• The Korean Journal of Ceramics
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• 제5권4호
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• pp.341-347
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• 1999

An ambiguity on the correct room temperature structure of $Bi_5Ti_3FeO_{15}$ was resolved using a combined Rietveld refinement of neutron and X-ray diffraction. The structure of this compound has been reported to have a space group of F2mm (adopting 2-fold rotation symmetry along the c-axis) or A21am. However, our diffraction, study reveals that some reflections would violate F-centering and confirm that the belong to $A2_1$am. Out refinement with the space group of $A2_1$am converged at $R_p=6.85%, R_wp=9.23%$ and $\chi^2$=1.66 for an isotropic temperature model with 85 variables. The lattice constants are a=5.4677(1) $\AA$, b=5.4396(1) $\AA$, and c=41.2475(8)$\AA$. In structure, Ti/Fe atoms at the oxygen octahedral sites of the perovskite unit are completely disordered, resulting in that these atoms are transparent in neutron diffraction. The octahedra of the perovskite unit are relatively displaced along the a-axis against the Bi atoms, which contribute as a major component to the spontaneous polarization of $Bi_5Ti_3FeO_{15}$.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.247-254
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• 2000

The base pressure of vacuum vessel of the KSTAR (Korea Superconducting Tokamak Advanced Research) Tokamak is to be a ultra high vacuum, $10^{-6}{\sim}10^{-7}Pa$, to produce clean plasma with low impurity containments. For this purpose, the KSTAR vacuum vessel and plasma facing components need to be baked up to at least $250^{\circ}C,\;350^{\circ}C$ respectively, within 24 hours by hot nitrogen gas from a separate baking/cooling line system to remove impurities from the plasma-material interaction surfaces before plasma operation. Here by applying the implicit numerical method to the heat balance equations of the system, overall temperature distributions of the KSTAR vacuum vessel and plasma facing components are obtained during the whole baking process. The model for 2-dimensional baking analysis are segmented into 9 imaginary sectors corresponding to each plasma facing component and has up-down symmetry. Under the resulting combined loads including dead weight, baking gas pressure, vacuum pressure and thermal loads, thermal stresses in the vacuum vessel during bakeout are calculated by using the ANSYS code. It is found that the vacuum vessel and its supports are structurally rigid based on the thermal stress analyses.

• 대한토목학회논문집
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• 제12권2호
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• pp.11-20
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• 1992

지진이나 바람과 같은 횡방향 하중이 가해졌을 때, 일반적으로 수직한 촉에 대해서만 대칭인 단면을 갖는 교량의 주형에는 횡방향 휨에 결합된 비틂이 유발되어 특히 사장교의 케이블등에는 예상치 못했던 추가응력이 유발될 수 있다. 이러한 거동은 일반적인 뼈대요소로는 해석할 수 없으므로, 임의의 단면 형상을 갖는 기하학적 비선형 3차원 뼈대요소를 사용하여야 한다. 본 연구에서 사용한 뼈대요소의 이론적인 배경과 검증은 이전에 발표된 논문에 수록되어 있다. 본 논문에서는 주형단면의 비대칭성을 고려한 지진해석을 수행하여 휨-비틂 결합작용에 의한 거동을 연구한다.

• 대한기계학회논문집B
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• 제22권12호
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• pp.1735-1745
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• 1998

Oil-film surface flow visualizations and three-dimensional flow measurements using a straight five-hole probe have been conducted for a circular impinging jet which is normally oriented to the crossflow in a channel. Throughout the experiments, the ratio of channel height to injection hole diameter, H/D, is fixed to be 1.0, and blowing ratio is varied to be 1.0, 2.0, 3.0 and 4.0. From the surface flow visualizations for both top wall(target plate) and bottom wall, impinging jet region on the target plate can be clearly identified, and for the small value of H/D = 1.0, presence of the bottom wall changes the near-hole flow structure, significantly. The three-dimensional flow measurements show that in the dawnstream region of the injection hole, there exist a pair of counter-rotating vortices, called "scarf vortices", and the strength of the vortices strongly depends on the blowing ratio. In addition, a new flow model in the flow symmetry plane has been proposed for H/D = 1.0.

• 한국레이저가공학회지
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• 제8권1호
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• pp.19-25
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• 2005

Pulse laser welding of AISI 304 stainless steel plate was simulated to optimize welding conditions by using commercial finite element code MARC. Due to geometric symmetry, a half model of AISI 304 stainless steel plate was considered. for the heat transfer analysis, user subroutines were applied to boundary condition. The material properties such as conductivity, specific heat, and mass density were given as a function of temperature and the latent heat associated with a given temperature range was considered. A moving heat source was designed on the basis of experimental data. As a result, Nd:YAG laser welding for AISI 304 stainless steel was successfully simulated and it should be useful to determine optimal welding condition.

