384 research outputs found
Influence of lamination orientation and stacking on magnetic characteristics of grain-oriented silicon steel laminations
Analytical and experimental investigations have been carried out upon the behaviour of flux in laminations, where the rolling directions of adjacent sheets are reversed. The paper clarifies the mechanism of the greatly different magnetic characteristics between such laminations and usual ones, where the rolling directions of adjacent sheets are coincident.</p
Investigation of effectiveness of various methods with different unknown variables for 3-D eddy current analysis
Computer codes using the A-φ, A-φ-Ω, A*-0Ω-E, T-Ω, and E-Ω methods were developed. The effects of the volume ratio of the conductor region to the whole region, the shape of the conductor, and the ratio of the hole region to the conductor region on the computer storage, the CPU time, and the accuracy of the methods are investigated systematically using a few simple models. The effect of the conductivity of the conductor is also examined. The computer storage, the CPU time, and the error are found to increase with increase of the volume ratio of the conductor region to the whole region. The computer storage and the CPU time are affected by the shape of the conductor in some methods of analysis. The error of the A*-Ω(E-Ω) method is larger than that of the other methods</p
New technique for producing a strong multi-pole magnet
A new technique for producing strong multipole magnet is developed. A cylindrical magnet oriented with its easy axis of magnetization perpendicular to the cylinder axis is magnetized by a multipole magnetizer. This procedure results in a multipole magnet with a flux density almost sixty percent greater than the flux density produced by a multi-pole magnet which is not oriented. The technique is especially effective for producing small cylindrical magnets with many poles and agreement of a theoretical analysis with experimental results is very good, with deviations of no more than a few percent.</p
The Effect of the Hall Term on the Nonlinear Evolution of the Magnetorotational Instability: II. Saturation Level and Critical Magnetic Reynolds Number
The nonlinear evolution of the magnetorotational instability (MRI) in weakly
ionized accretion disks, including the effect of the Hall term and ohmic
dissipation, is investigated using local three-dimensional MHD simulations and
various initial magnetic field geometries. When the magnetic Reynolds number,
Re_M \equiv v_A^2 / \eta \Omega (where v_A is the Alfven speed, \eta the
magnetic diffusivity, and \Omega the angular frequency), is initially larger
than a critical value Re_{M, crit}, the MRI evolves into MHD turbulence in
which angular momentum is transported efficiently by the Maxwell stress. If
Re_M < Re_{M, crit}, however, ohmic dissipation suppresses the MRI, and the
stress is reduced by several orders of magnitude. The critical value is in the
range of 1 - 30 depending on the initial field configuration. The Hall effect
does not modify the critical magnetic Reynolds number by much, but enhances the
saturation level of the Maxwell stress by a factor of a few. We show that the
saturation level of the MRI is characterized by v_{Az}^2 / \eta \Omega, where
v_{Az} is the Alfven speed in the nonlinear regime along the vertical component
of the field. The condition for turbulence and significant transport is given
by v_{Az}^2 / \eta \Omega \gtrsim 1, and this critical value is independent of
the strength and geometry of the magnetic field or the size of the Hall term.
If the magnetic field strength in an accretion disk can be estimated
observationally, and the magnetic Reynolds number v_A^2 / \eta \Omega is larger
than about 30, this would imply the MRI is operating in the disk.Comment: 43 pages, 8 tables, 20 figures, accepted for publication in ApJ,
postscript version also available from
http://www.astro.umd.edu/~sano/publications
Axisymmetric Magnetorotational Instability in Viscous Accretion Disks
Axisymmetric magnetorotational instability (MRI) in viscous accretion disks
is investigated by linear analysis and two-dimensional nonlinear simulations.
The linear growth of the viscous MRI is characterized by the Reynolds number
defined as , where is the Alfv{\'e}n
velocity, is the kinematic viscosity, and is the angular
velocity of the disk. Although the linear growth rate is suppressed
considerably as the Reynolds number decreases, the nonlinear behavior is found
to be almost independent of . At the nonlinear evolutionary stage,
a two-channel flow continues growing and the Maxwell stress increases until the
end of calculations even though the Reynolds number is much smaller than unity.
A large portion of the injected energy to the system is converted to the
magnetic energy. The gain rate of the thermal energy, on the other hand, is
found to be much larger than the viscous heating rate. Nonlinear behavior of
the MRI in the viscous regime and its difference from that in the highly
resistive regime can be explained schematically by using the characteristics of
the linear dispersion relation. Applying our results to the case with both the
viscosity and resistivity, it is anticipated that the critical value of the
Lundquist number for active turbulence
depends on the magnetic Prandtl number in
the regime of and remains constant when , where and is the magnetic diffusivity.Comment: Accepted for publication in ApJ -- 18 pages, 9 figures, 1 tabl
Direct finite element analysis of flux and current distributions under specified conditions
When the flux distribution of a magnetic circuit is analyzed by using the conventional finite element method, the magnetizing currents must be given. Therefore, if the flux distribution is specified, it is difficult to obtain the distributions of magnetomotive forces or configuration of magnets producing the specified field distribution by the conventional finite element method. New methods which are called the "finite element method taking account of external power source" and the "finite element method with shape modification" have been developed. The processes of calculation in these methods are contrary to the conventional technique. These new methods have the following advantages: (a) If there are many unknown independent magnetizing currents, these currents are directly calculated by the new method. (b) When a flux distribution is specified, the optimum shapes of the magnets can be directly calculated. (c) As these new methods need no repetition, computing time can be considerably reduced. The principles and the finite element formulations of these new methods are described, and a few examples of application of these methods are shown. These new methods make it possible to design the optimum magnetic circuits by using the finite element method.</p
New design method of permanent magnets by using the finite element method
A new method for determining the shapes and sizes of magnets which produce the prescribed flux densities by using the finite element method has been developed. In this paper, the new technique is explained briefly, and then the finite element formulation for non-linear analysis is derived. Finally, the usefulness of the technique is shown by applying this method to the design of magnetic circuits.</p
Application of the finite element method to the design of permanent magnets
A new method of determining the lengths of magnets in a magnetic circuit by using the finite element method has been developed. This method has the advantage that the lengths of magnets which produce the prescribed flux distribution can be directly calculated. In this paper, the error of this method is discussed at first, and then an example of application determining the shape of a magnet is shown. This method is effective for the design of magnetic circuits consisting of several permanent magnets and the determination of the shapes of magnets.</p
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
Finite element analysis of magnetic fields taking into account hysteresis characteristics
A new technique for taking into account hysteresis has been developed. The paper describes the details of the method and the usefulness of the technique is clarified by applying it to the analysis of magnetic fields in single-phase transformer cores. The calculated results are in good agreement with the measured ones.</p
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