• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.8
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• pp.1-7
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• 2005

This paper proposes a local improvement method that combines extended topological clean up and optimization-based smoothing for homogenizing triangular grid system. First extended topological clean up procedures are applied to improve the connectivities of grid elements. Then, local optimization-based smoothing is performed for maximizing the distortion metric that measures grid quality. Using the local improvement strategy, we implement the grid homogenizations for two triangular grid examples. It is shown that the suggested algorithm improves the quality of the triangular grids to a great degree in an efficient manner and also can be easily applied to the remeshing algorithm in adaptive mesh refinement technique.

• Nuclear Engineering and Technology
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• v.16 no.4
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• pp.202-216
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• 1984

The success of coarse-mesh nodal solution methods provides strong motivation for finding homogenized parameters which, when used in global nodal calculation, will reproduce exactly all average nodal reaction rates for large nodes. Two approximate theories for finding these ideal parameters, namely, simplified equivalence theory and approximate node equivalence theory, are described herein and then applied to the PWR benchmark problem. Nodal code, ANM, is used for the global calculation as well as for the homogenization calculation. From the comparative analysis, it is recommended that homogenization be carried out only for the unique type of fuel assemblies and for core boundary color-sets. The use of approximate homogenized cross-sections and approximate discontinuity factors predicts nodal powers with maximum error of 0.8% and criticality within 0.1% error relative to the fine-mesh KIDD calculations.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.261-264
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• 2011

유한 요소 기법을 이용한 허니콤 구조물의 해석은 모델링 작업 및 격자 생성의 어려움뿐만 아니라 과도한 해석 시간이 요구되기 때문에 균질화 기법은 계산상의 효율성을 증대시킬 수 있는 매우 유용한 방법이라 할 수 있다. 그러나 나노 크기의 구조물에서는 표면 효과로 인하여 거시적인 구조물에서와는 매우 상이한 기계적 거동 양상을 띠게 되며 균질화 기법을 나노 크기의 허니콤 구조물에 적용하기 위해서는 이러한 표면 효과를 반영해야만 한다. 본 논문에서는 표면 효과를 고려한 유한 요소를 제안하고 이를 이용하여 나노 크기의 3차원 허니콤 구조물을 균질화 기법을 이용하여 등가의 2차원 판으로 대체하였다.

• Proceedings of the Korea Society for Energy Engineering kosee Conference
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• 1999.05a
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• pp.127-132
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• 1999

우리나라의 도시폐기물 소각로는 화격자 위에 폐기물을 공급하고 화격자 밑에서 공기를 공급하는 스토커식 소각로를 대부분 채택하고 있다. 이러한 스토커 소각로 연소실내에서는 매우 복잡한 연소현상이 발생하는데, 연소실로 투입된 쓰레기는 먼저 건조부에서 수분의 건조가 일어나고, 화격자의 구동에 의해 쓰레기가 혼합 및 이송되면서 열분해, 가스화, 가연성분의 탈휘발화 및 연소, 일부 고정탄소의 표면연소 등의 반응이 일어난다. 그리고 1,2차 연소실에서는 휘발분 및 비산된 고체의 연소가 일어나는데, 이때 대류 및 복사열전달 등의 복잡한 현상을 수반하는 유동장이 형성된다. 더욱이 불균질한 특성을 갖는 쓰레기층 내에서의 복잡한 현상으로 인하여 발생하는 경계조건 설정의 불확실성으로 연소실내의 연소 현상을 전산해석하는 데에는 상당한 어려움이 있다.(중략)

• Journal of the Korean Society for Advanced Composite Structures
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• v.2 no.3
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• pp.1-11
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• 2011

The typical truss-lattice material successively packed by repeated cubic symmetric unit cells consists of sub-elements (SE) proposed in this study. The representative continuum model for this truss-lattice material such as the effective strain and stress relationship can be formulated by the homogenization procedure based on the notation of averaged mechanical properties. The volume fractions of micro-scale struts have a significant influence on the effective strength as well as the relative density in the lattice plate with replicable unit cell structures. Most of the strength contribution in the lattice material is induced by axial stiffness under uniform stretching or compression responses. Therefore, continuum based constitutive models composed of homogenized member stiffness include these mechanical characteristics with respect to strength, internal stress state, material density based on the volume fraction and even failure modes. It can be also recognized that the stress state of micro-scale struts is directly associated with the continuum constitutive model. The plastic flow at the micro-scale stress can extend the envelope of the analytical stress function on the surface of macro-scale stress derived from homogenized constitutive equations. The main focus of this study is to investigate the basic topology of unit cell structures with the cubic symmetric system and to formulate the plastic models to predict pressure dependent macro-scale stress surface functions.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.14 no.4
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• pp.495-503
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• 2001

