Science

Proposed 'fluid flow cloak' might greatly reduce ships' hydrodynamic drag

Proposed 'fluid flow cloak' mi...
A scientist has proposed a 'fluid flow cloak,' which might reduce the drag on ships' hulls by tricking the surrounding water into standing still
A scientist has proposed a 'fluid flow cloak,' which might reduce the drag on ships' hulls by tricking the surrounding water into standing still
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A scientist has proposed a 'fluid flow cloak,' which might reduce the drag on ships' hulls by tricking the surrounding water into standing still
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A scientist has proposed a 'fluid flow cloak,' which might reduce the drag on ships' hulls by tricking the surrounding water into standing still

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.

The Duke University research was recently published in the journal Physical Review Letters.

16 comments
Slowburn
If they can make it work more money to them.
Mr Stiffy
Just a thought...
While the professor said, heating the hull up to the point of creating a vapor lay between the ship and the water, would totally reduce the enormous amounts of drag be removing the wetted area., I am not sure that he meant that it could or should be done - simply because the energy requirements of keeping an enormous area of hull well above boiling point would be enormous - astronomically higher than the reduction in energy use to drive the ship, but in terms of friction....
I once traveled down about 700 miles of river in a kayak and I figured rather than stop and fish, I could sling a hand line over the edge and then just tow a lure behind me.
WOW - the increase in power required just from the drag of that - incredible.
It's like inflating car tyres properly - try riding a mountain / hybrid bike with good high pressure tyres - over inflate them by 20%, and then ride around the block. and then drop the pressure in the tyres by say 20%, then another 20% and then another 20% and then see just how hard it is to drive that bicycle with your energy being used to flex more and more rubber in the tyre, instead of driving you forward.
So the friction reduction issue on ships really is a HUGE deal - because it is SO incredibly power hungry....
Water is a VERY thick and viscous liquid, especially at SPEED and on HUGE wetted areas, AND worsera than anything, is all the crap that grows on the hull surfaces.
Go hire a kayak or canoe and then stick out a fishing lure that is about the size of a 20C piece / a quarter, or 2 or 3 cm across.... and then drag that behind you.
Daniel Plata Baca
tHIS HAS BEEN TRIED ON AIRCRAFT POUROUS WINGS...
Stein Varjord
I guess this article doesn\'t present all details of the idea, but what actually is said, seems like an idea based upon lack of understanding. Apparently the main point is to to reduce friction by avoiding contact between the hull and the water flow. The way to achieve it is making a layer of still water cover the surface. The trouble, apart from marine plants fouling the porous surface etc, is mainly three topics: 1. All objects moving through a fluid have such a layer of still fluid covering (most of) their surface. It\'s called \"the boundary layer\" and is basic knowledge in hydrodynamics. 2. Why would friction disappear because it\'s not acting directly on the hull? Answer: It doesn\'t. Water also has friction when \"rubbing\" on water. The forces from that friction will of course totally be relayed onto the hull as the boundary layer sticks to it. Still this layer is a good thing and a hull designer tries to make it as stable as possible, since the layer has a tendency to even out surface flaws and give a better general flow around the hull, with less turbulence. 3. Friction is not the main component of resistance for most boat hulls. Pressure patterns and wave formation is a way bigger problem. They are not influenced by friction at all, but by the fact that the water has to move somewhere so the hull can pass, and back again when it has passed.
Hydrodynamics is a big topic and nobody can pretend to know all the answers, but this article seems to present an idea based upon lack of basic knowledge and understanding, joining the steam idea...
@Mr Stiffy The fishing lure has all the same factors as a normal hull, but being totally submerged, the wave forming resistance is actually nearly gone. This phenomenon is noticeable with submarines. Their top speed submerged is way higher than it is at the surface. Still the lure will have a quite high resistance compared to much larger objects. Two reasons: 1. The scale effect. The lure is tiny compared to a boat. Thus it has a much larger surface to volume ratio. A bit hard to explain in an easy way, but the relationships may illustrate it sufficiently: A cube 1x1x1cm of course has a volume of 1 cubic cm and a surface of 6 square cm. A cube 100x100x100 cm is as easily 1 cubic metre and a surface of 6 square metres. Translating it to cm: 1 000 000 cubic cm and 60 000 square cm So the ratio difference is massive: 1 to 6 versus 100 to 6. This means the small cube has 100 times more surface for each measure of volume. Friction appears at the surface, so small objects have more resistance than one would think. 2.The line it hangs by has a considerable resistance. Maybe more than the lure itself. Lures are often shaped like a fish, which of course is quite an efficient hydrodynamic shape. The line is normally a round profile. According to an authority on the topic, CA Marchaj, a round section will have approximately the same drag as a 1 by 3 wing (or droplet) shaped section of 10 times the projected area. Meaning: Your line may be 0,5mm thick and would then have the same resistance as a 5mm x 15mm wing shaped rod in its place. The reason is that a round section will create a turbulence behind it way bigger than the line itself, which a wing shape will not. Of course it\'s not possible to use wing shaped fishing lines, since they have to be aligned with the flow. This is just an illustration of proportions.
Rkt9
Seems like a good idea, if sea water were just water. How does he propose to keep the porous medium from filling up with sea crud?
xfbiagent
This process already has a US Pat. 7004094 - Filed Jun 28, 2002 - Issued Feb 28, 2006
saxonprice
@stein varjord I read what you\'re saying, but I have to believe that the Duke professor responsible for the claim knows a thing or two about his chosen field and probably covered the topics you brought up. Simply put, I don\'t think this would be headline worthy if it could be debunked by dragging a lure through water and telling all his colleagues to feel how heavy it seems.
Ben Crumpacker
this is the same principle as the laminar wing design, wherein air exits small holes in the wing surfaces in order to reduce turbulence and wave formation, and thus smooth out the air flows above and below the wing. Result: higher speeds, more efficient travel. It\'s been used for decades in airplane design. Obviously water is much denser than air and behaves somewhat differently as an object moves on and through it, so the proposal probably addresses these issues. Agreed, foreign object damage [\"FOD\"] and clogging are issues for design & engineering.
Dana Lawton
Cool idea... you would not have to worry about water displacement drag because resistance is zero. There would be initial weight drag but once speed has leveled out it would dissapear.
Mr Stiffy
@ Stein Varjord
Good answer.....
Most of what I put up was to establish the sentient link, between pencil to paper and real world effects.
Like it\'s one thing to paddle a kyak over flat water - heading down river - and it\'s another thing to stand on the river bank and cast and wind the fishing lures back inshore, and it\'s another thing to FEEL just how much drag that lure creates, and how much power it requires to overcome the drag when paddling the boat - when YOU have to supply the power from your own effort.
It\'s being able to FEEL the difference - that is the establishment of the cognitive link, between power, and drag.
It\'s something all marine // aerospace // movement designers ought to be made to do as part of their training - the low pressure bike tyres and the kayak with drag....
Or better yet - push the car around the parking lot - with fully inflated tyres, and then push it when the tyres are significantly under inflated.
Getting the knowing from doing, is as different as talking about fuel efficiency and lists on paper and computations...