Duke engineers have already shown that they can “cloak” light
and sound, making objects invisible—now, they have demonstrated the
theoretical ability to significantly increase the efficiency of ships by
tricking the surrounding water into staying still.
“Ships
expend a great deal of energy pushing the water around them out of the
way as they move forward,” said Yaroslav Urzhumov, assistant research
professor in electrical and computer engineering at Duke’s Pratt School
of Engineering. “What our cloak accomplishes is that it reduces the mass
of fluid that has to be displaced to a bare minimum.
“We
accomplish this by tricking the water into being perfectly still
everywhere outside the cloak,” Urzhumov said. “Since the water is still,
there is no shear force, and you don’t have to drag anything extra with
your object. So, comparing a regular vessel and a cloak of the same
size, the latter needs to push a much smaller volume of water, and
that’s where the hypothesized energy efficiency comes from.”
The results of Urzhumov’s analysis were published online in the journal Physical Review Letters.
The research was supported by the U.S. Office of Naval Research and a
Multidisciplinary University Research Initiative (MURI) grant through
the U.S. Army Research Office. Urzhumov works in the laboratory of David
R. Smith, William Bevan Professor of electrical and computer
engineering at Duke.
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—which
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 make it so the water passing through the porous material leaves
the cloak at the same speed as the water surrounding by the vessel,”
Urzhumov said. “In this way, the water outside the hull would appear to
be still relative to the vessel, thereby greatly reducing the amount of
energy needed by the vessel to push vast quantities of water out of the
way as it progresses.”
While
the Duke invisibility cloak involved a man-made structure—or
metamaterial—based on parallel rows of fiberglass slats etched with
copper, Urzhumov envisions a different sort of metamaterial for his
fluid flow cloak.
“In
our case, 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. In
addition, some of the cells of this array would be equipped with
fluid-accelerating micro-pumps.”
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 like 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.
“When
you try to drag an object on a fishing line through water, it feels
much heavier than the object itself, right?” he said. “That’s because
you are dragging an additional volume of water with it.”
Based
on this understanding of the flow cloaking phenomenon, Urzhumov
believes that the energy expended by the micropumps could be
significantly less than that needed to push an uncloaked vessel through
the water, leading to the greatly improved efficiency.
