• Journal of the Korean Geotechnical Society
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• v.29 no.1
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• pp.23-27
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• 2013

The risks of debris flows caused by climate change have increased significantly around the world. Recently, landslide disaster prevention technology is more focused on the failure and post-failure dynamics to mitigate the hazards in flow-prone area. In particular, we should define the soil strength and flow characteristics to estimate the debris flow mobility in the mountainous regions in Korea. To do so, we selected known ancient landslides area: Inje, Pohang and Sangju debris flows. Firstly we measured physical and mechanical properties: liquidity index and undrained shear strength by fall cone penetrometer. From the test results, we found that there is a possible relationship between liquidity index and undrained shear strength, $C_{ur}=(1.2/I_L)^{3.3}$, in the selected areas, even though they were different in geological compositions. Assuming that the yield stress is equal to the undrained shear strength at the initiation of sliding, we examined the flow characteristics of weathered soils in Korea. When liquidity index is given as 1, 1.5 and 3.0, the debris flow motion of weathered soils is compared with that of mud-rich sediments, which are known as low-activity clays. At $I_L=1$, it seems that debris flow could reach approximately 250m after 5 minutes. As liquidity index increased from 1 to 3, the debris flow propagation of weathered soils is twice than that of low-activity clays. It may be due to the fact that soil masses mixed with the ambient water and then highly fragmented during flow, thereby leading to the high mobility. The results may help to predict the debris flow propagation and to develop disaster prevention technology at similar geological settings, especially for the weathered soils, in Korea.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.4
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• pp.369-376
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• 2016

In this study, a numerical analysis framework for investigating the nonlinear behavior of structures under fire conditions is presented. In particular, analysis procedure combining fire-driven flow simulation and thermo-mechanical analysis is discussed to investigate the mechanical behavior of fire-exposed representative volume structures made of steel and concrete, respectively. First of all, fire-driven flow analysis is conducted using Fire Dynamics Simulator(FDS) in a rectangular parallelepiped domain containing the structure. The FDS simulation yields the time history of temperature on the surface of the structure under fire conditions. Second, mechanical responses of the fire-exposed structure with respect to prescribed uniformly distributed loads are calculated by a coupled thermo-mechanical analysis using the time-varying surface temperature as boundary conditions. Material nonlinearities of steel and concrete have been considered in the thermo-mechanical analysis. A series of numerical results are presented to demonstrate the feasibility of the multiphysics structural fire analysis for investigating the structural behavior under fire conditions.

• Journal of the Korean Society of Groundwater Environment
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• v.6 no.1
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• pp.42-47
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• 1999

This study considers pseudosteady-state flow to a restricted-entry well in a single or multilayer aquifer with crossflow. A simple method for calculating the pseudoskin factor caused by partial penetration is presented to overcome a limited applicability in geometrical or computational aspects of previous methods. The computation is based on the solution of a simplified pseudosteady-state equation that describes the long-time behavior of the closed radial system. We illustrate the applicability of this method to various types of cylindrical systems and provide the results graphically. Comparisons with previously published results have indicated that this method yields highly accurate estimates of pseu-doskin factor with minimum computational effort. This method has also shown to be particularly useful for geometrically-complicated systems. Greatly improved computational efficiency of pseudosteady-state approach permits the engineer to easily account for the effect of partial penetration on the late-time performance of a well.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.7
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• pp.685-691
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• 2011

The aim of this study is to point out the uniqueness of Doppler optical coherence tomography (DOCT) for use in a probe station for (in vivo) visualization of microscale flow and structure and to maximize the effectiveness of DOCT by overcoming its limitations. Conventional DOCT produces images of only one of the velocity components that is parallel to the incident light. In this study, a multi-angle DOCT to quantify a velocity vector field is proposed; this is an extension from a velocity scalar field to a vector field. Quantifying an instantaneous three-dimensional velocity field in a pulsating flow is another challenge because of its limited frame rate. The in-vivo pulsating blood flow is measured by using an electrocardiogram-gated multi-angle DOCT in a hamster cheek pouch model. It is shown that the aliasing problem caused by a relatively low frame rate is resolved by using this method of measurement.

• The Optical Journal
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• s.143
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• pp.30-45
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• 2013

