Water waves exhibit negative gravity near a periodic array of buoys

May 09, 2011 by Lisa Zyga feature
Scientists have discovered a new mechanism to block water waves, which could be used to extract energy from ocean waves.

(PhysOrg.com) -- Ocean waves can be incredibly strong and very difficult to block completely. When a wave moving across the ocean interacts with a buoy, the wave can be slightly dampened, but will still pass by if its wavelength is long enough compared to the size of the buoy. But in a new study, scientists have calculated that a periodic array of resonators (such as vertical bottom-mounted split tubes or damping buoys) resonating at a low frequency can completely block water waves. The effect arises from the water having negative effective gravity.

The researchers, Xinhua Hu from Fudan University in Shanghai, China, and coauthors from China and the US, have published their study in a recent issue of .

Through and , the researchers have shown that, when a periodic array of vertical bottom-mounted split tubes resonates near a certain low frequency, the array strongly reflects approaching water waves. They found that such a strong reflection can dramatically modify the efficiency of the waves.

“It is a surprising result that a periodic structure can block long-wavelength water waves (namely, with wavelength longer than the periodic length) because conventional periodic structures such as a periodic array of bottom-mounted cylinders cannot block long-wavelength water waves,” Hu told PhysOrg.com. “In order to block long-wavelength water waves, the building block of the structure should have a low resonant frequency or a long resonant . Bottom-mounted split tubes or heaving buoys can present such a low-frequency resonance.”

As the researchers explain, because the water waves cannot pass through the periodic array of resonators, it’s as if the water has negative effective .

“The gravity is usually positive or pointed to the center of the Earth,” Hu explained. “Effective gravity is a parameter in our effective medium theory for long-wavelength water waves propagating through a periodic structure. The effective gravity is also usually positive for conventional periodic structures such as a periodic array of bottom-mounted cylinders.”

Although the researchers’ simulations involved the split tubes as resonators, they predict that other resonators such as damping buoys would have the same effect. that can block water waves could be used to extract ocean wave energy, and play a key role in future ocean wave power plants.

“Although current researches focus on improving the efficiency of a single resonator, an array of damping resonators is regarded as a key part of future ocean wave power plants,” Hu said. “Our work reveals that the absorption spectrum of an array of damping resonator (two absorption peaks) is quite different from that of a single damping resonator (one absorption peak). Such a modification is not expected by engineers on ocean wave energy extraction. Knowing such a modification is important for the future design of the resonator in ocean wave power plants.”

Hu added that the research group has experimentally verified the predicted results, which will be published in an upcoming paper.

Explore further: Study helps uncover mechanism behind solid-solid phase transitions

More information: Xinhua Hu, et al. “Negative Effective Gravity in Water Waves by Periodic Resonator Arrays.” Physical Review Letters 106, 174501 (2011). DOI:10.1103/PhysRevLett.106.174501

3.8 /5 (21 votes)

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User comments : 17

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Quantum_Conundrum
2.1 / 5 (9) May 09, 2011
One wonders why bother with ocean wave power generators, when you could, in the same area, build solar boiler platforms which will have a much higher maximum energy density by going directly to the source.

Normal ocean waves are a quarternary manifestation of solar power.

1) Direct Sunlight
2) Heats the land and ocean, driving convection (hard to harness).
3) Convection and temperature difference drive Wind Power.
4) Wind power produces wave action

The energy density in wave action is limited by the size and period of the waves. Under ordinary circumstances, not very much of this is going to be available to harvest in some sort of turbine or mill.

Additionally, waves are strongest where wind is strongest, with only a few exceptions of odd ocean floor topography and v-shaped bays. Therefore, you could as well invest in maximum wind turbine footprint in the area before considering wave harvesting.
ClevorTrever
4.6 / 5 (7) May 09, 2011
So, the wave can be slightly dampened, can it?

It's already dampened, you muppet, it's water.

I think you mean "damped".
Quantum_Conundrum
2.8 / 5 (6) May 09, 2011
Now something I thought of that would be easier to harvest, I think, and potentially have more consistent long term energy content, is to harvest channeled ocean currents.

Namely, the Gulf Stream as it passes through the Florida straights between Florida and Cuba, and between Florida and the Bahamas.

With a 2.5m/s flow rate, a cross-section of the Gulf Stream produces up to 7800 watts/m^2.

Even if a water turbine farm in the Gulf Stream was only 20% efficient, that would be 1562 watts/m^2...

