• 천문학회지
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• 제28권2호
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• pp.223-243
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• 1995

We describe the implementation of a multi-dimensional numerical code to solve the equations for idea! magnetohydrodynamics (MHD) in cylindrical geometry. It is based on an explicit finite difference scheme on an Eulerian grid, called the Total Variation Diminishing (TVD) scheme, which is a second-order-accurate extension of the Roe-type upwind scheme. Multiple spatial dimensions are treated through a Strang-type operator splitting. Curvature and source terms are included in a way to insure the formal accuracy of the code to be second order. The constraint of a divergence-free magnetic field is enforced exactly by adding a correction, which involves solving a Poisson equation. The Fourier Analysis and Cyclic Reduction (FACR) method is employed to solve it. Results from a set of tests show that the code handles flows in cylindrical geometry successfully and resolves strong shocks within two to four computational cells. The advantages and limitations of the code are discussed.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2003년도 하계학술대회 논문집 B
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• pp.804-806
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• 2003

In these days the Vacuum Circuit Breaker(VCB) is used in most medium voltage level because VCB has merits of simple structure, long life, free maintenance and environment friendly characteristics. Most of VCBs adopt mechanical spring drive mechanism to operate vacuum interrupter, but this mechanism is composed of many components and needs frequent maintenance works. In this paper, we study about the VCB drive mechanism with Permanent Magnet Actuator (PMA). Design methods and design flows about PMA are presented. The magnetic equivalent circuit is used for elementary and detailed design to determine the size of PMA. Finite Element Method (FEM) analysis is performed to evaluate the behavior characteristics of PMA in both static and transient state. Finally we manufacture sample PMA and verify FEM analysis through experiments.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1995년도 하계학술대회 논문집 A
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• pp.92-94
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• 1995

This paper is to describe a method for calculating resistance of cage rotor end-ring, based on 3-D finite element method using magnetic vector potential $\vec{A}$ and electric scalar potential ${\phi}$. The induced current of a cage rotor flows through the bars of a cage rotor. The current completes their closed paths by passing around the end-ring. The end-ring may contribute a significant influence to the performance of machine. The resistance under consideration of skin effect is calculate by using Joule's loss equation.

• 천문학회보
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• 제46권1호
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• pp.51.1-51.1
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• 2021

Spiral arms greatly affect gas flows and star formation in disk galaxies. We use local 3D simulations of vertically-stratified, self-gravitating, gaseous disks under a stellar spiral potential to study the effects of spiral arms on galactic star formation as well as formation of gaseous spurs/feathers. We adopt the TIGRESS framework to handle radiative heating and cooling, star formation, and ensuing supernova (SN) feedback. We find that more than 90% of star formation takes place inside spiral arms. The global star formation rate (SFR) in models with spiral arms is enhanced by less than a factor of 2 compared to the no-arm counterpart. This supports the picture that spiral arms do not trigger star formation but rather redistribute star-forming regions. Correlated SN feedback produces interarm feathers in both magnetized and unmagnetized models. These feathers live short, have parallel magnetic fields along their length, and are bounded by SN feedback in the lateral direction, in contrast to instability-induced feathers formed in our previous isothermal simulations.

• 천문학회보
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• 제46권2호
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• pp.64.1-64.1
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• 2021

Accretion flows around black holes and neutron stars emit high energy radiation with varying spectral and timing properties. Observed timing variations, both short and long-term, point to the existence of a mechanism, dictated by the flow dynamics, and not by the stellar surface or magnetic fields, that is common in both. Spectral energy distributions of multiple sources indicate that the Comptonization process, the dominant mechanism for changing states in X-ray, takes place inside the flow that has similar physical properties in both the objects. In a series of observational and numerical studies, we enquire about the following: 1. Is there a steady state configuration for accreting matter around black holes that can explain spectral and timing properties? 2. Could a similar formalism explain spectral and timing properties of accretion around neutron stars? 3. Could there be a generalized flow configuration for accreting matter around such compact objects? Furthermore, we show that a unified spectral model can be constructed based on the generalized flow configuration, common to black holes and neutron stars.

• 한국초전도ㆍ저온공학회논문지
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• 제16권2호
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• pp.42-46
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• 2014

In this study we investigated ac transport current losses in the face to face stack for the anti-parallel current flow, and compared the electromagnetic properties with those of the single SC tape as well as those of the same stack for the parallel current path. The gap between the SC tapes in the stack varied in order to verify the electromagnetic influence of the neighbors when current flows in opposite direction, and the model was implemented in the finite element method program by the commercial software, COMSOL Multiphysics 4.2a. Conclusively speaking, the loss was remarkably decreased for the anti-parallel current case, which is attributed the magnetic flux compensation between the SC layers due to the opposite direction of the current flows. As the gap between SC tapes was increased, the loss mitigation became less effective. Besides, the current density distribution is very flat cross the sample width for the narrower gap case, which is believed to be benefit for the power electric system. These results are all in good agreement with those predicted theoretically for an infinite bifilar stack.

