[LINK] Report: Advanced submarines could be detected by existing technology

[Stephen Loosley]

Science

US nuclear submarine weak spot in bubble trail: Chinese scientists

Extremely low frequency electromagnetic signal produced by speeding subs 
can be picked up by available devices, researchers say. The almost 
imperceptible bubbles could give away the position of most advanced 
submarines, but more research is needed


By Stephen Chen 14 Aug, 2023 
https://www.scmp.com/news/china/science/article/3230476/us-nuclear-submarine-weak-spot-bubble-trail-chinese-scientists



A study published in one of China’s most respected peer-reviewed 
journals suggests that advanced submarines could be detected by existing 
technology

The US is widely believed to operate some of the most difficult to 
detect submarines, with sophisticated acoustic and vibration reduction 
systems to blend them into the background noises of the ocean.

Carl Schuster, a retired US Navy captain and former director of 
operations at the Pacific Command’s Joint Intelligence Centre in Hawaii, 
told CNN in April that “submarines are one area where the United States 
retains unchallenged superiority over China”.

But the study, by researchers from the Chinese Academy of Sciences’ 
Fujian Institute of Research on the Structure of Matter, found an 
ultra-sensitive magnetic detector could pick up traces of the most 
advanced submarine from long distances away.

The team, led by Zou Shengnan, used computer modelling to determine 
whether it was possible to detect the almost imperceptible bubbles 
produced by a nuclear-powered submarine cruising at high speed.

The result “provides a new solution for the detection and tracking of 
submarines”, according to the paper published on August 1 by the Chinese 
Journal of Ship Research.

The journal is run by the China Ship Scientific Research Centre, which 
has a long and respected history of cutting-edge developments in ship 
and ocean engineering.

The researchers calculated that the extremely low frequency (ELF) signal 
produced by a submarine’s bubbles could be stronger than the 
sensitivities of advanced magnetic anomaly detectors by three to six 
orders of magnitude.

“The magnitudes of the induced electric field and magnetic field are ... 
well within the detection range of some top-notch sensors,” the paper said.

The bubbles are an inevitable consequence of the submarine’s cruising 
speed, which causes the water flowing around the hull to move faster as 
its kinetic energy increases and its potential energy – expressed as 
pressure – decreases.

This is because the total energy of a given system must remain constant.

In the case of a fluid in motion, the sum of the kinetic energy – caused 
by the motion of the fluid – and the potential energy will not alter, 
but the balance of the two forces will shift.

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When the pressure decreases sufficiently, small bubbles form on the 
surface of the hull as some of the water vaporises. This cavitation 
process is most likely to occur on areas with a sharp curvature or rough 
surface, which can create regions of low pressure.

As the water continues to flow around the hull, the bubbles grow bigger 
and move away from the surface, where the higher pressure – such as near 
the trailing edge of the hull – causes them to collapse violently.

This process causes turbulence and and can produce an electromagnetic 
signature, in a phenomenon known as the magnetohydrodynamic (MHD) 
effect. The faster the turbulence, the stronger the MHD voltage will be.

According to the computer modelling results, “significant induced 
electric field signals can be observed around the bow, stern, and rear 
of the hull”, the paper said.

The electromagnetic emissions produced by the cavitation bubbles 
fluctuate over time, generating a distinct signal in the extremely low 
frequency range, from 49.94 Hz to 34.19 Hz, according to the researchers.

Though faint, ELF signals can travel great distances, thanks to their 
ability to penetrate the water and reach the ionosphere, where they are 
reflected back to the Earth’s surface.

This quality is already exploited for submarine communication. China, 
for instance, has built the world’s largest antenna to transmit ELF 
signals to its submarine fleet hidden in deep waters.

According to Zou and her team, their findings can “also provide a 
reference for the selection of electromagnetic communication frequencies 
for high-speed submarines”.

Traditional electromagnetic detection technologies locate and track the 
disturbance in the Earth’s magnetic field created by the movement 
through the water of a submarine, which is typically made of a 
ferromagnetic material like steel.

By analysing the strength and direction of the electromagnetic field, it 
is possible to determine the location and movement of the submarine. But 
traditional detection technologies are increasingly limited by the 
countermeasures deployed by ship designers.

These include the use of low-magnetic or non-metallic materials in the 
design of the submarine shell, as well as other measures to continuously 
reduce the electromagnetic characteristics of the vessels.


There is an urgent need for research into a new type of signal source, 
according to Zuo and her team, but they cautioned that there are several 
practical challenges to detecting the electromagnetic signals produced 
by submarine cavitation.


These include the disappearance of the signal when the submarine slows 
down or stops, as well as interference from other sources – such as 
naturally occurring electromagnetic noise or human-made signals.

The flow of water around a submarine can also be highly turbulent and 
unsteady, which can affect the formation and evolution of cavitation 
bubbles and the resulting electromagnetic signals.

The researchers said further research is need to better understand the 
complex physics behind the flow-electromagnetic coupling, to develop 
more accurate and reliable models for predicting these induced 
electromagnetic submarine signatures.

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