June 03, 2011

Protecting coastlines and generating wave power by reflecting or absorbing 90 per cent of wave energy

Xinhua Hu and colleagues at Fudan University in Shanghai, China, have come up with a way to create shield against water waves that, unlike Enoch and Guenneau's set-up, could also double up as a wave-energy plant. Hu's team proposes using a rectangular array of stationary cylinders fixed to the sea floor in coastal waters. "The resonating cylinder array that we studied can be seen as a type of metamaterial for water waves," Hu claims.

Each hollow cylinder would be split vertically into quarter-circle arcs that fill up with water, and discharge it, depending on the water level surrounding them (see diagram). Although the cylinders are completely still, this constant filling and discharging is a form of oscillation and so is analogous to the electromagnetic oscillators that interfere with light waves in an invisibility cloak.

By adjusting the width of the vertical slits, the size of the cylinders, and their spacing, Hu calculates that the array could be tuned to water waves of a particular frequency so that it drains the peaks and then discharges to fill in the wave troughs - in effect dismantling those waves.

Using several arrays with different spacing and various sizes of cylinder, it might even be possible to block waves of several different frequencies - and perhaps even tsunamis, Hu claims. The result should be a huge reduction in waves within the array, and as a result, protection for any coastline or shore behind it, he says.

Physical Review Letters - Negative effective gravity in water waves by periodic resonator arrays

This action alone will not remove the waves' energy - "reflection" waves would be produced, but in the opposite direction. Alternatively, Hu says the energy from the dismantled waves could be used to generate power, if it was absorbed by a device inside the columns. Based on an idealized scenario in which waves are all one frequency, Hu calculates that 90 per cent of wave energy could be reflected or absorbed by the arr

Hu's team has demonstrated this idea using computer simulations and a table-top experiment. The researchers placed 15-millimeter-tall model resonators in water 8 mm deep, spaced 12 mm apart. When they tested waves of different frequencies on the mini-array, they were able to create a wave "no-go zone" at a frequency of 4.2 waves per second. Hu likens this zone to the band-gaps found in semiconductors, which are out-of-bounds for electrons thanks to quantum mechanics.

Pendry thinks Hu's design could probably work for regular waves, but not for tsunami defense. "Given the energy content of a tsunami, and also the very low frequency of the wave, it is definitely not a practical proposition," he says. Hu agrees that tsunami defense would require an "unprecedented" engineering effort: columns would need to be placed every 100 meters and be strong enough to survive the tsunami.

Hu adds that arrays of buoys already used to generate wave energy could be adjusted to double up as coastal shields.

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