• 대한조선학회논문집
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• 제32권1호
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• pp.83-93
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• 1995

사각형의 MHD 추진 덕트 내부유동을 수치해석과 실험적인 방법으로 연구하였다. 수치해석연구에서는 전기장과 자기장의 영향하에 있는 비압축성 3차원 통전유체에 대하여 유한 차분법으로 계산하였다. 수치계산의 결과 전자기력이 약할때 층류유동의 전형적인 포물선 유동 형태가 전극 부근에서 M자 형상으로 변하였고, 균일분포의 일정 전자기력하에서 MHD 덕트 내부 압력은 입구에서부터 하류로 나아감에 따라 선형적으로 증가하였다. 실험에서는 MHD 추진 덕트 내부 유동을 해석할 수 있는 실험장치를 제작 하였으며, MHD 추진은 전류를 변화 시킴으로써 추력을 쉽게 조정할 수 있음을 알았다. 또한 MHD 덕트내 유동방향의 압력구배는 전자기력에 비례하여 증가하며 수치계산 결과와 잘 일치하였다.

• 한국자기학회지
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• 제3권3호
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• pp.229-233
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• 1993

광자기 디스크의 제조시 기록막내부의 산소함량에 따른 기록 및 자기적 특성의 변화를 연구하였다. 기록막내의 산소함량 변화는 반응성 스퍼터링에 의해 조절하였으며 스퍼터링 전과정 동안 산소의 거동을 in-situ 잔류가스분석기로 조사하였다. 산소함량의 증가에 따라 보자력은 감 소하였으나 수직이방성 및 수직 각형비등은 변화하지 않았다. 대부분의 첨가된 산소는 스퍼터링 과정중 소모되어 Tb원자와 안정한 산화물을 형성하여 자기적 조성의 변화를 초래하였지만 임계기 록크기와 임계외부자계크기에만 약간의 변화를 가져왔을 뿐 디스크의 기록신호잡음비에는 별로 영향을 미치지 않았다. 또한 디스크의 신뢰도면에서도 크게 감소하지 않았다.

• 대한기계학회논문집A
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• 제21권5호
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• pp.819-827
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• 1997

In this paper, a new modeling of a fine actuator for an optical pick-up has been proposed and multiobjective optimization of the actuator has been performed. The fine actuator is constituted of the bobbin which is supported by wire suspension, the coils which wind around the bobbin, and the magnets which cause the magnetic flux. If current flows in the coils, magnetic force is so produced as to be balanced with spring force of wire, so the bobbin is pisitioned. In this model the transfer function from input voltage to output displacementof bobbin has been obtained so that we can describe this integrated system with electromagnetic and mechanical parts. Wire suspension is regarded as a continuous Euler beam, damper as distributed viscous damping, and bobbin as a rigid body which can move up- and down- ward motion only. According to the model, the high frequency dynamic characteristics of the fine actuator can be known and the effect of damping can be investigated while the conventional second order model cannot. In multiobjective optimization, two objective functions have been chosen to maximize the fundamental frequency and the sensitivity with respect to the input voltage of the actuator so that Pareto's optimal solutions have been obtained using .epsilon.-constraint method. These objective functions will satisfy the trends which will enhance the access speed and reduce the tracking error in the optical pick-up technology of next generation. In the result of optimization, we obtain the designs of the optical pick-up fine actuator which has high speed, high sensitivity and low resonant peak. Furthermore, we offer the relation between two object functions so that the designer can make easy choice.

• Journal of Magnetics
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• 제18권4호
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• pp.481-486
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• 2013

An electromagnetic launcher (EML) system accelerates and launches a projectile by converting electric energy into kinetic energy. There are two types of EML systems under development: the rail gun and the coil gun. A railgun comprises a pair of parallel conducting rails, along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail, but the high mechanical friction between the projectile and the rail can damage the projectile. A coil gun launches the projectile by the attractive magnetic force of the electromagnetic coil. A higher projectile muzzle velocity needs multiple stages of electromagnetic coils, which makes the coil gun EML system longer. As a result, the installation cost of a coil gun EML system is very high due to the large installation site needed for the EML. We present a coil gun EML system that has a new structure and arrangement for multiple electromagnetic coils to reduce the length of the system. A mathematical model of the proposed coil gun EML system is developed in order to calculate the magnetic field and forces, and to simulate the muzzle velocity of a projectile by driving and switching the electric current into multiple stages of electromagnetic coils. Using the proposed design, the length of the coil gun EML system is shortened by 31% compared with a conventional coil gun system while satisfying a target projectile muzzle velocity of over 100 m/s.