It comes from the observation that ships expend a great deal of energy pushing the water around them out of the way as they move forward.
‘When you try to drag an object on a fishing line through water, it feels much heavier than the object itself — that’s because you are dragging an additional volume of water with it,’ said project lead Prof Yaroslav Urzhumov of Duke. ‘What our cloak accomplishes is that it reduces the mass of fluid that has to be displaced to a bare minimum.’
While the cloak postulated by Urzhumov differs from other cloaks designed to make objects seem invisible to light and sound, it follows the same basic principles — the use of a man-made material that can alter the normal forces of nature in new ways.
In Urzhumov’s fluid-flow cloak, he envisions the hull of a vessel covered with porous materials — analogous to a rigid sponge-like material — that would be riddled with holes and passages. Strategically placed within this material would be tiny pumps, which would have the ability to push the flowing water along at various forces.
‘The goal is to make it so that the water passing through the porous material leaves the cloak at the same speed as the water surrounding the vessel,’ Urzhumov said. ‘In this way, the water outside the hull would appear to still be relative to the vessel, greatly reducing the amount of energy needed by the vessel to push vast quantities of water out of the way as it progresses.’
Urzhumov explained that when a regular vessel moves through fluid, it also pushes and displaces a volume of water that greatly exceeds the volume of the vessel itself. That is because, in a viscous fluid such as water, an object cannot just move a single layer of water without all others; the shear force effectively attaches an additional mass of water to the object.
’Since the water is still, there is no shear force and you don’t have to drag anything extra with your object,’ said Urzhumov, who now plans to build prototypes based on the fluid-flow cloak principle.