• Journal of Ocean Engineering and Technology
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• v.7 no.2
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• pp.150-161
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• 1993

원초변수를 이용한 Navier-Stokes 방정식의 수치계산기법을 개발하고, 이를 응용하여 backward facing step의 층류 유동을 계산하였다. 직교좌표계에서의 비압축성 Navier-Stokes방정식을 풀기위해 시간과 공간항을 2차 정도의 유한 차분을 사용하여 이산화하였고 비교차격자계를 사용하여 양해법으로 수치 계산하였다. 운동량방정식과 연속방정식으로 부터 유도된 압력방정식(pressure-poisson equation)을 이용하여 무발산 조건을 만족시켰ㄲ다. Backward facing step의 층류 유동을 100.$\leq$R$_e$$\leq$1000 범위에 대해서 수치 계산하였으며 실험결과와 잘 일치하는 결과를 구할 수 있었다. 특히 step뒤에서 생기는 박리구간의 길이는 다른 계산결과들보다 실험치에 가까운 값을 얻을 수 있었으며, Re가 600보다 클때는 위쪽 벽에 또 다른 박리 유동이 발생되는 현상이 예측되었다.

• Journal of computational fluids engineering
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• v.5 no.2
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• pp.20-27
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• 2000

An unstructured implicit Euler solver is parallelized on a Cray T3E. Spatial discretization is accomplished by a cell-centered finite volume formulation using an upwind flux differencing. Time is advanced by the Gauss-Seidel implicit scheme. Domain decomposition is accomplished by using the k-way n-partitioning method developed by Karypis. In order to analyze the parallel performance of the solver, flows over a 2-D NACA 0012 airfoil and 3-D F-5 wing were investigated.

• Korean Journal of Materials Research
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• v.7 no.12
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• pp.1041-1046
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• 1997

본 연구에서는 분자선 에피택시 방법으로 비대칭 AIAs/GaAs(001)이중 장벽, 삼중장벽구조를 성장한 수 이를 이용하여 2단자 소자를 제작하여 전기적 특성을 분석하였다. 에피층은 쌍결정 X-ray회절 분석과 단면투과 전자현미경을 이용하여 결정성 및 격자 정합성을 확인하였다. 전기적 성능을 보다 향상시키기 위해 n-GaAs에 대한 오믹 접촉등의 소자 제작 공정을 최적화하였다. 삼중장벽 구조를 이용하여 제작한 소자의 전기적 특성 연구 결과 두개의 주요 공진 터널링 전류 피크 사이에 X-valley에 의한 구조를 확인할 수 있었으며, 이중 장벽구조에 제2의 양자우물 구조를 첨가함으로써 낮은 전압위치에서 전류 피크가 향상하는 결과를 얻었다.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.2
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• pp.90-97
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• 2004

Flow mechanism of contractive and dilative motion is numerically investigated to obtain a propulsive force in highly viscous fluid. An axisymmetric code is developed with unstructured grid system based on cell-centered scheme. It is validated by comparing with the results of Stokes approximation for the problem of uniform flow past a sphere in low Reynolds number(R$_{n}$=1). The validated code is applied to the simulation of contractive and dilative periodic motion of body whose results are quantitatively compared with the two dimensional case. In order to investigate the grid dependency, two different grids are applied to the present computations. The present study provides key information for the development of an axisymmetric Micro-hydro-robot.t.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.3
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• pp.13-19
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• 2021

This study examined the effects of stacking faults on the thermoelectric properties for n-type SiC semiconductors. Porous SiC semiconductors with 30~42 % porosity were fabricated by the heat treatment of pressed ��-SiC powder compacts at 1600~2100 ℃ for 20~120 min in an N2 atmosphere. XRD was performed to examine the stacking faults, lattice strain, and precise lattice parameters of the specimens. The porosity and surface area were analyzed, and SEM, TEM, and HRTEM were carried out to examine the microstructure. The electrical conductivity and the Seebeck coefficient were measured at 550~900 ℃ in an Ar atmosphere. The electrical conductivity increased with increasing heat treatment temperature and time, which might be due to an increase in carrier concentration and improvement in grain-to-grain connectivity. The Seebeck coefficients were negative due to nitrogen behaving as a donor, and their absolute values also increased with increasing heat treatment temperature and time. This might be due to a decrease in stacking fault density, i.e., a decrease in stacking fault density accompanied by grain growth and crystallite growth must have increased the phonon mean free path, enhancing the phonon-drag effect, leading to a larger Seebeck coefficient.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.3 no.1
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• pp.18-25
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• 1978

