When our oceans are hit by tsunamis, earthquakes, explosions and other violent events, they send acoustic-gravity waves rippling through their deepest depths at the speed of sound. Now, scientists from the Massachusetts Institute of Technology have identified another source of acoustic-gravity waves, although it's a bit less scary: surface ocean waves, including those seen from the beach and while you're on the deck of a boat.
Although these waves do not travel quite as fast or deep as standard acoustic-gravity waves, these waves, also referred to as surface-gravity waves, can create powerful, low-frequency sound waves at the right conditions. Additionally, the team found that when two of these waves head toward each other at a similar but not identical frequency, they release up to 95 percent of their initial energy, increasing the speed and depth that the resulting wave travels.
Usama Kadri and his team closely examined acoustic-gravity waves, which are typically excluded from standard water wave equations due to the fact that they do not factor in compressibility and gravity effects
"Without compressibility and gravity, we cannot describe low-frequency sound waves correctly," Kadri said in a press release. "This is one of the reasons why researchers have mostly overlooked acoustic-gravity waves."
Using his findings, Kadri and his team thus created a wave equation that includes compressibility, gravity and other higher-order nonlinear terms.
"In linear theory, two surface-gravity waves traveling toward each other do not feel each other; they get closer, pass each other, and then move away without exchanging any form of energy, as if they have never met," he explained. "However, in reality the picture is more complicated, and nonlinear effects may come into play, resulting in energy exchange and even generation of new waves, sometimes. Here, at specific frequency ranges, gravity waves can actually produce an acoustic wave that has completely different properties-and that is amazing."
This new wave equation allowed Kadri and his steam to study the unique characteristics of both acoustic and gravity waves, shedding light on the amplitudes of each wave and how they exchange energy with each other.
Kadri is now attempting to combine his team's new findings with the field of tsunami detection and hopes to design a system that detects the acoustic-gravity waves that are released before a tsunami, waves that travel more than 10 times as fast as the actual tsunami wave itself.
"Severe sea states, such as tsunamis, rogue waves, storms, landslides, and even meteorite fall, can all generate acoustic-gravity waves," Kadri said. "We hope we can use these waves to set an early alarm for severe sea states in general and tsunamis in particular, and potentially save lives."
The findings were published online in the Dec. 22 issue of the Journal of Fluid Mechanics.
