Military Embedded Systems | New electromagnetic formulation helps Navy protect ships from mines

Working with the Navy, Sherbrooke Consulting used electromagnetic simulation to accurately determine the magnetic signature of a warship used for full-scale electromagnetic signature tests. The success of this simulation should make it possible in the future to reduce or eliminate physical testing when measuring the signature of other warships in order to avoid detection and mines.

The movement of a naval ship in the Earth’s magnetic field generates an electromagnetic field that can be used to detect the presence of the ship or to set off mines. Stealth can be achieved only if the ship generates an equal and opposite magnetic field that will cancel out the original field and prevent the ship from being detected. The challenge is determining what sort of a field the ship will generate in a wide range of positions on the Earth, speeds, and wave conditions. Obtaining this information through physical experiments would be costly and time-consuming, so the U.S. Navy is turning to computer simulation.

Electromagnetic warship signatures
The sizeable mass of iron that makes up most of a ship attains a significant level of magnetization in the presence of the magnetic field at the Earth’s surface, which has a magnitude of approximately 0.5 Gauss. The electromagnetic signatures of warships first became important during World War II when sensors capable of detecting a ship’s magnetic field were discovered and used for detonating a charge close enough to the hull to cause major damage. This development prompted the United States and the United Kingdom to begin developing countermeasures for mines triggered by magnetic fields. The Navy developed DC compensating coils that degauss or null out the ship’s ferromagnetic signature in order to prevent detection.

However, as well as the ferromagnetic signature, an additional electromagnetic field is created when electrically conducting materials, such as steel, aluminum, and copper, move in a magnetic field. Movement of a conducting material in a magnetic field induces currents in the conductor (called eddy currents) that generate their own reaction field. The most difficult part of canceling out the magnetic field is quantifying the reaction magnetic field generated by any type of rocking motion so that algorithms can be developed to drive the degaussing coils based on the ship’s motion. As naval ships operate more frequently in shallow water destinations, the problem becomes greater because rolling motion tends to increase as water depth decreases.

The traditional approach to canceling a ship’s magnetic field employed by the Navy since World War II is to use instruments to measure the electromagnetic fields generated by ships in a fixed slip or pier. Due to the high cost of performing experiments on actual ships, measurements have also been performed on scale models. For example, the Magnetic Ship Models Laboratory at the former Naval Surface Warfare Center, White Oak, Maryland, was used to test physical models up to 12 feet long. The lab was capable of generating magnetic fields to replicate the magnetic environment at any point on the Earth’s surface. Physical models provide useful information and are cost-effective but are limited to some degree by the assumptions required in constructing the model and capabilities of test facilities.

More accurate results can be obtained from testing actual naval vessels, but, of course, it’s very expensive to bring ships to fixed facilities where a DC background field can be applied for ferromagnetic signature measurements or an AC background field can be applied to simulate wave motion for eddy current measurements. Previously, the Navy purchased a German gunboat primarily for the purpose of performing full-scale electromagnetic signature tests at the Charleston Naval Station, Charleston, South Carolina. While these measurements provide a higher level of confidence than scale models, it still is only possible to test this single ship under a narrow range of possible motions. The expense of actual vessel measurements and limits associated with scale modeling has led the Navy to seek alternative approaches.

Simulation is faster and less expensive
The Navy has overcome these physical limitations by using COTS software that can calculate the electromagnetic field generated by any object under any set of conditions to higher accuracy and at less expense. Using this software, they can determine ferromagnetic and eddy current signatures and aid in the development of the algorithms needed to drive electromagnetic countermeasures to cancel out a ship’s magnetic field to avoid setting off mines. The key advantage of electromagnetic simulation is that it greatly reduces the time and expense required for each test, which is critical when considering that the Navy’s ultimate goal is to countermeasure a ship’s electromagnetic signature during every possible wave-induced motion, and at any place on Earth it is likely to visit.

There are two major issues involved in electromagnetic simulation:

Sherbrooke has been working with the Navy to overcome these challenges.

