• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.20 no.4
• /
• pp.1437-1448
• /
• 1996

This paper presents a simplified model for approximate analysis of the solute redistribution in coarsening dendrite arms during solidification of binary metal alloys. By introducing a quadratic concentration profile with a time-dependent coefficient, the integral equation for diffusion in the solid phase is reduced to a simple differential relation between the coefficient and the solid-liquid interface position. The solid fraction corresponding to the system temperature is readily determined from the relation, phase equilibrium and the overall solute balance in which the liquid phase is assumed to be completely mixed. In order to validate the developed model, calculations are performed for the directional solidification of Al-4.9 mass Cu alloy. The predicted eutectic fractions for a wide range of the cooling rate reasonably agree with data from the well-known experiment as well as sophisticated numerical analyses. Also, the results for the back diffusion limits are consistent with available references. Additional calculations show that the characteristic parameters such as the coarsening, density variation and nonlinarity in the phase diagram significantly affect the microsegregation. Owing to the simplicity, efficiency and compatibility, the present model may be suitable for the micro-macroscopic solidification model as a microscopic component.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.22 no.11
• /
• pp.1530-1537
• /
• 1998

Mixture formation is one of the significant factors to improve combustion performance of an spark ignition engine. This is affected by spray and atomization characteristics of injector. In the case of EGI system, air-fuel mixing period is so short that a lot of fuel-film and liquid-fuel flow into cylinder. Since this fuel-film is not burnt perfectly in cylinder, it is exhausted in the form of HC emission. In this paper, three measurement techniques were utilized to measure spray characteristics and the amount of fuel-film in the cylinder. At first, PMAS was used to measure the spray characteristics such as size distributions, SMD, and spray angle. Secondly the amount and distribution of fuel-film which flow into through intake valve could be measured quantitatively using the fuel-film measuring device. And lastly, by optical fiber type spark plug used to detect the diffusion flame, the amount of unburned HC was measured. As the result of these experiments, the information of optimal spray characteristics and injection condition to minimize fuel-film could be built up.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2011.05a
• /
• pp.257-257
• /
• 2011

현업에서 사용하는 유출해석 기본이론은 연속방정식과 운동방정식으로서 운동파가정(kinematic wave analogy)을 기반으로 한 집중수문모형(lumped hydrologic model)에 의하여 수행되고 있지만 집중형 모형은 한 매개변수에 여러 가지의 물리적 과정을 개념화하여 담고 있기 때문에 유출과정에 대한 섬세한 모형화의 제약으로 인하여 유역고유의 매개변수값을 찾기가 쉽지 않은 단점을 가지고 있다. 이에 본 연구에서는 물리적 기반의 2차원 분포형 강우-유출모형을 개발하고자 하며 이는 완전분포형 수문동역학적 모형으로 지표흐름과 침투과정, 기저유출과 관련된 과정을 모의한다. 본 모형은 공간적으로 변화하는 침투량과 소규모 및 대규모의 지형학적 특성을 사용하는 St. Venant 방정식을 사용하고 개발될 모형은 모든 스케일에서의 수심과 유량을 계산할 수 있으며 Richard 방정식(또는 선택적으로 Green-Ampt 방정식 채택)을 이용하여 정밀한 침투량 모의가 가능하다. 또한 레이다등의 고해상도 강우관측자료를 지점자료와 합성하여 입력자료로 사용할 수 있도록하고자 하며 강우-유출모형에 다목적댐이나 보등에서의 유량조절효과를 반영하고, 다목적댐군에서의 연계운영모의가 가능케 함으로서 현업의 운영자들이 실무에서 실질적으로 활용할 수 있는 형태의 모형을 개발하고자 한다. 이는 국내에서의 2차원 분포형 강우-유출모형을 자체 개발함으로서 연구역량을 제고하고, 국내 현업기관에서의 분포형 모형기반의 홍수모니터링 및 전망시스템의 확산에 기여할 것으로 예상된다.

• Water for future
• /
• v.28 no.4
• /
• pp.195-204
• /
• 1995

New dispersion coefficient equation which can be used to estimate dispersion coefficient by using only hydraulic data easily obtained in natural streams has been developed. Dimensional analysis was performed to select physically meaningful parameters, One-Step Huber method, which is one of the nonlinear multi-regression method, was applied to derive a regression equation of dispersion coefficient. 59 measured hydraulic data which were collected in 26 streams in the United States and were analyzed in the Part I of this study, were used in developing new dispersion coefficient equation. Among 59 measured data sets, 35 data sets were used in deriving regression equation, and 24 data sets are used for verification. The new dispersion coefficient equation, which has been developed in this study was proven to be superior in explaining dispersion characteristics of natural streams more precisely compared to existing dispersion coefficient equations.

• International Journal of Highway Engineering
• /
• v.8 no.2 s.28
• /
• pp.95-104
• /
• 2006

Hysteresis is a common feature exhibited in hydraulic properties of an unsaturated soil. This study focuses on hysteresis observed in a compacted weathered granite subgrade soils based on the pressure plate laboratory tests. It was found that the Soil-Water Characteristics Curve of a soil is hysteretic and unique. The results also show that the wetting and drying curves predicted using the Fredlund and Xing model is quite close to the laboratory-measured results. For a specific matric suction, water content or coefficient of permeability on a wetting curve is always lower than those found on a drying curve.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.19 no.9
• /
• pp.1782-1792
• /
• 1994

Ti : $LiNbO_3$ traveling-wave phase optical modulators at wavelength 1.3㎛ have been designed and fabricated, focusing on the optical waveguide and asymmetric coplanar electrode structure. To improve the phase-mismatch of traveling-wave ACPS electrode, the characteristic impedance, effective microwave index, and electrode loss have been presented as a function of geometric parameters including electrode and buffer layer thickness. Low-loss channel optical waveguides on $LiNbO_3$ were fabricated by the Ti diffusion method with $O_2$ water-vapor environment. $2.5{\mu}m$ thick electrode was successfully fabricated by double-spin image reversal process. Modulation bandwidth was limited by a resonance at 2.9 GHz and modulation bandwidth up to 2.5GHz was approxirnately measured.

