Advanced Algorithms and Tools for Permanent Magnetization Prediction

ABSTRACT Reduction of the underwater electromagnetic signature of a submarine is critical to its survivability. A steel–hulled submarine has a permanent magnetization dependent on the surrounding magnetic field, and, as the vessel moves through the water, it generates a magnetic signature possibly detectable by a hostile ordnance or device. Degaussing systems comprising current-carrying coils are one of the primary methods to reduce a ship’s magnetic signature by partial cancellation. Since the Earth’s magnetic field changes relative to the ship’s axis and position, degaussing coil currents must be controlled to compensate for these predictable changes. As ferromagnetic steel is a hysteretic, magnetostrictive magnetic material, changes in the magnetic field and dynamic stresses alter the vessel’s magnetic properties. Another physical mechanism that may affect a vessel’s magnetic signature is electric current flowing through the vessel’s hull and seawater due to corrosion-related phenomena. The objective of this research is to continue to refine and extend a suite of tools enabling the more accurate and more efficient prediction of the variations in induced and permanent magnetization of a vessel under dynamic changes in magnetic field and mechanical stresses. These capabilities will provide Naval engineers with an effective tool to estimate a submarine’s magnetic signature and in-turn improve the design of systems to reduce this field. Other systems requiring knowledge of the electromagnetic (EM) fields would also potentially benefit from these tools. Under prior support by the Office of Naval Research, the University of Kentucky’s Computational Electromagnetics Group (UK-CEMG) together with the University of Colorado- Denver’s Magnetic Material’s Library (UCD-MML) has developed a suite of tools for the prediction of the permanent and induced magnetization in non-linear, hysteretic magnetic- conducting materials, e.g., a steel vessel, under low-frequency time-varying applied magnetic fields and mechanical stress. The primary tool developed for magnetic signature prediction is called Magström, which is currently being used by Naval engineers at NSWCCD to aid in degaussing coil design. Magström (Magnetostatic-Nyström), an analysis tool based on a locally corrected Nyström discretization of the quasi-magnetostatic volume integral equation, can predict the magnetization (and, hence, the magnetic signature of) in non-linear, hysteretic magnetic materials with arbitrary geometries under time-varying applied magnetic fields and stresses. The same suite of tools can also predict the effects of time-varying eddy currents in magnetic-conducting materials. Various magnetostrictive, hysteretic magnetic material models have been incorporated, including the Cooperative model and the Jiles-Atherton model. Furthermore, stresses in a ship’s hull due to a specified loading can be computed using external tools and then utilized by Magström to predict the influence of stress on magnetization. For analysis of large-scale geometries, Magström interfaces to a physics-based fast-direct solver library called MFDLib also developed at the University of Kentucky. MFDLib provides a suite of data-sparse methods for use with general integral equation formulations. For efficient solution of large problems on distributed computing clusters, MFDlib is parallelized using the MPI (Message Passing Interface) standard. 1