Hyun-Ju Chung

Implementation of Material Sensitivity Analysis for Determining Unknown Remanent Magnetization of a Ferromagnetic Thin Shell

This paper deals with the numerical implementation of the material sensitivity analysis, which is used to efficiently determine an optimal magnetization distribution on a ship hull. A material sensitivity formula for the forward problem formulated in terms of the equivalent magnetic charge method is analytically derived. Then, the components of the adjoint system including the magnetic pseudo-source are thoroughly investigated in order to obtain the first-order gradient information of an objective function with respect to the magnetic charge. The formula facilitates applying the deterministic approach to searching for an optimal charge distribution and also it yields stable and reliable solutions to magnetostatic inverse problems in three dimensions. Finally, the proposed method has been successfully applied to a scale model ship and the predicted results on the underwater field disturbance due to the remanent magnetization of the hull have been compared to real measurements.

Comparison of Three Modeling Methods for Identifying Unknown Magnetization of Ferromagnetic Thin Plate

This study presents three different magnetization models for identifying unknown magnetization of the ferromagnetic thin plate of a ship. First, the forward problem should be solved to accurately predict outboard magnetic fields due to the magnetization distribution estimated at a certain time. To achieve this, three different modeling methods for representing remanent magnetization (i.e., magnetic charge method, magnetic dipole array method, and magnetic moment method) were utilized. Material sensitivity formulas containing the first-order gradient information of an objective function were then adopted for an efficient search of an optimum magnetization distribution on the hull. The validity of the proposed methods was tested with a scale model ship, and field signals predicted from the three different models were thoroughly investigated with reference to the experimental data.

A Study on the Deperm Protocols Considering Demagnetizing Field of a Ferromagnetic Material

(Received 23 September 2013, Received in final form 7 March 2014, Accepted 14 March 2014)Magnetic materials with large coercive force and high squareness ratio are currently developing to meet anindustrial demand. Since a ferromagnetic material has hysteresis characteristics, it is hard to demagnetize aferromagnetic material precisely. In this paper, we describe deperm processes and conduct an analysis ofresidual magnetization of ferromagnetic material using the Preisach modeling with a two-dimensional finiteelements method (FEM). From the results, it was shown that an exponential decrement form of depermprotocol is more efficient than a linear decrement form because of the demagnetizing field in the ferromagneticmaterial. Keywords : deperm, ferromagnetic material, preisach model, Anhysteretic Deperm, Deperm-ME, demagnetizationeffect, FEM

Efficient Methodology for Optimizing Degaussing Coil Currents in Ships Utilizing Magnetomotive Force Sensitivity Information

This paper presents an efficient methodology for optimizing degaussing coil currents in ships to minimize the anomaly of underwater magnetic fields due to the hull magnetization induced under the Earths magnetic field. To achieve this, first, the shielding effect of the hull on the underwater fields is thoroughly examined. Then, for a fast search of optimum degaussing currents, a sensitivity formula of an objective function with respect to the magnetomotive force is adopted. The feature of the proposed method is that it does not require any numerical field analyses to assess an objective function during optimization process providing experimental field data for each coil are given. The validity and effectiveness of the method has been tested with a model ship.

Accurate prediction of unknown corrosion currents distributed on the hull of a naval ship utilizing material sensitivity analysis

This paper presents an efficient approach to determining the corrosion currents distributed on the hull of a naval ship, which offers useful information for the prediction of underwater electric fields and diagnosis of hull corrosion state. To achieve this goal, a material sensitivity formula containing the first-order gradient information of an objective function with respect to the corrosion current is analytically derived by exploiting the augmented Lagrangian method and the adjoint variable method. The validity of the proposed method has been proved through the comparison of the predicted and the measured fields for a test ship model.

Magnetic Dipole Modeling Combined With Material Sensitivity Analysis for Solving an Inverse Problem of Thin Ferromagnetic Sheet

This paper presents an efficient methodology for determining the underwater field anomaly due to the remanent magnetization of a ferromagnetic ship hull by utilizing a magnetic dipole modeling technique combined with material sensitivity analysis. The complicated 3D structure of the hull is replaced with an equivalent magnetic dipole array placed in a 2D plane, of which the optimal dipole moment values will be easily sought out with the aid of material sensitivity analysis. To achieve this, a material sensitivity formula, which contains the first-order gradient information of an objective function with respect to the magnetic dipoles, is analytically derived by exploiting the augmented objective function and adjoint variable method. The proposed method leads to easy numerical implementation and also dramatically reducing system unknowns of the 3D inverse problem considered. Finally, the validity of the method has been tested with real measurements of a scale model ship as well as numerical results of our previous work, which adopted the magnetic charge method in conjunction with material sensitivity analysis.

Optimization of Degaussing Coil Currents for Magnetic Silencing of a Ship Taking the Ferromagnetic Hull Effect Into Account

This paper presents an efficient methodology for optimizing degaussing coil currents in a ship with ferromagnetic hull to minimize magnetic field anomaly underwater. For fast search for an optimum, a magnetomotive force sensitivity formula is adopted. The feature of the method does not require any numerical field analyses to assess an objective function during optimization process providing experimental field data on each coil with respect to the reference magnetomotive force are given. Especially, the shielding effect of the hull on degaussing fields is thoroughly investigated. The validity of the proposed method has been tested with a model ship.

Indirect Fault Detection Method for an Onboard Degaussing Coil System Exploiting Underwater Magnetic Signals

This paper proposes an indirect fault detection method for an onboard degaussing coil system, installed to reduce the underwater magnetic field from the ferromagnetic hull. The method utilizes underwater field signals measured at specific magnetic treatment facilities instead of using time-consuming numerical field solutions in a three-dimensional space. An equivalent magnetic charge model combined with a material sensitivity formula is adopted to predict fault coil locations. The purpose of the proposed method is to yield reliable data on the location and type of a coil breakdown even without information on individual degaussing coils, such as dimension, location and number of turns. Under several fault conditions, the method is tested with a model ship equipped with 20 degaussing coils.

Prediction for Underwater Static Magnetic Field Signature Generated by Hull and Internal Structure for Ferromagnetic Ship

Underwater static magnetic field signature for the naval ship has been widely used as the detonating source of the influence mine system because it is possible to make an accurate target detection in the near field although the magnetic field falls off relatively fast with distance in comparison with the underwater radiated noise signal. In this paper, we describe the prediction results about the underwater static magnetic field by the ferromagnetic hull, the internal structures and the main on-board equipment for the target vessel using the commercial FEM software. Also we analyze the degaussing effectiveness for the target vessel through the degaussing coils arrangement.

A Study on Dipole Modeling Method for Ship`s Magnetic Anomaly using Singular Value Decomposition Technique

This paper describes the mathematical modeling method for the static magnetic field signature generated by a magnetic scale model. we proposed the equivalent dipole modeling method utilizing a singular value decomposition technique from magnetic field signatures by magnetic sensors are located special depths below the scale model. The proposed dipole modeling method was successfully verified through comparisons with the real measured values in our non-magnetic laboratory. Using the proposed method, it is possible to predict and analyze static magnetic field distributions at any difference depths generated from the real ships as well as a scale model ship.