• 한국결정성장학회:학술대회논문집
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• 한국결정성장학회 1996년도 제11차 KACG 학술발표회 Crystalline Particle Symposium (CPS)
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• pp.285-309
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• 1996

Non oxide ceramics such as nitrides of transition metals have shown significant potential for future economic impact, in diverse applications in ceramic, aerospace and electronic industries, as refractory products, abrasives and cutting tools, aircraft components, and semi-conductor substrates amid others. Combustion synthesis has become an attractive alternative to the conventional furnace technology to produce these materials cheaply, faster and at a higher level of purity. However he process os highly exothermic and manifests complex dynamics due to its strongly non-linear nature. In order to develop an understanding of this process and to study the effect of operational parameters on the final outcome, numerical modeling is necessary, which would generated essential knowledge to help scale-up the process. the model is based on a system of parabolic-hyperbolic partial differential equations representing the heat, mass and momentum conservation relations. The model also takes into account structural change due to sintering and volumetric expansion, and their effect on the transport properties of the system. The solutions of these equations exhibit steep moving spatial gradients in the form of reaction fronts, propagating in space with variable velocity, which gives rise to varying time scales. To cope with the possibility of extremely abrupt changes in the values of the solution over very short distances, adaptive mesh techniques can be applied to resolve the high activity regions by ordering grid points in appropriate places. To avoid a control volume formulation of the solution of partial differential equations, a simple orthogonal, adaptive-mesh technique is employed. This involves separate adaptation in the x and y directions. Through simple analysis and numerical examples, the adaptive mesh is shown to give significant increase in accuracy in the computations.

• Wind and Structures
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• 제15권1호
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• pp.17-26
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• 2012

Modeling swirling wakes is of considerable interest to wind farm designers. The present work is an attempt to develop a computational tool to understand free, far-wake development behind a single rotating wind turbine. Besides the standard momentum and continuity equations from the boundary layer theory in two dimensions, an additional equation for the conservation of angular momentum is introduced to study axisymmetric swirl effects on wake growth. Turbulence is simulated with two options: the standard ${\kappa}-{\varepsilon}$ model and the Reynolds Stress transport model. A finite volume method is used to discretize the governing equations for mean flow and turbulence quantities. A marching algorithm of expanding grids is employed to enclose the growing far-wake and to solve the equations implicitly at every axial step. Axisymmetric far-wakes with/without swirl are studied at different Reynolds numbers and swirl numbers. Wake characteristics such as wake width, half radius, velocity profiles and pressure profiles are computed. Compared with the results obtained under similar flow conditions using the computational software, FLUENT, this far-wake model shows simplicity with acceptable accuracy, covering large wake regions in far-wake study.

• 설비공학논문집
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• 제23권12호
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• pp.805-814
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• 2011

In the present study, the detailed flow structure and heat transfer characteristics in the newly-designed heat transfer surface geometry were investigated. The surface geometry proposed in the present study is a traditional dimple structure combining with a protrusion inside the dimple, which is named a protrusion-in-dimple in this study. The basic idea underlying the present surface geometry is to enhance the flow mixing and the corresponding heat transfer in the flow re-circulating region generated by a conventional dimple cavity. The present study was performed by the direct numerical simulation at a Reynolds number of 2800 based on mean velocity and channel height and Prandtl number of 0.71. Three different protrusion heights for protrusion-in-dimples were considered as the main design parameter of the present study. The calculated pressure drop and heat transfer capacity were assessed in terms of the Fanning friction factor and Colburn j factor. The overall performances estimated in terms of the volume and area goodness factor for protrusion-in-dimple cases were higher than the conventional dimple case.

• 멤브레인
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• 제6권1호
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• pp.44-52
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• 1996

평판형 모듈 설계의 최적화를 목적으로 feed 흐름 조건에 따른 feed 온도 및 유속분포를 예측할 수 있는 모델식을 확립하였고 모델 모사를 통해 흐름 조건들이 온도 분포에 끼치는 영향들을 조사하였다. 모델내의 유체의 Re 크기가 커지면 채널 두께방향으로의 유속 구배가 커질 뿐 아니라 투과물 증발을 위한 에너지원인 feed 흐름 속도가 커져 물질 및 열흐름이 증가하여 투과물 증발로 인한 feed 온도 강하가 증어든다. 반면에 채널 간격이 작아지면 feed 흐름량이 상대적으로 작아져 급격한 온도 강하를 야기시킨다. Re 크기에 따른 feed 온도 변화는 실험결과와 일치함이 관찰되었다.

