How to defuse a tsunami. Perfect absorption reproduced in the laboratory

AGI – Physicists have shown that two cavities dug into the side of a canal can completely dissolve the energy of the incident waves. In a model setup with real water, the researchers were able to “perfect absorption“, in which the waves completely cancel each other out when bouncing off the walls of the cavity, with waves with a frequency of 2.9 Hertz.

The discovery suggests the possibility of designing structures to protect shorelines or harvest wave energy. With further development, researchers believe the effect could be exploited Reduce erosion or protect sensitive structures using a series of elements distributed near the coasts. “Our motivation was the need to control or absorb waves in rivers or protect coastlines,” explained Agnes Maurel, a physicist and mathematician at ESPCI in Paris.

“Completely absorbing wave energy is even better than redirecting it, and one can even imagine harvesting that energy,” Maurel continued. However, it is not easy to achieve perfect absorption because, according to common knowledge, the water, light or sound waves that hit a hypothetical object, a so-called perfect absorber, are neither reflected nor transmitted, but simply disappear.

In recent years, Researchers who work with light or sound waves They showed something very close to perfect absorption. But no one has created a perfect absorber for water waves. Maurel and colleagues were inspired to try a new approach based on their recent mathematical work, which showed that perfect absorption could be achieved by developing a particular type of resonant structure that incoming water waves would interact with.

This work showed that when waves travel along a straight channel, when excited by a passing wave, these structures emit secondary waves that perfectly cancel out the reciprocating waves. To demonstrate this theory in practice, Maurel and his colleagues followed a two-step process: first, they analyzed how the cavities cannot produce transmission at certain frequencies, and then optimized this configuration to produce no reflection at the same frequencies.

They considered a water channel 1.4 m long, 6 cm wide and 5 cm deep. In preliminary calculations, they showed, based on the equations for an ideal fluid that has no energy losses due to friction, that two small cavities attached to the side of the channel could function as resonance chambers. The calculations assumed that the two cavities were identical in size, set back 4 cm from the canal wall and extending 3 cm along the canal.

For waves in the frequency range studied by the team of scientists, these calculations predicted zero transmission at 2.7 and 3.3 Hz. The research team then tested these predictions using water waves in the laboratory. In close agreement with expectations, the researchers found two dips in the sending frequency curve: the first to almost zero and the second to around 40%. According to the researchers, the deviations from theory reflect frictional losses in the fluid and the approximations used in the analysis.

This arrangement did not produce perfect absorption at any of the frequencies because the reflection was significant. However, the researchers found that by distorting the two cavities so that one is slightly further from the channel wall than the other, perfect absorption, meaning no transmission and reflection, could be achieved at 2.9 Hz.

This required trial-and-error adjustment of the exact asymmetry of the cavity so that the transmission and reflection zeros appear at exactly the same frequency. Maurel and colleagues hope to develop this idea into a practical system for protecting coastlines from erosion. “The mechanism we demonstrated for guided waves can be expanded into a kind of protective belt in the seasaid Maurel.

“This is important work,” said Sèbastien Guenneau of Imperial College London, an expert in wave propagation in a wide range of applications. “The two closely spaced water cavities communicate and behave like pistons in an engine,” added Guenneau, who foresees future applications of the technology new types of dams that could contain water with lower risk of overlap. “Resonant structures,” Guenneau concludes, “could also be used for this.” harvest the energy of the ocean waves”.

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