Today, we touch a shark. The
University of Houston's College of Engineering presents this series about the
machines that make our civilization run, and the people whose ingenuity created them.
Last fall, I was snorkeling on Belize's barrier reef, and I
had the rare opportunity to pet a shark. I still remember its skin with its fine
sandpapery texture. The structure of the shark’s skin is made up of tooth-like
elements and of tiny grooves running parallel with its body. The grooves somehow
mute the development of turbulence in the water that’s being dragged by the moving shark.
The water that a boat or a fish or a swimmer drags with it is one of the major reasons
it takes so much energy to move through water. A dolphin also has a special skin.
It's soft and compliant. It yields to your touch.
People who can do the mathematics of fluid flow, struggle to describe the action of
either a shark's or a dolphin's skin. If they ever manage to do so accurately, they
might be able to create artificial skins for ships -- skins that would save an enormous
amount of energy and greatly reduce fuel consumption.
The skin of either beast affects what we call the boundary layer. That's the region of
water, near the moving object, that's dragged along with it. On a minnow, that layer
might stay under a millimeter thick. On a large tanker, the boundary layer might be
fifty feet thick or more near the stern.
As more and more water is dragged along, it starts tumbling in eddies and vortices. It
becomes turbulent. When that happens, drag greatly increases. Neither the shark's,
nor the dolphin's, skin prevents turbulence from taking place. But it somehow mutes
its effect. Just how, is very hard to say, because the mathematics of turbulence is
terribly complex. And the problem gets worse because the hull of fish, unlike the
hull of a ship, flexes as it swims.
There's so much to understand before we'll have effective coatings for ships. America's Cup
contenders have tried coating their boats, but the results have been uncertain.
So we look for other ways of undoing the drag upon the hull of a ship or a boat. Maybe
we can find a way to keep liquid from adhering to the hull. Then there'd be no boundary layer.
Liquid sticks to anything immersed in it. But, if we could insert a layer of air between
the water and the hull then the ship would be dragging air, and air exerts far less drag
than water. That may sound crazy, but now several organizations are experimenting with
air bubbles pumped into the water next to ships.
The hope is that we'd get a huge savings in the energy used to drive propellers, at a much
smaller cost in energy driving air pumps. But like artificial sharkskin, it's harder to
design than you might think. One variation is actually being used in speedboats. A step,
partway back on the hull, creates a long bubble along much of its length. That bubble
entirely breaks contact with the water.
Well we're getting there, but I remember the Cheshire cat stare of that Belizian shark.
I felt as though he was reminding me that I was only a visitor in a domain, which,
so far, was still his.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
For a major article on the state of fluid mechanical research on the role of skin in swimming
speed reduction, see, E. F. Fish and G. V. Lauder, Passive and Active Flow Control by Swimming
Fishes and Mammals. Annual Review of Fluid Mechanics, Vol. 38, 2006, pp. 193-224.
And here's a recent article on air injection as a means of drag reduction. T. Thwaites, Ocean Flyer.
New Scientist, Feb. 19-24, 2006, pp. 49.
I am grateful to Ralph Metcalfe, UH Mech. Engr. Dept, for his counsel.
For a schematic sketch of the structure of a boundary layer, click on the thumbnail below:
Source: A Heat Transfer Textbook, pg. 273.
The Engines of Our Ingenuity is
Copyright © 1988-2005 by John H.