• International Journal of Fluid Machinery and Systems
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• 제9권1호
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• pp.66-74
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• 2016

Concerning the numerical simulation of high-speed water jet with intensive cavitation this paper presents a practical compressible mixture flow method by coupling a simplified estimation of bubble cavitation and a compressible mixture flow computation. The mean flow of two-phase mixture is calculated by URANS for compressible fluid. The intensity of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the mean flow, and the effect of cavitation on the flow turbulence is considered by applying a density correction to the evaluation of eddy viscosity. High-speed submerged water jets issuing from a sheathed sharp-edge orifice nozzle are treated when the cavitation number, ${\sigma}=0.1$, and the computation result is compared with experimental data The result reveals that cavitation occurs initially at the entrance of orifice and bubble cloud develops gradually while flowing downstream along the shear layer. Developed bubble cloud breaks up and then sheds downstream periodically near the sheath exit. The pattern of cavitation cloud shedding evaluated by simulation agrees experimental one, and the possibility to capture the unsteadily shedding of cavitation clouds is demonstrated. The decay of core velocity in cavitating jet is delayed greatly compared to that in no-activation jet, and the effect of the nozzle sheath is demonstrated.

• 한국환경보건학회지
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• 제27권2호
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• pp.145-152
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• 2001

The purpose of this study was to determine nitrogen dioxide(NO$_2$) decay rate by reaction between NO$_2$ and interior materials in Korean residence. The results of this research could be helpful to choose the interior construction materials and to study on reduction of indoor air pollutants. The results of this research are as follows; For 30 residences in Seoul and Incheon from October 2000 to march 2001, the mean of infiltration rate was 0.70$\pm$0.44 ACH, and single-detached houses (7 houses) and apartments (19 houses) were 0.97$\pm$0.55 ACH and 0.61$\pm$0.34 ACH, respectively. The $CO_2$ decay followed approximately first-order process ($R^2$=0.97$\pm$0.02). There existed a statistic significance in filtration rate between houses built in 1980’s and built in 1990’s by t-test (p<0.02). Mean of NO$_2$ decay rates in 26 residence3s except 4 residences was 0.94$\pm$0.49hr$^{-1}$ , and also 0.86$\pm$0.49hr$^{-1}$ , 0.97$\pm$0.50hr$^{-1}$ in single-detached houses and apartments, respectively. Mean NO$_2$ decay rates in houses built in 1980’s were 0.78$\pm$0.37hr$^{-1}$ , 1.33$\pm$1.03hr$^{-1}$ , respectively. Nothing were showed statistical significance among indoor temperature, indoor humidity, and NO$_2$ decay rate. However, NO$_2$ decay rates had a tendency to increase by increase of temperature and humidity. Average volume/surface of participated houses was 0.55$\pm$0.07m and mean NO$_2$ deposition velocity was calculated as 1.46$\pm$0.59msec$^{-1}$ .

• 환경영향평가
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• 제9권1호
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• pp.39-46
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• 2000

Removal of ammonia using the porous ceramic biofilter inoculated with earthworm casts was characterized. By assuming a plug air flow in the biofilter and applying the Michaelis-Menten equation, the maximum removal rate of $NH_3$ was $280.7g-N{\cdot}m^{-3}{\cdot}h^{-1}$($18.0g-N{\cdot}kg^{-1}{\cdot}d^{-1}$) at $30^{\circ}C$. $NH_3$ removal rate was increased as temperature increases from $15^{\circ}C$ to $35^{\circ}C$. The maximum removal rate was $285.8g-N{\cdot}m^{-3}{\cdot}h^{-1}$($18.8g-N{\cdot}kg^{-1}{\cdot}d^{-1}$) at $35^{\circ}C$. At $15^{\circ}C$, the $NH_3$ removal rate was $122.8g-N{\cdot}m^{-3}{\cdot}h^{-1}$($8.1g-N{\cdot}kg^{-1}{\cdot}d^{-1}$). When 210 ppm $NH_3$ was supplied to the biofilter at space velocity of $220h^{-1}$, the removal efficiency of $NH_3$ at 15, 25, 30 and $35^{\circ}C$ was 80, 90, 95, and 96%, respectively. The removal rate of the ceramic biofilter was 3 to 15 times higher than other biofilters comparing the removal efficiency of $NH_3$ per unit volume of carrier. This result indicates that earthworm casts and porous ceramics are very good inoculum source and carrier, respectively, for the $NH_3$-degrading biofilter.

