Effortless sailing with fluid flow cloak

Aug 11, 2011

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.

" 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 comes from."

The results of Urzhumov's analysis were published online in the journal . 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 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.

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User comments : 19

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Techno1
1 / 5 (2) Aug 11, 2011
If true, this would cut fuel consumption to about half.

Have they done any scale model tests yet? Or is this just an "on paper" theory?
Scottingham
not rated yet Aug 11, 2011
Where do you get your numbers Techno1?

"Cutting fuel consumption to about half" is wild speculation.
Techno1
1.6 / 5 (5) Aug 11, 2011
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.


Here.

Figure the displacement for a large ship:

If a ship displaces 100,000 tons, then it's basically dragging that much extra mass of water with it everywhere it goes.

===

However, in all honesty, I cannot see how his plan will work, but I guess we'll all just need to wait and see.

Anyway, It's not clear how the Navy would benefit from this any time soon, as cuts in military spending are imminent within a few years, and it would take decades to phase in yet ANOTHER class of Carriers and destroyers implementing any technology resulting from this research.
eachus
5 / 5 (1) Aug 11, 2011
Rediscovering the wheel. Imagine a hull which consists of two flat sides (flat on the outside, that is), connected by a flat bottom. Instead of propellers below the ship you pull enough water through the ship so that there is no wake.

There have been a number of floating drydocks designed like this, and they move very efficiently through calm water. Of course, they handle like dogs in a heavy cross-chop. The drydock can close the doors and pump out the well when necessary, but now they move very slowly.

Adding side thrusters to compensate for cross-currents might make it a more reasonable design, but I still can't see building warships that way.
that_guy
not rated yet Aug 11, 2011
There's an ice breaker that already has this idea in use. But the researcher wanted to be smart and make new ideas rather than admit he's second to the idea. The key distinction between the one out there and this guy's idea is that the primary motivation for the icebreaker was that they didn't want it getting stuck on the ice...but it's the same physics/principle.

In conclusion, Urzhumov is a hack who is using the 'cloaking' buzzword to make it look like he's on the bleeding edge.

@techno. wrong.
The drag of a ship is based on its drag coefficient. Weight is a factor in design - it determines how much is in the water, but it does not create drag by itself. We both know you pulled that out of your ass.

Read Up http://en.wikiped...physics)

That's why ships have keels that are shaped like knives, rather than like a 1988 cadillac el dorado. Because drag is not directly related to displacement.
Techno1
not rated yet Aug 11, 2011
That's why ships have keels that are shaped like knives, rather than like a 1988 cadillac el dorado. Because drag is not directly related to displacement.


Of course not. It's related to shape and size.

I was not concerned with that at the time of the post, and was referencing what was said in the article.

This is why I don't think this will work, or won't be useful for Navy vessels, unless he's claiming to have discovered some entirely new physics principle nobody else has ever thought of: Like inertial dampeners or something...

He did make the reference to cloaking technology, but I figure that was hyperbole...

If this actually worked the same way the cloaking materials works, then this really would be an inertial dampener, well at least of a sort, not like the sci-fi kind anyway...

From the article, it seems to just be an over-complicated way of saying, "Cut some pipelines in the hull and let the water go directly through it..."

maybe I missed it...
Techno1
1 / 5 (1) Aug 11, 2011
But the implication from his statement in the paragraph I quoted is that it's going to significantly change the way the ship interacts with water so that it doesn't need to move the water around.

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


I don't know how that would be possible without some sort of freaky physics, so it must be something more exotic than simply allowing water to flow through the hull. That's pathetic and isn't really even anything new....
Techno1
1 / 5 (1) Aug 11, 2011
Like, if water flowed through the hull, then that would not make water outside the hull "appear to be still relative to the vessel," like he said.

That would only change how the water moved relative to the vessel.

Maybe his choice of wording is bad or something, but its' almost as if he's saying the ship will be moving and not moving at the same time...
Doc_aymz
not rated yet Aug 11, 2011
When a ship is planing it no longer obeys Archimedes principle.
The inertia of the fluid starts to work in your favour in this case.

This coupled with the fact that the less you disturb the water the less energy you are wasting. I thought these were well known and I don't see what this adds.

I'm not that taken with the description of how such a thing would work.
Callippo
1 / 5 (1) Aug 11, 2011
IMO this cloak is already working in various "shark-skin" technologies, the main purpose of which is to reduce hydrodynamic drag & friction. It's not a new technology, rather a new approach to its formal description.

Because in aether theory the magnetic field corresponds the hydrodynamic flux of vacuum, I can see the similarity of the hydrodynamic cloaking with magnetic flux cloaking device, described recently.

http://arxiv.org/abs/1107.1647
sstritt
5 / 5 (1) Aug 11, 2011
This should also work for aircraft, although obviously not on the lifting surfaces.
Techno1
not rated yet Aug 11, 2011
When a ship is planing it no longer obeys Archimedes principle.
The inertia of the fluid starts to work in your favour in this case.


Well, that's not what the article says.

I know what hydrofoil boats are, silly, yes it's well known. They have races in the small, one-man versions too, and lots of people get hurt too.

Anyway, that's not what the article is talking about.

It says in two places that it works by stopping water flowing relative to the boat:

they have demonstrated the theoretical ability to significantly increase the efficiency of ships by tricking the surrounding water into staying still.


And

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


Either this is a very bad choice of words, or it's something entirely different from hydroplaning...

ronaldk13
5 / 5 (1) Aug 12, 2011
Not only should it be applicable to aircraft, but I've thought for a few years this type of technology (reduced drag) will make supersonic transport acceptable to the public (reduced boom).
antialias_physorg
not rated yet Aug 12, 2011
Link to the original article here:
http://arxiv.org/...82v2.pdf

From what I gather by quickly skimming it there is an upper limit to the ratio of inertial to viscous forces for which this works (Reynolds number)

I'm not pretending to understand the math here, but it sounds like a very elegant way to reduce the energy need of a vessel.
_nigmatic10
not rated yet Aug 12, 2011
not practical due to the expenditure of energy to accomplish the effect. Could be used in future applications of space flight and high speed aircraft, where such an effect could reduce the stress on the structure during high speeds.
antialias_physorg
not rated yet Aug 12, 2011
not practical due to the expenditure of energy to accomplish the effect.

How do you know whether the expenditure of energy outweighs the savings?

Could be used in future applications of space flight and high speed aircraft, where such an effect could reduce the stress on the structure during high speeds.

Hardly, because a porous material would get ripped to shreds in no time with the kinds of forces (and temperatures) imparted at those high speeds. Current Scramjet designs already incorporate a solid ablative copper tip.
Think about what the kinds of forces that can melt away a copper substrate during flight would do to a foamy substance.
xznofile
not rated yet Aug 13, 2011
the pumps would have to be turbulence free too. Darpa has a kind of jet wing surface with holes to reduce drag.
Isaacsname
not rated yet Aug 13, 2011
Great idea, but how does it transition between differing temps, salinity, pressure, ie: water density ?
Pete1983
not rated yet Aug 14, 2011
There was a cloak of a similar nature on physorg a few weeks ago, and someone in the comments came up with exactly this idea. Can't find the old article, but whoever had that idea, kudos!