• 천문학회지
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• 제49권1호
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• pp.1-8
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• 2016

The aim of this study is to describe the physical processes taking place in the solar photosphere. Based on 3D hydrodynamic simulations including a detailed radiation transfer scheme, we investigate thermodynamic structures and radiation fields in solar surface convection. As a starting model, the initial stratification in the outer envelope calculated using the solar calibrations in the context of the standard stellar theory. When the numerical fluid becomes thermally relaxed, the thermodynamic structure of the steady-state turbulent flow was explicitly collected. Particularly, a non-grey radiative transfer incorporating the opacity distribution function was considered in our calculations. In addition, we evaluate the classical approximations that are usually adopted in the onedimensional stellar structure models. We numerically reconfirm that radiation fields are well represented by the asymptotic characteristics of the Eddington approximation (the diffusion limit and the streaming limit). However, this classical approximation underestimates radiation energy in the shallow layers near the surface, which implies that a reliable treatment of the non-grey line opacities is crucial for the accurate description of the photospheric convection phenomenon.

• Ocean Systems Engineering
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• 제13권3호
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• pp.269-286
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• 2023

Autonomous Underwater Gliders (AUGs) are a type of Underwater Vehicles that move without the help of a standard propeller. Gliders use buoyancy engines to vary their weight or buoyancy and traverse with the help of the Lift and Drag forces developed from the fuselage and the wings. The Lift and Drag Coefficients, also called Hydrodynamic coefficients (HDCs) play a major role in glider dynamics. This paper examines the effect of the different types of glider fuselages based on the bodies of revolution (BOR) of NACA sections. The HDCs of the glider fuselages are numerically estimated at a low-speed regime (10⁵ Reynolds Number) using Computational Fluid Dynamics (CFD). The methodology is validated using published literature, and the results of CFD are discussed for possible application in the estimation of glider turning motion.

• Ocean Systems Engineering
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• 제4권3호
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• pp.185-200
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• 2014

The objective of the present simulations is to evaluate the applicability of the standard $k-{\varepsilon}$ turbulence model in engineering practice in the subcritical to supercritical flow regimes. Two-dimensional numerical simulations of flow around a circular cylinder at $Re=1{\times}10^5$, $5{\times}10^5$ and $1{\times}10^6$, had been performed using Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with the standard $k-{\varepsilon}$ turbulence model. Solution verification had been studied by evaluating grid and time step size convergence. For each Reynolds number, several meshes with different grid and time step size resolutions were chosen to calculate the hydrodynamic quantities such as the time-averaged drag coefficient, root-mean square value of lift coefficient, Strouhal number, the coefficient of pressure on the downstream point of the cylinder, the separation angle. By comparing the values of these quantities of adjacent grid or time step size resolutions, convergence study has been performed. Solution validation is obtained by comparing the converged results with published numerical and experimental data. The deviations of the values of present simulated quantities from those corresponding experimental data become smaller as Reynolds numbers increases from $1{\times}10^5$ to $1{\times}10^6$. This may show that the standard $k-{\varepsilon}$ model with enhanced wall treatment appears to be applicable for higher Reynolds number turbulence flow.

• Journal of Advanced Research in Ocean Engineering
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• 제4권2호
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• pp.86-95
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• 2018

The fluid flow over structures has been widely investigated by many researchers because its extensive application in offshore structures, skyscrapers, chimneys and cooling towers, brides. In the viewpoint of reducing the drag for offshore structure, it becomes challenging problem in the field of hydrodynamic of offshore structure. The purpose of this study is to investigate a flow over a square cylinder with an attached splitter plate using RANS method. First, RANS turbulent models such as a standard $k-{\omega}$ model, SST $k-{\omega}$ model, RNG $k-{\varepsilon}$ model, realizable $k-{\varepsilon}$ model, standard $k-{\varepsilon}$ model were used for choosing suitable turbulent model which has the best agreement with available experimental result. Drag of single cylinder estimated by using standard $k-{\omega}$ has a good agreement with published experimental result. Therefore, the stand $k-{\omega}$ was selected for simulation for flow over a square cylinder with an attached plate. Second, the numerical results of drag of square cylinder with an attached splitter plate in various length of an attached plate were performed using RANS method in ANSYS Fluent. In this paper, the numerical simulations were conducted at a Reynolds number of 485 and the thickness of the splitter plate is chosen as a constant value about 10% of cylinder width. The numerical results of drag coefficient of square cylinder are compared with experimental result published by other researchers. Finally, the effect of the splitter plate attached to the rear side of the square cylinder has been investigated numerically with a focus on the drag coefficient and flow characteristic. As a result, the drag coefficient decreases with an increase in splitter plate length.

