Three-dimensional eddy current analysis by the boundary element method using vector potential

A boundary-element method using a magnetic vector potential for eddy-current analysis is described. For three-dimensional (3-D) problems, the tangential and normal components of the vector potential, tangential components of the magnetic flux density, and an electric scalar potential on conductor surfaces are chosen as unknown variables. When the approximation is introduced so that the conductivity of the conductor is very large in comparison with the conductivity of air, the number of unknowns can be reduced; also, for axisymmetric models the scalar potential can be eliminated from the unknown variables. The formulation of the boundary-element method using the vector potential, and computation results by the proposed method, are presented </p

Computation accuracies of boundary element method and finite element method in transient eddy current analysis

The computation accuracies of the boundary-element method (BEM) and finite-element method (FEM) in transient eddy-current problems are compared by using a slot-embedded conductor model and a diffusion model that can be solved theoretically. For computing the vector potential or magnetic flux density it is shown that larger time-step width can be chosen in the BEM than in the FEM method for the same accuracy </p

A Systematic Study of X-Ray Flares from Low-Mass Young Stellar Objects in the Rho Ophiuchi Star-Forming Region with Chandra

We report on the results of a systematic study of X-ray flares from low-mass young stellar objects, using Chandra observations of the main region of the Rho Oph. From 195 X-ray sources, including class I-III sources and some young brown dwarfs, we detected a total of 71 X-ray flares. Most of the flares have the typical profile of solar and stellar flares, fast rise and slow decay. We derived the time-averaged temperature (kT), luminosity (L_X), rise and decay timescales (tau_r and tau_d) of the flares, finding that (1) class I-II sources tend to have a high kT, (2) the distribution of L_X during flares is nearly the same for all classes, and (3) positive and negative log-linear correlations are found between tau_r and tau_d, and kT and tau_r. In order to explain these relations, we used the framework of magnetic reconnection model to formulate the observational parameters as a function of the half-length of the reconnected magnetic loop (L) and magnetic field strength (B). The estimated L is comparable to the typical stellar radius of these objects (10^{10-11} cm), which indicates that the observed flares are triggered by solar-type loops, rather than larger ones (10^{12} cm) connecting the star with its inner accretion disk. The higher kT observed for class I sources may be explained by a higher magnetic field strength (about 500 G) than for class II-III sources (200-300 G).Comment: 33 pages, 7 figures, accepted for publication in PASJ, the complete version of tables are available at ftp://ftp-cr.scphys.kyoto-u.ac.jp/pub/crmember/kensuke/PASJ_RhoOph/KI_all.tar .g

Computaton of 3-dimensional Eddy current problems by using boundary element method

The boundary element method for computing 3-dimensional eddy current distributions is presented. This method is based on Vector Green's Theorem, and unknown electric field vectors and magnetic flux density vectors are assumed on the boundaries of two materials, and unknown electric field vectors are assumed in the conductor regions. After determining these unknown vectors, 3-dimensional eddy current distributions in the conductors are computed. The computation results of a conducting sphere model by this method were examined in contrast to those of a coupled circuit model. </p

An analysis of 3-dimensional magnetic field distributions in a small-sized synchronous motor with a permanent magnet rotor

This paper describes an analysis of 3- dimensional magnetic field distributions in a small-sized synchronous motor with a permanent magnet rotor and a simulation of the rotor behavior. Here, the concept of a surface magnetic charge is introduced, and then the magnetic field distributions are computed by using the integral equation method. Next, the rotor displacement is computed by using Newmark's &#946;-parameter method. By the use of these techniques, simulation of the rotor behavior is performed. The results of the simulation are examined in contrast to those of the experiments. </p

The optimum design of electrode and insulator contours by nonlinear programming using the surface charge simulation method

A new method is presented for optimizing electrode and insulator contours. The contours are modified by using the iteration methods of nonlinear programming until the desired electric field distribution is obtained. The Gauss-Newton, quasi-Newton, conjugate gradient, or steepest descent method is used for the iteration. The electric-field distributions are computed by means of the surface charge simulation method. It is shown that the Gauss-Newton method gives very fast convergence </p

Techniques for boundary element analysis of three-dimensional eddy current distribution

An analysis of three-dimensional eddy-current distribution by a boundary-element method using field vector variables is described. A triangular element is used as a boundary element. The electric field vector and magnetic flux density vector are defined as the unknown vectors and are assumed to be constant on each triangular element. For forming simultaneous equations, the computation point on the triangular element is set at the null point, where the triangular element itself does not induce tangential components of the electric field and the magnetic flux density </p

GGD 27: X-rays from a Massive Protostar with an Outflow

We report the discovery of a cluster of Class I protostars in GGD 27. One of these protostars is the previously known, centrally located, GGD 27-ILL, which powers a massive bipolar outflow. We show that GGD 27-ILL, which is known to be the bright infrared (IR) source, IRAS 18162-2048, and a compact radio continuum source, is also the newly discovered hard X-ray source, GGD 27-X. The observations were made with the ACIS instrument on the Chandra X-ray Observatory. The X-rays from GGD 27-X are variable when compared with 4 years earlier, with an unabsorbed 2-10 keV X-ray luminosity in this observation of 1.5-12 × 10^31 erg s^–1 and a plasma temperature of ≥ 10^7 K. The X-rays are probably associated with the underlying B0 star (rather than outflowing material), providing a rare glimpse in hard X-rays of an optically obscured massive protostar with an outflow. The X-ray luminosity and spectrum appear to be consistent with stars of its type in other star formation regions. Several other variable X-ray sources are also detected in the IR cluster that contains GGD 27-X. We also discuss another nearby cluster. In each of the clusters there is an object that is X-ray hard, highly absorbed at low energies, in a blank optical/IR/radio field, and variable in X-ray intensity by a factor of ≥ 10 on a timescale of 4 years. These latter objects may arise from more recent episodes of star formation or may be "hidden" Class III sources

Eddy current and deflection analyses of a thin plate in time-changing magnetic field

Eddy current and deflection analysis of a thin-plate model in a time-changing magnetic field is described. The model is solved as a coupled problem in which the time-changing magnetic field induces eddy currents and the eddy currents cause deflection of the thin plate by the Lorentz force. The eddy current analysis and deflection analysis are performed by an integro-differential method using a current vector potential and a structural finite element method using beam elements, respectively. The formulations of the motional electromotive force and the Lorentz force for the thin-plate model are presented. In addition, the applicability of the proposed method is verified by using a cantilevered-beam model </p

Computation of three-dimensional electromagnetic field in the eddy-current testing of steel pipes

The computation of three-dimensional electromagnetic field distributions in the eddy-current testing for holes in steel pipes is described. A boundary element method with vector variables was used to compute the eddy-current and magnetic-flux-density distributions. Computed and experimental results for the magnetic flux density on the inner surface are compared, and the mechanism of defect detection in steel pipes is clarified </p