• 대한기계학회논문집B
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• 제24권11호
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• pp.1523-1534
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• 2000

The performance of a number of existing dynamic subgrid-scale(SGS) models is evaluated in large-eddy simulations(LES) of two prototype transitional and turbulent shear flows, a planar jet and a channel flow. The dynamic SGS models applied include the dynamic Smagorinsky model(DSM);Germano et al. 1991, Lully 1992), the dynamic tow-component model(DTM; Akhavan et al. 2000), the dynamic mixed model(DMM;Zang et al, 1993). and the dynamic two-parameter model(DTPM; Salvetti & Banerjee 1995). The results are compared with those for DNS for their evaluation. The LES results demonstrate the superior performance of DTM with use of a sharp cutoff filter and DMM with use of a box filter, as compared to their respect counterpart DSM, in predicting the mean statistics, spectra and large-scale structure of the flow, Such features of DTM and DMM derive from the construction of the models in which tow separate terms are included to represent the SGS interactions; a Smagorinsky edd-viscosity term to account for the non-local interactions, and a local-interaction term to account for the nonlinear dynamics between the resolved and subgrid scales in the vicinity of the LES cutoff. As well, overall the SGS models using a sharp cutoff filter are more successful than those using a box filter in capturing the statistics and structure of the flow. Finally, DTPM is found to be compatible or inferior to DMM.

• 한국진공학회지
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• 제10권2호
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• pp.219-224
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• 2001

N형 InGaAs에 대한 Pd/Ge/Ti/Pt 오믹 접촉 특성을 조사하였다. $450^{\circ}C$까지의 급속 열처리에 의해 우수한 오믹 특성을 나타내어 $400^{\circ}C$, 10초의 급속 열처리 조건에서 최저 $3.7\times10^{-6}\; \Omega\textrm{cm}^2$ 의 접촉 비저항을 나타내었다. 이는 열처리에 의해 생성된 Pd-Ge계 화합물의 형성 및 Ge의 InGaAs 표면으로의 확산과 관련이 있었다. 그러나 열처리 시간을 연장할 경우 접촉 비저항이 $low-10^5\; \Omega\textrm{cm}^2$로 약간 증가하였다. 고온 열처리 후에도 오믹 재료와 InGaAs의 평활한 계면을 유지하면서 우수한 오믹 특성을 나타내어, 화합물 반도체 소자의 오믹 접촉으로 충분히 응용 가능하다고 판단된다.

• 대한기계학회논문집A
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• 제36권11호
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• pp.1353-1360
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• 2012

병렬로봇의 구동을 계산하기 위해서 자코비안과 특이점 해석이 필요하다. 기존의 자코비안을 구하는 미분의 방식은 그 계산과정이 복잡하고 기하학적인 의미도 찾기 어렵다. 본 논문에서는 스크류 이론을 사용하여 병렬로봇의 자코비안을 쉽게 구하고 그것의 기하학적인 의미도 구하였다. 뿐만 아니라 특이점도 간단한 형태로 식을 구성할 수 있으며 기하학적인 의미도 가진다. 또한, 스크류 이론의 적용이 5 링크와 같이 간단한 형태의 비 여유구동 로봇뿐만 아니라 다양한 형태의 평면형 여유구동 병렬로봇에도 적용 가능하다는 것을 본 논문에서 제시하였다.

• 한국자기공명학회논문지
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• 제5권1호
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• pp.1-12
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• 2001

ZSM-5 gallosilicate molecular sieves was synthesized and cupric ion was ion-exchanged into the gallosilicate. The locations of Cu(ll) species in the framework and their interactions with various adsorbates were characterized by combined electron spin resonance(ESR) and electron spin echo modulation(ESEM) methods. It was found that in a fresh hydrated material, Cu(II) is octahedrally coordinated to six water molecules. This species is located in the channel intersections of two sinusoidal channels and rotates rapidly at room temperature. Evacuation removes some of these water molecules, leaving the Cu(II) coordinated to less water molecules and anchored to of oxygens in the channel wall. Dehydration produces two Cu(II) species, both of which are located in sites inaccessible to oxygen as evidenced by non-broadening of its ESR lines by oxygen. Adsorption of adsorbate molecules such as water, alcohols, ammonia, acetonitrile and ethylene on dehydrated CuNaH-ZSM-5 gallosilicate materials causes changes in the ESR spectrum of Cu(II), indicating the migration of Cu(II) into main channels to form complexes with these adsorbates there. Cu(II) forms a complex with two molecules of methanol, ethanol and propanol, respectively as evidenced by ESR parameters and ESEM data. Cu(II) also forms a square planar complex with four molecules of ammonia, based on the resolved nitrogen superhyperfine interactions and their ESEM parameters. Cu(II) forms a complex with two molecules of acetonitrile based on the ESR parameters and ESEM data. Interestingly, however, only part of Cu(II) interacts indirectly with one molecule of nonpolar ethylene based on ESR and ESEM analyses.