In a functionally graded materials(FGM), two constituent material particles are mixed up according to a specific volume fraction distribution so that its thermoelastic behavior is definitely characterized by such a material composition distribution. Therefore, the designer should determine the most suitable volume fraction distribution in order to design a FGM that optimally meets the desired performance against the given constraints. In this paper, we address a numerical optimization procedure, with employing interior penalty function method(IPFM) and FDM, for optimizing 2D volume fractions of heat-resisting FGMs composed of metal and ceramic. We discretize a FGM domain into finite number of homogenized rectangular cells of single design variable in order for the optimization efficiency. However, after the optimization process, we interpolate the discontinuous volume fraction with globally continuous bilinear function in order to enforce the continuity of volume fraction distributions.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.2
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• pp.79-88
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• 2005

Evaluating the effective properties of materials containing various types of in-homogeneities is an important issue in the analysis of structures composed of those materials. A simple and effective method for the purpose is to impose the periodic displacement boundary conditions on the finite element model of a unit cell. Their theoretical background is explained based on the purely kinematical relations in the regularly spaced in-homogeneity problems, and the strategies to implement them into the analysis and to evaluate the homogenized material constants are introduced. The creep behavior of a thin sheet with square arrayed rectangular voids is characterized, where the orthotropy is induced by the presence of the voids. The homogenization method is validated through the comparison of the analysis of detailed model with that of the simplified one with the effective parameters.

• Analytical Science and Technology
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• v.6 no.2
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• pp.197-205
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• 1993

Blends of elemental Ni and 20 weight % Cr powder were milled for different period in a laboratory attritor. Powder size distribution, microstructure and X-ray diffraction characteristics were investigated as a function of processing period. Saturated magnetization, Ms and coercive force, Hc we also measured and compared with plasma melted ingot to confirm the mechanically alloyed states. Mechanical alloying occurred as a consequence of the partition of powders and the increase of interfacial area driving diffusing of Cr into Ni. However, magnetic properties of chemically homogeneous solid solution like melted ingot has not been observed even though steady state of submicron grain size has been achieved after milling over 15 hrs. Further mechanical alloying period gave refinement of grain size, which resulted in the increase of alloyed layer. It is concluded that homogenization should be controlled by the increase of interfacial area between constitutive powders caused by plastic particle deformation and by the diffusion of Cr within the alloyed phase into Ni-rich phase through lattice defects.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.4
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• pp.365-371
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• 2014

A numerical simulation was conducted for the combustion chamber of a 4-step grate-firing boiler for wood pellet fuel. The flame is extended to the exit of combustion chamber, which is reproduced by present numerical method based on a homogeneous reaction model. Flow field from the simulation shows a strong recirculation flow at the upstream corner of the chamber, along which the flame is extended to the exit. These combustion and flow characteristics remain unchanged for partial load operations, which suggest modification of the combustion chamber structure rather than resizing should be effective to improve combustion characteristics. Possible modifications for combustion chamber are suggested such as relocating its exit, increasing the number of grate steps or installing internals such as guide baffles.

• Korean Journal of Materials Research
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• v.4 no.5
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• pp.600-607
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• 1994

Optimum exper~mental conditions were established for the growth of heteroepitaxial GaAs layers on InP using liquid phase epitaxy (LPE) technique. Results showed that the optimum growth temperature was $720^{\circ}C$ at a cooling rate of $0.5^{\circ}C$/min. Surface morphology of the grown layers significantly improved by addition of about 0.005wt% Se to the Ga growth melt, which effectively suppressed melt-back of InP substrates into the melt during the initial stage of growth. It was observed that the quality of GaAs layers also improved substantially when the substrates patterned with grating structure were used, as determined by the (400) double crystal X-ray diffraction. The transmission electron microscopy observation indicated t.hat the misfit dislocations interact with each other at the grating region, resulting in a lower dislocation density in the upper GaAs layer.