The first order of business was to ensure that we could replicate the actual magnetic fields generated by a ship using software. We used the FEM (Finite Element Method) OPERA and ELEKTRA software from Vector Fields, Aurora, Illinois, because this software was built from the ground up for electromagnetic simulation and offers many advantages over other software programs that were originally designed for structural or thermal modeling. For example, we like the fact that the geometries of the coils are independent from the structure of the (finite element) mesh. This means we can mesh the ship without even thinking about where we are going to put the coils that represent the Earth’s magnetism. The Navy technical team began by modeling the German gunboat because it had reliable physical measurements that it could use to verify the accuracy of the simulation.

Overcoming the technical demands of modeling a warship
Warships are very difficult to model, both because of their complexity and their shape. An automatic modeler is essential to handle the many and varied structural elements in a warship, yet automatic modelers tend to resolve a ship into elements with high aspect ratios, which means they are also long and narrow. This kind of element tends to be very difficult for the analysis module to obtain a converged solution. To overcome these issues, we assembled a team of people with years of experience modeling Navy ships. The Navy, Anteon, and Sherbrooke worked with detailed drawings and simplified the geometry of the German ship by including only those elements with a significant impact on the magnetic field such as the hull and decks and other major structural details. These major features can be more easily meshed with elements whose aspect ratio is less extreme. The aspect ratio of an element is its height divided by its width. The biggest technical problem was determining the permeability and conductivity of each element used to model the ship, which is a necessary prerequisite of the simulation.

Figure 1 (click to zoom)

The hull, deck, internal bulkheads, forward superstructure, and many of the internal elements were made of permeable steel. The remaining conducting elements, including the rear superstructure and some of the decks, were made of aluminum. One of the big uncertainties was the electrical conductivity of the actual material, so a range of possible values were run. A constant uniform vertical field of 23,000 nT was imposed on the model to determine its magnetic signature. The field was generated by a large diameter, long solenoid to produce a homogenous field of the correct magnitude over the volume of the ship.

The thickness of the hull is known, making it easy to estimate conductivity at this point. But in many other areas where many different pieces are coming together, it’s much more complex. For example, it may be difficult to determine from a drawing whether a joint is welded and providing a closed path or whether there is a small gap that would prevent current from flowing. Another concern is that there are numerous gratings in many areas of the ship that needed to be simplified in order to reduce the complexity of the model. Instead of modeling each little hole, we typically adjust the material properties, such as by defining a grating as 10 percent metal.

Edge variable method reduces solution time
The model needed to be analyzed multiple times in order to account for the rolling motion of the ship at a period of 3 to 12 seconds for a complete roll. When originally solved, the model results correlated very well with physical testing as shown in Figures 2 and 3. But using the traditional nodal finite element formulation within ELEKTRA, the model took 19 days to converge to a solution on a high-end personal computer. A finite element model consists of a system of many equations that describe the physical system. The system of equations is solved by substituting approximate values for the variables in each equation, solving the system, determining the errors, and then repeating the process by substituting new values for the variables. With this method, scalar and vector electromagnetic potentials are calculated at the nodes of the elements by the analysis module. This approach works very well for static fields but tends to be relatively slow for time varying fields such as those produced by a rolling ship.

Figure 2 (click to zoom)

Figure 3 (click to zoom)

Fortunately, while this work was underway, Vector Fields, the company that developed ELEKTRA, upgraded the software by providing the alternative of using the edge-variable finite element method, which they had previously used in their high-frequency electromagnetic analysis module called Soprano. In an edge-variable finite element, the potential or field is no longer solved at the nodes of the element but on its edges. The nodes are points that define the element, typically by serving as its corners. This makes a formulation based entirely in vector potential viable, as only one value per edge is required and the vector direction is implied by the edge. Simply switching to the edge-variable method reduced the solution time to only one day, making it possible to dramatically increase the speed with which results were obtained and analyzed.

The success of this simulation project should help the Navy save time and money in the future by using simulation to replace some or all of the physical testing normally required to determine the electromagnetic signature of a new warship. The edge variable method played a key role by reducing solution time for the large models required in this application.

Dr. Pillsbury has more than 30 years of experience developing advanced computational algorithms and programs for the design and analysis of conventional and superconducting magnet systems. He has been a consultant in electromagnetics and the design and analysis of magnet systems for applications in fusion, Maglev, and plasma technologies. He has more than 50 technical reports and publications in journals and conference proceedings.

1700 N. Farnsworth Avenue
Aurora, IL 60505
Tel: 630-851-1734
Website: www.vectorfields.com

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