• 한국연소학회지
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• 제1권1호
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• pp.65-73
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• 1996

Flame propagation in a four-valve spark-ignition optical engine was visualized under lean-bum conditions with A/F=18 at 2000rpm. The early flame development in a four-valve pentroof-chamber single-cylinder engine was examined with imaging of the laser-induced Mie scattered light using an image-intensified CCD camera. Flame profiles along the line-of-sight were also visualized through a quartz piston window. Two-dimensional flame structures were visualized with a Proxitronic HF-1 fast motion camera system by Mie scattering from titanium dioxide particles along a planar laser sheet generated by a copper vapor laser. The flame propagation images were subsequently analysed with an image processing programme to obtain information about the flame structure under different tumble flow conditions generated by sleeved and non-sleeved intake ports. This allowed enhancement of the flame images and calculation of the enflamed area, and the displacement of its center, as a function of the tumble flow induced by the pentroof-chamber in the vicinity of spark plug. Image processing of the early flame development quantified the correlation between flame and flow characteristics near the spark plug at the time of ignition which has been known to be one of the most important factors in cyclic combustion variations in lean-burn engines. The results were also compared with direct flame images obtained from the natural flame luminosity of the lean mixture.

• Journal of Welding and Joining
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• 제16권5호
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• pp.56-66
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• 1998

During solidification or welding of alloys, the solute redistribution brings out microsegregation. The microsegregation causes the formation of non-equilibrium second phases, shrinkage and porosity degrading mechanical/chemical properties Therefore, it has been required to predict microsegregation quantitatively. To predict the degree of microsegregation, more exact and appropriate computer simulation technique has been actively used during last two decades. To predict the degree of microsegregation in weld metal, an advanced two dimensional model was suggested. In the new model, both primary and secondary arm regions were defined for the analysis region. The growth in the primary arm regina was assumed to be a planar for effective calculation. Especially, for the growth of a secondary arm, a simple and effective mathematical function was established to show the growing pattern, the solute diffusion in the solid phase was calculated by finite difference method (FDM). The solid-liquid interface movement was considered to be in local equilibrium state. The experiments for welding of 310S stainless steel were carried out in order to examined the reasonability and feasibility of this model. The concentration profiles of the solute predicted by this model were compared with those obtained from experimental works.

• 한국진공학회지
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• 제11권1호
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• pp.43-49
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• 2002

N형 InGaAs에 대한 Pd/Ge/Pd/Ti/Au 오믹 접촉의 급속 열처리 조건에 따른 오믹 특성을 조사하였다. $450^{\circ}C$까지의 열처리에 의해 우수한 오믹 특성을 나타내어 $400^{\circ}C$/10초의 급속 열처리 후에 최저 $1.1\times10^{-6}\Omega\textrm{cm}^2$의 접촉 비저항을 나타내었다. $425^{\circ}C$ 이상의 열처리 후에 접촉 비저항이 점점 증가하여 $450^{\circ}C$에서는 오믹 재료와 InGaAs의 반응에 의해 오믹 특성의 열화가 나타났다. 그러나 high-$10^{-6}\Omega\textrm{cm}^2$ 정도의 비교적 우수한 오믹 특성을 유지하였고, 양호한 표면 및 계면이 얻어져 화합물 반도체 소자에의 응용 가능성이 충분한 것으로 판단된다.

• 한국광학회지
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• 제21권4호
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• pp.161-167
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• 2010

단축 이방성 매질에서 다른 단축 이방성 매질로 진행하는 빛의 유효투과계수 표현을 유도하였다. 두 이방성 매질 사이에 가상의 등방성 매질을 도입한 후, 가상의 등방성 매질 내의 다중반사 효과를 반영함으로써 두 단축 이방성 매질간의 경계면에서 유효 투과계수 표현을 제시하였다. 균일하지 않은 단축 이방성 매질을 얇고 균일한 단축 이방성 층들의 합으로 대치하여 각각의 두 경우에 대하여 투과하는 빛의 편광상태 변화를 계산한 후 비교함으로써 이 방법의 타당성을 확인하였다.

• 센서학회지
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• 제14권6호
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• pp.374-380
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• 2005

Vibration sensors have a wide scope of applications in the field of monitoring systems that needs to perceive an undesirable physical vibration before a critical failure occurs in a system, and then costly unplanned repairs can be avoided. The conventional vibration sensors developed so far have many disadvantages, such as complex manufacturing process, bulkiness, high cost, less reliability and so on. This paper reports a simple-structured vibration sensor, which has been developed using a commercialized conductive ball and silicon bulk-micromachining technology. The sensor consists of a conductive ball placed in $600{\mu}m$-deep micromachined silicon groove, in which Au thin film has been patterned using a shadow mask technique. Prior to the formation of the Au thin film, the sharp convex corner was rounded for smooth meatl deposition on the non-planar surface at the edge of the groove. The measurement results of the fabricated vibration sensor demonstrate a stable response characteristic to low-frequency vibration range ($1{\sim}30{\;}Hz$).

• 한국정밀공학회지
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• 제30권11호
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• pp.1161-1169
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• 2013

In this paper an omni-directional mobile robot is suggested for educational robot platform. Comparing to other robots, a mobile robot can be easily designed and manufactured due to its simple geometric structure. Moreover, since it is required to have low DOF motion on planar space, fabrication of control system is also simple. In this research, omni-directional wheels were adopted to remove the non-holonomic characteristic of conventional wheels and facilitate control system design. Firstly, geometric structure of a Mecanum wheel which is a most frequently used omni-directional wheel was demonstrated. Then, the organization of the mobile platform was suggested in aspects of mechanism manufacturing and electronic hardware design. Finally, a methodology of control system development was introduced for educational purpose. Due to an intuitive motion generating ability, simple hardware composition, and convenient control algorithm applicability, the omni-directional mobile robot suggested in this research is expected to be a promising educational platform.