미국 QED Technologies(이하 QED)가 세계 최초로 상용화한 자기유동유체연마장치(이하 MRF$^{(R)}$-Magnetorheological (Fluid) Finishing)는 역시 QED가 개발한 간섭계 응용장비인 SSI$^{(R)}$(Subaperture stitching interferometer)와 같이 운용하면 1um 이하의 나노광학산업에서 ${\lambda}/10$ 또는 ${\lambda}/100$ 이상 더 정밀한 영역을 손 쉽게 제조할 수 있는 획기적인 공정을 가능하게 한다. 이러한 능력 때문에 미국, 유럽 및 일본 등의 첨단광학회사들은 96년 상업화된 이후 MRF$^{(R)}$와 SSI$^{(R)}$ 공정을 앞다투어 도입하며 기존공정이 지니고 있던 많은 한계들을 극복했다. 우리나라의 MRF와 SSI의 도입은 선진국에 비해 10년 이상 늦어진 2008년에 이르러서야 최초의 장비가 도입되었으며 이후 꾸준히 그 응용분야가 확장되어 가고 있다. 2012년에는 한국천문연구원과 그린광학에 SSI의 최신장비인 ASI(Aspheric Stitching Interferometer)와 MRF가 각각 도입됐다. 이를 계기로 해서 한국사무소에서 제안을 하여 QED사의 총괄대표이사인 Dr. Andrew Kulawiec, 응용기술부장인 Paul Dumas, QED 한국사무소의 오한석 박사와 신지식 박사가 그간의 기술동향을 발표하는 기술강연회가 지난 2012년 10월 23일 개최되었다. 한국천문연구원, 그린광학과 한국광학기기산업협회의 후원으로 한국천문연구원 은하수홀에서 개최된 이번 강연회에서는 다음과 같은 세가지 주제를 다루었다. 1. MRF의 원리 및 응용분야의 현황과 장비개발 현황 2. SSI의 원리 및 응용분야의 현황과 장비개발 현황 3. 광학설계와 가공실현성에 도움이 되는 비구면을 표현하는 새로운 식 Q-Polynominal 이번 강연회는 특히 QED의 공정 및 장비개발자들이 직접 참가한 기술강연회로 지난 2004년 및 2008년에 열린 기술강연회와 차별화되며, 25개 업체 약 50여 명이 참석하여 성황리에 마무리되었다. 향후 국내의 사용자 그룹에 대한 세미나를 정기적으로 개최해 QED의 첨단광학기술에 대한 정보교환의 장을 확대할 예정이며, 기술강연회도 가능한 한 정기화할 예정이다. 이번 강연회의 주제 중 우선 MRF와 SSI에 대한 논의를 광학세계 1월호에 소개하고자 한다.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.7
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• pp.237-244
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• 2016

The workability of mortar determines its construction performance in a structure showing its designed resistance to external loads. Measuring the rheological properties of mortar is one way of quantifying its workability, but its field-applications are limited due to economical and spatial issues. The robustness of the slump flow test allows its use for evaluating the workability of mortar, even though it is a rather qualitative test method. This paper proposes a channel flow test and develops a correlation between its result and the rheological properties of mortar. The volume-of-fluid simulation for the channel flow test was accomplished, and a numerical database for the correlation was composed. A correlation model to estimate the rheological properties of mortar using the results of the channel flow test as inputs is proposed.

• Journal of Aerospace System Engineering
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• v.12 no.4
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• pp.57-63
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• 2018

IR thermography is a non-invasive method and used for the visualization of the surface temperature of the model. However, this technique only derives 2D results and not quantitative data. The goal of this study is to apply the 3D mapping technique for IR thermography. The wind tunnel model is an ogive-cylinder with a wind speed of 20 m/s ~ 80 m/s and the angle of attack ranging from $0^{\circ}$ to $90^{\circ}$. The real location of the model was made to correspond with the position of the IR image using the makers. Based on this result, quantitative results were obtained. The 3D mapping method was verified by comparing the separation point and the theoretical value.

• Journal of the Korean Geotechnical Society
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• v.19 no.1
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• pp.21-29
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• 2003

In this study, practical guidelines to check the possibility of some lateral movement of piled abutment were investigated. In these tests, both the depth of soft clay and the rate of embankment construction are chosen to examine the effect on lateral soil movements. The depth of soft clay layer varies from 5.2 m to 11.6 m, and the rate of embankment construction has two types : staged construction(1m/30days, 1m/15days) and instant construction. Various measuring instruments such as LVDTs, strain gauges, pressure cells, and pore pressure transducers are installed in designed positions in ordo. to clarify the soil - pile interaction and the short and long term behavior f3. piled bridge abutments adjacent to surcharge loads. The validity of the proposed guidelines by centrifuge test was compared with the observed performance by lateral movement index, F(Japan Highway Public Corporation) and modified I index(Korea Highway Corporation). Based on the results obtained, the critical values off and modified I, as a practical guidelines, are proposed as 0.03 and 2.0, respectively.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.9
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• pp.817-824
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• 2010

An improved spray model of a pressure-swirl atomizer was developed and the grid dependency of the model was investigated. Since the Lagrangian-Eulerian approach was adopted for tracking droplets, very small grids could not be used. However, in order to detect swirl flow accurately, small grids were needed because of the consideration of swirl injection. In order to overcome these limitations, numerical studies were performed by using various grids with cell sizes ranging from 10.0 $\times$ 10 mm to 0.625 $\times$ 0.625 mm. From these calculated results, it was observed that the most efficient grid cell size was 1.25 $\times$ 1.25 mm.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.5
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• pp.513-520
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• 2014

An unmanned aerial vehicle (UAV) should be designed to be as small and lightweight as possible to optimize the efficiency of changing the blade shape to enhance the aerodynamic performance, such as the thrust and power. In this study, a computational fluid dynamics (CFD) simulation of an unmanned multi-rotor aerial vehicle in hover mode was performed to explore the thrust performance in terms of the blade rotational speed and blade shape parameters (i.e., taper ratio and twist angle). The commercial ADINA-CFD program was used to generate the CFD data, and the results were compared with those obtained from blade element theory (BET). The results showed that changes in the blade shape clearly affect the aerodynamic thrust of a UAV rotor blade.