Dec 12 2017
Scientists have created a water cloaking concept hinged on electromagnetic forces that has the ability to eliminate the wake of an object, thereby considerably minimizing its drag while at the same time assisting it in evading detection.
A schematic for a prototype of the proposed water cloaking device. It consists of wires and coils that create an electromagnetic field that acts on dissolved ions to move water around the object. (Credit: Duke University)
The concept emerged in the year 2011 at Duke University when scientists defined the general concept. If the acceleration of the water surrounding a moving object is matched with the object’s movement, its propulsion efficacy could theoretically be greatly increased without disturbing the surrounding sea. The theory was a development over the team’s pioneering study on metamaterials, where the structure of a material, and not its chemistry, provides desired characteristics.
After 6 years, Yaroslav Urzhumov, adjunct assistant professor of electrical and computer engineering at Duke, has provided an update on the theory by describing a prospective approach in detail. However, instead of using a complicated system of tiny pumps as originally considered, Urzhumov has used electromagnetic fields and the dense concentration of charged particles observed in saltwater.
The research has been published online in the Physical Review E journal on December 7, 2017.
The original idea was so big that it enticed colleagues at the Naval Undersea Warfare Center to help us pursue it, even though they were incredibly skeptical, since then, we have identified a path to materializing this seemingly impossible proposal.
Yaroslav Urzhumov, one of the scientists who contributed to the original 2011 paper.
The crux of the problem being handled is that water is a comparatively viscous liquid that, if moved, likes to drag its surroundings along for the ride via shear forces. A fish weighs more upon being pulled out of the water than when it is lifted in open air due to the water pulled along with it.
Apart from actually dragging extra water, drag might also be increased by the way water flowing around an object. When a fluid flows smoothly along the surface of a hydrodynamic object, very less drag is generated as against a blocky object that gives rise to turbulent, chaotic flows in its wake state.
Moving the water out of the way is the solution to address these issues. When the water surrounding the object is accelerated to match its speed, turbulent flows and shear forces can both be eliminated.
“There are many ways to reduce wake and drag, like surrounding an object with low-friction bubbles, which is actually done with some naval torpedoes,” stated Urzhumov. “But there’s only so much you can do if you’re just applying forces at the surface. This cloaking idea opens a new dimension to create forces around an underwater vessel or object, which is absolutely required to achieve full wake cancellation.”
At first, Urzhumov proposed a kind of truss-like frame covering an object with tiny pumps and thin structures to speed up its flow when it passed through. However, as time passed, he decided that using magnetohydrodynamic forces would be the more practical approach.
The movement of a charged particle through an electromagnetic field renders the field to create a force on the particle. Since ocean water contains ions such as potassium, sodium, and magnesium, a large amount of charged particles exist to push. The concept is not as crazy as it might sound—Japan developed a prototype passenger ship known as the Yamato 1in 1991 by using these forces as a source of propulsion; however, but discovered that the method was not more effective than conventional propellers.
In their new paper, Urzhumov and his graduate student Dean Culver employ fluid dynamics simulations to demonstrate how this approach could be used to achieve a water cloak. By controlling direction and velocity of the water around a moving object, the simulations reveal that such a system is capable of matching the movement of the water inside the cloak to that of the surrounding sea.
This would seem as if the water within the cloak is completely stagnant with respect to the water outside the cloak, avoiding the wake and drag. Since practical applications are not perfect, some amount of wake and drag would exist in any realization of the device.
Although the simulations were performed with a cloaking shell with a width half of that of the object itself, the computations reveal that the shell can theoretically be made very thin as one desires it to be. Another significant outcome of the study was that the forces within the shell would not have to alter their directions when the object accelerated, but they would only require more power.
“That is one of the major achievements of this paper,” stated Urzhumov. “If you don’t have to adjust the distribution of forces, you don’t need any electronic switches or other means of dynamic control. You can set the structure with a specific configuration and simply crank up the current as the object speeds up.”
According to Urzhumov, using such a device in an actual submarine or ship would mandate a nuclear reactor for powering it up due to the extensive energy required to cloak an object of that size. However, this does not mean that a smaller cloaking device could not be powered using a smaller diesel vessel to shield potentially susceptible protrusions from detection.
He further stated that his hypotheses and computations have a number of potential uses outside of the ocean. It is possible to use similar designs to develop a distributed ion propulsion system for use in spacecraft or to curb plasma instabilities in thermonuclear fusion reactor prototypes.
“I believe these ideas are going to flourish in several of these fields,” stated Urzhumov. “It is a very exciting time.”
The Office of Naval Research supported this study through the Naval Undersea Research Program (N00014-13-1-0743) and the Multidisciplinary University Research Initiative (MURI) (N00014-13-1-0631).