## Reducing the Magnetic Signature of a Submarine

##### Mark Fowler February 24, 2014

Submarines can be detected by enemy weapon systems due to their magnetic signatures. By designing vessels with reduced magnetic signatures, detection can be avoided, but the composition and size of most submarines often make simulation difficult. COMSOL software helps you overcome this problem.

### Detecting Magnetic Signatures

Magnetic signatures are measurable disturbances in Earth’s magnetic field. Submarines produce their own magnetic signatures, for example. However, the magnetic signatures they produce make them vulnerable to enemy weapon systems. During submarine warfare, enemies use increasingly sophisticated magnetic sensors and signal processing systems that make disturbances in Earth’s magnetic field easy to detect.

Submarine. Image courtesy of Kockums AB.

To combat this, engineers use degaussing techniques, i.e. methods to suppress submarines’ magnetic signatures to safe levels. One such technique involves generating a counteracting magnetic field of sufficient strength and direction that matches the background field. In order to accomplish this, one must be able to predict a submarine’s magnetic signature based on its magnetic properties.

### Standing up to the Submarine Simulation Challenge

Submarines are essentially long steel tubes, and their magnetic signature is primarily due to the permeability of the steel hull. Although it is built to withstand enormous pressure, a submarine hull is still relatively thin compared to the size of the vessel. Simulation of such a structure would be difficult using standard finite element analysis because volume meshes of thin, long structures are difficult to generate and tend to become very large.

However, using COMSOL Multiphysics and the AC/DC Module, you can overcome this issue by modeling thin metal sheets as 2D faces embedded in a 3D geometry.

A model geometry of a submarine where face objects represent the submarine and a 3D box represents the surrounding water.

#### The Reduced Field Formulation

Once you have built the geometry of the submarine and you understand its magnetic properties, you can work towards predicting its magnetic signature. The AC/DC Module contains the Reduced Field formulation, which is a very helpful feature for obtaining the magnetic signature. The feature allows you to define the background field as a predefined quantity and solve only for disturbances in the field.

If you follow the step-by-step instructions in the Model Gallery, you can set up and solve the model.

The total magnetic flux density in a horizontal slice plot 7.5 m below the keel.

A distinct field disturbance is apparent due to the presence of the submarine. Shown, using arrows, are the magnitude and direction of the tangential magnetic field in the submarine’s hull. The reduced field is seen as isosurfaces of the reduced magnetic potential.

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1. antoine viana January 5, 2016   10:45 am

Hello Mark

I don’t agree with you when stating that the magnetization is null.
It is not : the magnetization can be considered as tangential (it is wrong at some places of the hull) but in no case it is null. The magnetization is M=(mur – 1)H = (mur-1)(Hred + Hbackground) (1) or, alternatively, B=mu0(H+M) (2).
The ferromagnetic material implies a non-null magnetization.

The mfnc model is unable to compute the magnetization (try to represent Mx, My and Mz) but because it only computes the reduced field. To access the value of magnetization, we need to use one of the previous relations (1) or (2)

Thanks

Antoine

2. Bridget Cunningham January 5, 2016   1:22 pm

Hi Antoine,