The 1/f noise spectrum of short-circuited output drain current due to the Shockley-Read-Hal] recombination centers with a single lifetime in homogeneous nondegenerate MOS-field effcte transtors with n-type channel is calculated under the assumptions that the quasi-Fermi level for the carriers in each energy band can not be defined if we include the fluctuation for time varying quantities. and so 1/f noise is a majority carrier effect. Under these assumptions the derived 1/f noise in this paper show some essential features of the 1/f noise in MOS-field effect transistors. That is, it has no lowfrequency plateau and is proportionnal to the channel cross area A and to the driain bias voltage Vd and inversely proportional to the channel length L3 in MOS field effect transistors. This model can explain the discrepancy between the transition frequency of the noise spectrum from 1/f- response to 1/f2 and the frequency corresponding to the relaxation time related to the surface centers in p-n junction diodes. In this paper the results show that the functional form of noise spectrum is greatly influenced by the functional forms of the electron capture probability cn (E) and the relaxation time r (E) for scattering and the case of lattice scattering show to be responsible for the 4 noise in MOS fold effect transistors. So we canconclude that the source of 1/f noise is due to lattice scattering.

• Korean Chemical Engineering Research
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• v.55 no.2
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• pp.156-162
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• 2017

CuO(x)/0.3Al-0.7Ce catalysts with different CuO loadings (x = 2~20 wt%) were prepared by impregnation method and investigated the effects of CuO loadings on the low temperature CO oxidation. Of the used catalysts, the CuO(10)/0.3Al-0.7Ce catalyst showed the highest catalytic performance in the absence or presence of water vapor. In the presence of water vapor, the catalytic performance was drastically decreased, with a temperature of 50% CO conversion ($T_{50%}$) shifted to higher temperature by $50^{\circ}C$ compared to the those in dry conditions because of the competitive adsorption of water vapor on the active sites. The copper metal surface area calculated from $N_2O$-titration analysis and the oxygen capacity from CO-pulse experiments were increased with the CuO loadings and showed a maximum at 10 wt%CuO/0.3Al-0.7Ce catalyst. These trends are in good agreement with the tendency of $T_{50%}$ of the catalysts. From these characteristic aspects, it could be deduced that the catalytic performance was closely related to the oxygen capacity and the copper metallic surface area.

• Journal of the Korean Magnetics Society
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• v.16 no.1
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• pp.28-33
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• 2006

La substituted perovskite $BiFeO_3$ have been prepared by a sol-gel method. Magnetic and structural properties of the powders were characterized with Mossbauer spectroscopy, XRD, SEM, and TG-DTA. The crystal structure is found to be a rhombohedrally distorted perovskite structure with the lattice constant $\alpha=3.985{\AA}\;and\;\alpha=89.5^{\circ}.\;Bi_{2/3}La_{1/3}FeO_3$ powders that were annealed at and above $600^{\circ}C$ have a single-phase perovskite structure. However, powders annealed at $900^{\circ}C$ have a typical perovskite structure with small amount of $Bi_2O_3$ phase. The Neel temperature of $Bi_{2/3}La_{1/3}FeO_3$ is found to be $680\pm3K$. The isomer shift value at room temperature is found to be 0.27 mm/s relative to the Fe metal, which is consistent with high-spin $Fe^{3+}$ charge states. Debye temperature far$Bi_{2/3}La_{1/3}FeO_3$ is found to be $305\pm5K$. The average hyperfine field $H_{hf}(T)$ of the $Bi_{2/3}La_{1/3}FeO_3$, shows a temperature dependence of $[H_{hf}(T)-H_{hf}(0)]/H_{hf}(0)=-0.42(T/T_N)^{3/2}-0.13(T/T_N)^{5/2}$ for $T/T_N<0.7$ indicative of spin-wave excitation.

• The KIPS Transactions:PartA
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• v.19A no.2
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• pp.85-92
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• 2012

In this paper, a highly scalable real-time locating system which can measure location of fast moving targets is proposed. Within the system, the location service area is partitioned into grids with squares which is referred to as a macro-cell. Also, a macro-cell is further partitioned into $N{\times}N$ micro-cells. In a micro-cell, location reference nodes are placed on every vertex and an arbitration node is placed on the center. When a mobile node tries to measure its location, it should first communicate with the arbitration nodes for granting location measurement operation. Therefore, within a micro-cell, only one granted mobile node can calculate its location by a series of communication with location reference nodes. To evaluate performance of the proposed system, the system is modeled and simulated. The simulation result shows that the proposed system requires small communication time for location measurement operation and produces small location calculation error for fast moving targets.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.13 no.4
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• pp.179-191
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• 2013

The quincunx lattice is a non-separable sampling method in image processing. It treats the different directions more homogeneously and good frequency property than the separable two dimensional schemes. The high density discrete wavelet transformation is one that expands an N point signal to M transform coefficients with M > N. In two dimensions, this transform outperforms the standard discrete wavelet transformation in terms of shift-invariant. Although the transformation utilizes more wavelets, sampling rates are high costs. This paper proposed the high density discrete wavelet transform using quincunx sampling, which is a discrete wavelet transformation that combines the high density discrete transformation and non-separable processing method, each of which has its own characteristics and advantages. Proposed wavelet transformation can service good performance in image processing fields.