• 한국군사과학기술학회지
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• 제16권4호
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• pp.523-536
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• 2013

A recent air defense missile system is required to have a capability to intercept short-range super-high speed targets such as tactical ballistic missile(TBMs) by performing engagement control efficiently. Since flight time and distance of TBM are very short, the missile defense system should be ready to engage a TBM as soon as it takes an indication of the TBM launch. As a result, it has to predict TBM trajectory accurately with cueing information received from an early warning system, and designate search direction and volume for own radar to detect/track TBM as fast as it can, and also generate necessary engagement information. In addition, it is needed to engage TBM accurately via transmitting tracked TBM position and velocity data to the corresponding intercept missiles. In this paper, we proposed a method to estimate TBM trajectory based on the Kepler's law for the missile system to detect and track TBM using the cueing information received before the TBM arrives the apogee of the ballistic trajectory, and analyzed the bias of prediction error in terms of the transmission period of cueing data between the missile system and the early warning system.

• Tribology and Lubricants
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• 제30권6호
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• pp.337-342
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• 2014

The reduction of drag torque is an important research issue in terms of improving transmission efficiency. Drag torque in a wet clutch occurs because of the viscous drag generated by the transmission fluid in a narrow gap (clearance) between the friction plate and a separate plate. The objective of this paper is to observe the effects of the friction plate area and the clearance on the drag torque using finite element simulation. The two-phase flow of air and oil fluid is considered and modeled for the simulation. The simulation analysis reveals that as the rotational speed increases, the drag torque generally increases to a critical point and then decreases sharply at a high speed regime. The clearance between the two plates plays an important role in controlling drag torque peak. An increase in the clearance causes a decrease in shear stress; thus, the drag torque also decreases according to Newton's law of viscosity. An observation of the effect of the area of contact between transmission fluid and friction plate shows that the drag torque increases with the contact area. The flow vectors inside the flow channel present clear evidence that the velocity of the fluid flows is faster with a larger friction plate, that is, in the case of a larger contact area. Therefore, the optimum size of the friction plate should be determined carefully, considering both the clutch performance and drag reduction. It is expected that the results from this study can be very useful as a database for clutch design and to predict the drag torque for the initial design with respect to various clutch parameters.

• 한국산학기술학회논문지
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• 제15권5호
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• pp.2587-2593
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• 2014

세라믹 사출공정(CIM)은 산업 분야 전반에 걸쳐 널리 사용되고 있는 공정 중 하나로, 점차 의료용 전자기기의 부품 등으로 확대 적용되고 있다. 본 연구에서는 FEM 해석을 통해 CIM의 공정변수가 제품의 품질에 미치는 영향을 분석했다. 단순평판 형상의 해석결과를 기초로 구멍이 있는 형상, 모서리부가 둥근 형상 및 측벽 구조가 있는 형상 등과 비교 분석했다. 구멍이 있는 형상의 경우, 구멍 주변에 밀도분포가 고르지 못하며 용접선(weld-line)과 같은 결함이 발생할 수 있음을 예측할 수 있었다. 반면 제품의 모서리부 반경이 크면 성형성 및 유동성이 좋아지는 것을 확인했다. 따라서 CIM 공정변수 뿐만 아니라 제품의 형상변수도 고려해야 한다. 해석결과 온도, 초기분율, 속도 등의 공정변수는 제품의 품질 향상을 위한 중요한 설계 변수가 될 수 있음을 확인할 수 있었다.