By comparison, a wind turbine experiencing 15kts winds (7.66m/s) at 30% efficiency produces only 81 watts/m^2...

Now a "Current" is much different than a "Wave" because a current is to water what wind is to atmosphere. The key difference is the mass of liquid water is so much more than that of air, so there is more kinetic energy from the mass component...about 1000 times more to be exact.
Deadbolt
2 / 5 (2) May 09, 2011
Could ships protect themselves in this way?
bugmenot23
3 / 5 (2) May 09, 2011
Could this be used to prevent a Tsunami from reaching the shore?
Quantum_Conundrum
1.7 / 5 (6) May 09, 2011
Could ships protect themselves in this way?


No.

Could this be used to prevent a Tsunami from reaching the shore?


No.
_ilbud
1.7 / 5 (6) May 09, 2011
Ohh Quanty you have the grasp on science of a five year old. Hopefully one day you'll reach the age of reason.
J-n
not rated yet May 09, 2011
Is it easier to derive electricity from movement or from photons?
Quantum_Conundrum
4 / 5 (5) May 09, 2011
Ohh Quanty you have the grasp on science of a five year old. Hopefully one day you'll reach the age of reason.


How about if I said something incorrect, you actually provide a rebuttal.

My calculations above were done using the formula for power of a moving fluid, given it's density, cross section of the flow, and velocity.

If you have a problem with that then maybe you should bother doing some actual research.

What else would you be disputing from my posts above?

Sunlight causes heat, which causes wind? Hope you don't disagree with that...

Wind causes wave action? Again, hope you know that, because if not...

so um...what do you disagree with?

Maybe you're the guy who gave his tsunami question a 5...

Must not know much about tsunamis.

Try watching some of the videos from Japan.

Your pathetic man made oscillator isn't going to stop a 9300 gigaton wave...

My goodness.
DigiMc
5 / 5 (4) May 09, 2011
If the water has 'negative' gravity, wouldn't that actually increase height of the waves (and if the negative gravity is strong enough, cause the water to flow upwards)? Since exactly the opposite is described, that sounds more like locally reinforced 'positive' gravity. Not that the entire article actually has anything to do with gravity...
hellblade
May 09, 2011
This comment has been removed by a moderator.
mrlewish
not rated yet May 09, 2011
The "negative gravity" they are talking about is just if you are under a wave and a peak of a wave passes over you. There will be a miniscule by measurable reduction in your weight compared to the trough. Meaning more mass overhead more pull that way. My question is why did the stupid scientist try to complicate it up? They seem to be trying to not just work with the energy of a single wave but also the frequency of multiple waves (like ripples) to enhance energy collection much the same way that solar energy researchers sometimes concentrate on certain light spectra to improve their collectors. Waves are waves.
SemiNerd
not rated yet May 10, 2011
QC - While a man made apparatus couldn't stop a tsunami, I wonder if it could be used to 'reflect' one. There are a few places on the west coast, like Crescent Bay CA and Newport bay OR that have natural resonators and have very high local tsunami danger.

Its like a mirror reflecting laser light a million times more powerful than anything can absorb without being obliterated.

This isn't obviously wrong, although I admit to its improbability.
SantaBJ
not rated yet May 10, 2011
One wonders why bother with ocean wave power generators, when you could, in the same area, build solar boiler platforms which will have a much higher maximum energy density by going directly to the source.


You're making a flawed assumption. Energy from the sun is not the only driver of oceanic current / ocean waves. Gravity plays a major role here; the moon's gravitational pull, for example, creates a difference in tidal water levels of many meters in many parts of the world. Ocean wave generators harness gravity at least as much as they indirectly harness heat from the sun.
Ricochet
not rated yet May 10, 2011
My question is why did the stupid scientist try to complicate it up?

Probably to increase publicity for their article, so it'll get featured on sites like these...
n0ns3ns0r
not rated yet May 11, 2011
Is it easier to derive electricity from movement or from photons?

How do you define what is easier? Anybody can build a generator that harnesses motion. Can you manufacture a photovoltaic cell?
DavidMcC
not rated yet May 12, 2011
This looks like the hydrodynamic equivalent of photonic crystals. Interesting!
tkjtkj
not rated yet May 17, 2011
..researchers have shown that ...


No. They have shown that their SIMULATION demonstrated what they perhaps wanted it to demonstrate.

'Simulations' are not 'proof' of anything other than that the sim produces a result.

Not to say they are not useful, but what they show is not connected to reality except thru the preconceptions of the designers of the sim.