Chinese Scientists Develop New Techniques to Hunt Submarines in Deep Ocean

Submarines have gotten stealthier thanks to better sound dampening and magnetic shielding. This poses a challenge for finding them with traditional sonar or magnetism-detecting methods. However, scientists have discovered another way.

When submarines glide swiftly through water, they disturb the fluid in ways that create tiny electric currents. These induced currents then generate weak electromagnetic fields. Unlike a submarine’s emitted sounds or magnetism, these induced fields depend solely on water movement.At first, researchers thought these submarine-stirred fields were too faint to detect from afar. But with improving tools, they saw potential. Studies of waves, tides and ship wakes showed distinct induced fields that sunlight detectors could spot from miles away.Yet problems remained. Existing theories oversimplified water’s flow patterns near fast-moving submarines. They also ignored “cavitation” – air bubbles that periodically cloaked speeding submarines due to pressure changes. To overcome this, scientists from Fuzhou University and Chinese Academy of Sciences decide to try something nobody has done before.Imagine the ocean as a giant, invisible energy field. When a submarine cuts through the water, it’s like dropping a pebble in a pond. It sends ripples – electromagnetic signals – through this energy field. These ripples can be picked up by special detection technologies, giving us important info about what’s happening underwater without needing to hear anything.

However, to pick up and interpret these signals are anything but straightforward. The interaction between the moving submarine, that is, the flow field and the electromagnetic field is complex and behaves in pretty unpredictable ways.

One thing that makes this interaction complex is something called natural cavitation – basically, the formation of bubbles in the water caused by the submarine’s movement. These bubbles can mess with the electromagnetic signals, so we need to understand their effects more deeply.

To do that, the scientists first create a mathematical model that combines fluid dynamics with electromagnetics, which explain how the energy field behaves. Then, they use a computer program called Fluent to simulate how the electromagnetic signals change with different levels of cavitation.

The simulations show that the electromagnetic signals have a kind of repeating, yet unsteady pattern due to the way the cavitation changes. The strength of the electric and magnetic signals they found are weak, but they’re just within the range of what our most advanced sensors can detect. They found that as bubbles grow and shed from the hull, it disturbs local fluid flows and the velocity field. This leads to intense transfer of mass, triggering detectable electromagnetic signals in the medium on scales of 10-1 mV/m for electric fields and 10-2 nT for magnetic fields.

The study also observed periodic changes in the electromagnetic fields at the tail wing area with a frequency of 0.021 seconds. This matched the shedding cycle of bubbles breaking away from the submarine. Such consistent frequencies could help in tracking submarines electromagnetically.

The number of cavitation bubbles, or “cavitation number”, also influenced the frequency characteristics of the induced fields. As more bubbles formed, field oscillation amplitudes increased strongly. A main frequency emerged in the electric field spectra between 34-50 Hz, depending on cavitation number. Additionally, shedding of large bubbles created smaller bubbles, boosting low frequencies near the main frequency.The study is recently published in the Chinese Journal of Ship Research. This knowledge could help in creating new underwater sensors and making decisions about communication methods for advanced submarines. The findings are also valuable for detecting vessels that move quickly underwater without using sound.