Proposed 'fluid flow cloak' might greatly reduce ships' hydrodynamic drag
By Ben Coxworth
August 12, 2011
North Carolina's Duke University has been grabbing some headlines over the past few years, due to research carried out there involving the use of metamaterials for creating functioning invisibility cloaks. Just this month, Duke researchers announced that they had developed another such material that could be used to manipulate the frequency and direction of light at will, for use in optical switching. Now, Duke's Prof. Yaroslav Urzhumov has proposed that metamaterials could also be used to drastically reduce the drag on ships' hulls, "by tricking the surrounding water into staying still."
When you pull an object such as a fishing lure through the water, it tends to feel like it's much heavier than it actually is. This is because of the friction that occurs between the moving object and the stationary (or at least less quickly-moving) water around it. Not only does the object drag against the layer of water that's touching it, but to a lesser extent, it also has to pull along the water that's adjacent to that layer.
In Urzhumov's scenario, the layer of water touching the hull would be moving at the same speed as the ship, so it would be as if the water wasn't moving relative to the vessel - most of the friction would occur between that layer and the surrounding water, and not be focused on the ship. This would be made possible through a metamaterial coating the ship's hull, that was full of tiny holes and passages, kind of like a rigid sponge. Water would enter the material, then be ejected by small pumps, at a speed matching that of the water surrounding the vessel.
"I see this porous medium as a three-dimensional lattice, or array, of metallic plates," he said. "You can imagine a cubic lattice of wire-supported blades, which would have to be oriented properly to create drag and lift forces that depend on the flow direction."
Essentially, the moving ship would be surrounded by a cloak of still water. While some energy would be required to run the micro-pumps, Urzhumov believes that it would be more than made up for by the energy that the system would save.
A different system, recently proposed by the University of Melbourne's Prof. Derek Chan, would see ships made more hydrodynamic by enveloping their hulls in a layer of super-heated water.