traveling at less than one per cent of the speed of the molecules touching the solid surface. Although layers of fluid at different depths may slip smoothly past each other in what is called laminar flow, they may also get tangled up, creating turbulently swirling eddies.
The behavior of turbulence in boundary layers remains one of the great mysteries of science. Turbulence is made up of an infinite number of random microscopic events that are unpredictable by their very nature. Within this chaos, however, eddies coalesce into drag-producing bursts that seem to erupt at fairly regular intervals, like the rhythmic flickers of a candle flame in still air. Why and how does this semblance of order arise spontaneously from a disordered system?
Despite scientists’ bafflement at the theoretical aspects of turbulence, they have discovered new ways to reduce or even prevent it in the boundary layer.
In 1975, Soviet scientists began publishing reports of experiments in which they claimed to have achieved drag reductions of nearly 90 percent by pumping ordinary air through a porous plate. Some of the current American work is based on the Soviet reports.
A submarine equipped to use micro-bubbles would have a double hull, the outer layer consisting of porous metal or some similar material, with compressed air between the two hull layers. Since the air supply aboard a submarine is limited, and since all gases seem to work equally well, the bubbles might conceivably consist of steam generated as a by-product of the propulsion system.
Bubbles rise, of course, and if they were expelled from the upper surfaces of a submerged boat they might rise above the thin boundary layer where they reduce skin-friction drag. But at even moderately high speeds, the bubbles seem to remain in the boundary layer long enough to do their job.
Why does it work? No one knows for sure, but, says Reischman, who supervises some of the Navy’s research contracts, “when the air bubbles get in among the little turbulent eddies in the boundary layer, I believe that those eddies get kind of confused and forget what they’re trying to do. The natural process of turbulence generation is sort of interrupted by the air in the fluid.”
Another approach that has excited scientists involves the injection of liquid polymers into the boundary-layer flow. Since the dawn of history, sailors have recognized that slippery hulls slide through water better than ordinary ones. (Ancient Phoenician efforts may have included the application of animal tallow to wooden hulls.) But the small drag reduction afforded by simple lubricants is more than off-set by the fact that they are quickly washed away, especially in sea water. A better technique seems to have been developed by fish, which have skins that secrete mucus continuously. Scientists have long surmised that mucus secretion helps fish swim faster with less effort.
The theory led to searches for an artificial substance with some of the characteristics of natural mucus. In the 1970s, researchers in several countries (including the Soviet Union and Great Britain) hit upon the family of long-chain, carbon-based molecules called polymers, which are also the basis of plastics. One of these, polyethylene oxide, can be dispersed in water to produce a liquid almost indistinguishable from water except for its slightly slimy feel. Polyethylene oxide is now the object of intense scrutiny by the Navy. Scientists found that when even as little as 150 parts of polyethylene oxide per million parts of water is injected into a pipeline, the drag of the pipe wall on the fluid passing through it drops dramatically, so that fluids can be pumped faster with less work.
Could polymer ejection from a ship’s hull into the surrounding water also reduce drag? In the mid-70’s, the Soviets took the lead, publishing a series of papers claiming success. Results of the u.s. Navy’s latest series of tests are secret, but the scientists involved say they are extremely encouraging. One of them speculates that some Alfa-class submarines — the fastest in the Soviet fleet — may already be squirting polymers from their skins.
In practice, a submarine would probably eject polymers through a ring of slots around its hull near its nose, right at the spot where a turbulent boundary layer normally forms. The liquid would flow back along the entire length, perhaps reinforced by additional rings of slots farther back. Gerald Lauchle, the polymer project leader at Penn State, says drag reductions of up to 35 percent have been achieved for the flow of fluid through pipes.
One obvious disadvantage: submarines don’t have much room to spare for storing polymers. ONR’s Reischman notes, however, that polymer ejection need not be continuous, and that it could be used for emergency bursts of speed. Why polymers reduce drag remains a puzzle, but some speculate that the long molecular chains may somehow interfere with the tiny fluid eddies that combine into drag-producing bursts.