• Journal of Gifted/Talented Education
• /
• v.12 no.4
• /
• pp.108-119
• /
• 2002

We have analyzed students' reports on a problem for 'adhesive tape' among ten assigned problems in the first Korean youth physics tournament (KYPT). There were five team' s reports on this problem. Students reported that the reasons for lightening coming out from the stripping down adhesive tape are caused by electrostatic friction, electron transfer, energy transformation, etc. However, only few teams suggest new creative ideas with divergent thinking during their inquiry process. Because the KYPT is a debate based on the problem solving by inquiries, the reports should include the creative investigations by their own ideas in every steps of inquiry up to their final conclusions.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
• /
• 2003.04a
• /
• pp.67-72
• /
• 2003

제주도 동부지역에 설치한 심부관측정과 지하수 개발공에 대한 시추코아 지질검층, EC 및 수온검층, 지하수위 관측, 수질조사 등의 자료를 기초로 이 지역의 지하지질 분포와 지하수 부존특성에 대해 해석을 실시하였다. 제주도 동부 해발 200m 이하지역은 평균 해수면 하 90~120m 깊이까지 투수성이 좋은 용암류 누층으로 이뤄져 있으며, 일부지역에서는 서귀포층 상부에 유리쇄 설성 각력암층(hyaloclastite breccia)이 두껍게 분포하고 있어 담수지하수의 확산과 해수의 유입이 잘 일어날 수 있는 지질상태를 이루고 있다. 특히 서귀포층은 G-H비에 의한 담.염수 경계면 분포 깊이보다 깊은 위치에 존재하고 있어 제주도 서부지역에서처럼 담수지하수를 저류 해주는 역할을 기대하기 어려운 것으로 판단된다. 제주도 동부지역 지하수체는 수직적으로 담수지하수, 저염지하수, 염수지하수로 구분할 수 있으며, Ghyben-Herzberg 원리가 적용되는 담수렌즈(기저지하수) 두께는 일반적인 G-H비 보다 훨씬 얇은 평균 1:20의 비율을 나타냈다. 담수렌즈는 대체로 해안으로부터 내륙쪽 8km 지역까지 광범위하게 발달하고 있으며, 담수렌즈 포장량은 822백만톤(공극율 5%)에서 1,970백만톤(공극율 12%)의 범위로 추정되었다.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2007.05a
• /
• pp.874-878
• /
• 2007

최근 들어 하구의 이용도가 높아져 감에 따라 용수 취수, 염해 방지공, 하도 계획, 수질 개선 등의 문제와 관련하여 하도 내로 침입하는 염수쐐기를 적절히 제어해야 될 필요성이 대두되고 있다. 이를 위해 염수쐐기의 형태와 거동특성을 정확하게 예측하고 내부유동 구조를 구명하는 것이 필요하다. 염수쐐기의 수리학적 특성에 관한 국내외 연구는 미흡한 실정이다. 국내에서는 하구에서의 유동장 해석을 위한 연구와 염수침입 현상과 반대로 담수 유출이 해양환경에 미치는 영향에 관한 해양공학적 연구가 주로 수행되었다. 국외에서는 1950년대 초반에 하구에서의 염도 혼합양상과 확산계수의 결정 및 수학적 모형에서 각 항들의 상대적 중요성 평가와 같은 기초적인 연구가 수행되었으며, 1970년대에 컴퓨터의 급속한 발전에 힘입어 다양한 수치적 기법이 개발되어 폭이 좁고 성층화된 하구에서의 수치계산이 수행되었다. 본 연구에서는 정상 염수쐐기의 형태 및 거동특성을 파악하기 위하여 수리실험을 수행하였다. 실험은 염수수조, 담수유입부, 수로부로 구성된 실험수로에서 수행되었으며, 염수와 담수간의 밀도차에 근거하여 실험 조건을 설정하였다. 실험을 통하여 밀도차와 담수유입량에 따라 염수쐐기의 형태 및 거동특성이 지배됨을 관찰할 수 있었다.

• Fire Science and Engineering
• /
• v.17 no.3
• /
• pp.20-25
• /
• 2003

Measurements with filaments and thermocouples and computations with Oppdif and FDS were carried out to investigate the impact of flame strain, agent addition, and buoyancy on the structure and extinction of nonpremixed counterflow flames. Measurements through 2.2 s drop tests in microgravity conditions and experiments in normal gravity conditions were compared with the results of computations. For the global strain rates 7 s$^{-1}$ through 100 s$^{-1}$ , the turning point behavior in the critical nitrogen concentration at O-g was confirmed. The effects of buoyancy, that is, changes in the flame curvature and thickness were also confirmed by the computations with FDS. There was agreement in the peak flame temperature and its position between the computations and the measurements in the near extinction methane/air diffusion flames in microgravity.