• Journal of Magnetics
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• 제18권2호
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• pp.130-134
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• 2013

Most electrical machines like motor, generator and transformer are symmetric in terms of magnetic field distribution and mechanical structure. In order to analyze these problems effectively, many coupling techniques have been introduced. This paper deals with a coupling scheme for open boundary problem of symmetric and periodic structure. It couples an analytical solution of Fourier series expansion with the standard finite element method. The analytical solution is derived for the magnetic field in the outside of the boundary, and the finite element method is for the magnetic field in the inside with source current and magnetic materials. The main advantage of the proposed method is that it retains sparsity and symmetry of system matrix like the standard FEM and it can also be easily applied to symmetric and periodic problems. Also, unknowns of finite elements at the boundary are coupled with Fourier series coefficients. The boundary conditions are used to derive a coupled system equation expressed in matrix form. The proposed algorithm is validated using a test model of a bush bar for the power supply. And the each result is compared with analytical solution respectively.

• 혜화의학회지
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• 제19권1호
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• pp.1-7
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• 2010

Melasma is a common disorder that causes dark colored patches. It generally causes brown spots on the face, especially on the forehead, cheeks, and upper lips. The pattern of patches has bilateral symmetry in yellowish-brown to gray-brown colors. It is much more common in women than in men. Melasma is considered to be caused by environmental and physical constitutional factors and often occurs when a woman's hormone changes by pregnancy or the use of oral contraceptive pills. It is important to combine oriental medicine and Sasang constitutional medicine for treatment of Melasma. Normal treatment of Melasma includes warming oneself, removal of the emotional stress factors, and good nutrition.

• 대한토목학회논문집
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• 제11권2호
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• pp.15-26
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• 1991

지진이나 바람과 같은 횡방향 하중이 가해졌을 때, 일반적으로 수직한 축에 대해서만 대칭인 단명을 갖는 교량의 주형은 횡방향 휨과 비틀림이 결합된 거동을 하게되어 특히 사장교의 케이블등에는 예상치 못했던 추가응력이 유발될 수 있다. 이러한 거동은 일반적인 뼈대요소로는 해석할 수 없으므로, 본 연구에서는 가상일의 원리와 운동에너지로 부터 임의의 단면형상을 갖는 기하학적 비선형 3차원 뼈대요소의 강도매트릭스와 질량매트릭스를 유도하여 주형을 모델링하고, 케이블요소는 Ernst가 제안한 등가탄성계수를 사용한다. 그리고 해석예를 통하여 이론의 타당성을 검증한 후, 3차원 사장교 모델의 고유진동해석을 수행하여 주형의 휨-비틀림 결합작용을 연구한다.

• Smart Structures and Systems
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• 제12권5호
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• pp.523-545
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• 2013

Piezoelectric and dielectric behaviors of a piezoceramic patch adhesively centered on a carbon composite plate are identified using a robust multi-objective optimization procedure. For this purpose, the patch piezoelectric stress coupling and blocked dielectric constants are automatically evaluated for a wide frequency range and for the different identifiable behaviors. Latters' symmetry conditions are coded in the design plans serving for response surface methodology-based sensitivity analysis and meta-modeling. The identified constants result from the measured and computed open-circuit frequencies deviations minimization by a genetic algorithm that uses meta-model estimated frequencies. Present investigations show that the bonded piezoceramic patch has effective three-dimensional (3D) orthotropic piezoelectric and dielectric behaviors. Besides, the sensitivity analysis indicates that four constants, from eight, dominate the 3D orthotropic behavior, and that the analyses can be reduced to the electromechanically coupled modes only; therefore, in this case, and if only the dominated parameters are optimized while the others keep their nominal values, the resulting piezoelectric and dielectric behaviors are found to be transverse-isotropic. These results can help designing piezoceramics smart composites for various applications like noise, vibration, shape, and health control.

• 한국철도학회논문집
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• 제7권3호
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• pp.193-199
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• 2004

Numerical analysis of aerodynamic characteristics was differently performed according to the running situation of the Korean Tilting Train eXpress(TTX) that would be introduced for an improvement in efficiency of the used railroad track. Fluent 6.0 was used for the analysis of Non-tilting case, Tilting case and Passing-by case with the model of TTX. As a result, the aerodynamic drag had little difference between Tilting and Non-tilting case. However, pressure contour under the train of Tilting case was not symmetry because the gap between a train and the ground was different at both sides. In Passing-by case attraction and counterattraction occurred alternately and affected to the opposite train. When two trains were side by side, the maximum attraction was generated especially. Through an analysis of pressure wave in tunnel a large variation of pressure was generated by the bluff nose of TTX. The results in this study would be good data for the aerodynamic characteristic on TTX and provide important information to judgment of running safety.