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While some scientists develop microbubble and polymer injection, which suppress turbulent bursts in the boundary layer, others are working on ways to prevent the onset of turbulence altogether. One promising method involves sucking fluid out of the boundary layer while it is still non-turbulent, or laminar, so as to delay turbulence until the boundary layer passes the end of the hull. NASA’s approach for airplanes, has been to drill microscopic holes near the leading edges of flying surfaces and bleed air out of the boundary layer through them. The air passes under the skin of the plane and exits at the tail. The Navy is working on a comparable idea for submarines.
Still another approach involves heating the entire surface of the hull to about 70 degrees Fahrenheit warmer than the surrounding surface. The heating changes the rate at which the viscosity of the water varies with the distance from the hull, and this produces a smooth laminar flow in the boundary layer. Hull heating would be feasible only for a vessel with energy to spare, but the nuclear power plants used by submarines must dump excess heat anyway — heat that could be put to use.
Unfortunately, this technique doesn’t work in the field. The reason, according to Mohammed Gad-el-Hak is that the ocean contains swarms of small organis!IB called plankton, and when these run into the laminar boundary layer of a vessel moving through the water, they trigger turbulence, nullifying the beneficial effect of hull heating. It may seem strange that such tiny objects could so greatly affect the drag of a big ship, but Gad-el-Hak notes that even crushed insects do the same thing to airliners. Is there a solution to the plankton problem? Says one of the scientists involved, “I can’t even comment. The whole subject is one of the most sensitive Navy secrets.”
Scientists and engineers, both naval and aeronautical, have long regarded perfectly smooth skins as vi tal to drag reduction. Designers demand joint-free surfaces, flush riveting, and mirrorlike polishes for their aerodynamic and hydrodynamic creations. But the latest research has turned up surprises. In January, NASA’s Langley facility reported experiments showing that, in fact, fine grooves extending along the surface of a moving skin seem to reduce drag better than perfectly smooth finishes. Jerry Hefner says, it turned out that certain fast-swimming sharks have small patches on their skin — dermal denticles, they’re called — which are covered with little ridges running along their surfaces in the direction of the water flow.” Others have noted that the parallel grooves covering part of a baleen whale 1 s skin may alsoc reduce skin drag, allowing the whale to swim faster with less work.
Langley’s experiments have caught the Navy’s eye, and naval scientists are already hard at work on the riblet idea.
Another direction for reducing drag is being pursued by Alfred Buckingham of the Lawrence Livermore Laboratories. He has put to work a Cray supercomputer to discern patterns in turbulence that could help speed up America’s submarine fleet. If, for example, a turbulent burst could be predicted with even partial accuracy, the hull skin under it might be made to change shape in anticipation, perhaps canceling the burst’s drag effect. Buckingham believes that this might be accomplished with a flexible, or “compliant” coating — one that would form little dimples just ahead of the approaching bursts. (Buckingham and others speculate that the dolphin’s skin may do something like this.) He has suggested that such a coating on a submarine’s hull might have a fairly stiff outer membrane, backed by some supporting structure and filled with a gooey fluid .
The Navy likes the idea enough that it has spent the past several years testing a variety of compliant coatings. None of them have worked so far. The main problem seems to be a lack of theoretical understanding of turbulence.
To those who contend that no real progress in skin-drag reduction can be achieved until the mathematical underpinnings are unraveled, Gad-el-Hak of Flow Research, replies, “True academicians would say that theoretical understanding must precede practical results, but if that were strictly true, the ancient Egyptians could never have built the pyramids.” There’s a theoretical approach and an engineering approach, and the truth falls somewher e in between.
“Anyway, I think we’re going to see some pretty fast submarines.”
This article is condensed and reprinted by permission of Discover magazine : Malcolme Browne, Discover, April 1984, Time, Inc.