• 대한환경공학회지
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• 제31권5호
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• pp.364-370
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• 2009

정수처리공정에서 안정적인 탁도 관리가 날이 갈수록 중요해지고 있다. 따라서 침전 효율 향상을 위한 플록형성 공정의 최적화는 매우 중요하다. 우리는 전산유체역학적 방법을 이용하여 플록큐레이터의 회전방향(시계방향, 반시계방향)에 따른 플록 형성지 내 유동특성을 평가하였다. 평가 결과, 플록큐레이터가 시계방향으로 회전하는 경우는 플록큐레이터 회전방향과 수류 흐름이 일치하는 수표면 쪽에서 유동이 강해지고 유출량의 분산 및 표준편차는 각각 8.5, 2.9로 나타났다. 반면에 플록큐레이터가 반시계 방향으로 회전하는 경우는 플록큐레이터 회전방향과 수류 흐름이 일치하는 바닥 쪽에서 유동이 강해지고 유출량의 분산 및 표준편차는 각각 5.3, 2.3으로 나타났다. 또한 플록형성지 유출량은 1,2단 플록큐레이터 회전방향보다는 3단 플록큐레이터의 회전방향에 의해 주로 영향을 받으며 3단 플록큐레이터가 반시계방향으로 회전하는 경우가 시계방향으로 회전하는 경우보다 유출량의 편차가 적기 때문에 침전지의 균등한 흐름을 위해 유리하다.

• 한국수자원학회논문집
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• 제37권5호
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• pp.363-373
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• 2004

본 연구에서는 교각이 설치된 미공병단의 Waterways Experiment Station (WES)의 표준 월류형 여수로에 대한 동수역학적인 흐름거동을 FLOW-3D를 이용하여 해석하였다. 모의 결과는 WES의 실험자료와 비교하였으며, 교각이 없는 월류형 여수로에 대한 2차원 모의결과와도 상호 비교하였다. 특히 교각이 설치된 월류형 여수로에서의 월류수맥과 압력분포에 대한 특성을 상세히 고찰하였다. 유량, 월류수맥 및 압력분포에 대한 모의결과, 부분적으로 WES의 실험결과와 차이가 발생하는 곳이 있으나, 대부분 그 경향이 일치하였다. 교각이 설치된 월류형 여수로에서의 주요 흐름거동을 정리하면, 첫째 교각이 위치하는 여수로 측면부에서의 수위에 비해 여수로 중앙부에서의 수위가 높고, 둘째 실제수두가 여수로의 설계수두보다 높은 경우 웨어마루 상류에서는 여수로 측면부에서 부압이 크게 발생하며 웨어마루 하류에서는 여수로 중앙부에서 부압이 크게 발생하는 것으로 밝혀졌다.

• 기술사
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• 제27권6호
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• pp.72-84
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• 1994

This is about an environmental study of groundwater interference by hydrodynamic dispersion between the well A and well C in Dongsam-dong, the Yeongdo Island, Pusan, Korea. The groundwater in the study wells come from the fracture zones deeply seated in welded lapilli rhyodacitic tuff of the Late Cretaceous Yucheon Group. The boring depth at the well A is 190 meters, and the optimal pumping rate of the well A is about 100 cubic meters per day therein. The fractured aquifers in impermeable welded tuff show the conjugate fracturing type and are of anisotropic. The aquifers along two fracture zones in the well A are 80 and 100 meters in depth, respectively. It is not suggested that those fractured aquifers are simply connected between the well A and C. The sea level fluctuation by ebb and high tides in a day is not effective to the groundwater table in the well A. The pumping for 15 days at the well A doesn't give rise to any changes of the groundwater levels in the neighbor well C. The radius of influence of the well A is measured as less than 200 meters. The measuring electric conductivity for the test of salt tracer doesn't testify any relationship between the well A and the well C. There is the main difference between the well A and the well C on the basis of the water analysis of those wells. the well A is located in the high content zone of salt much over the standard value for drinking, whereas the other wells B. C. D are in the low content area of salt below the standard value. It is elucidated for the high content zone of salt in Yeongdo around the well A to have been uplifted over 20 meters.

• 한국해양공학회지
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• 제35권3호
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• pp.173-182
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• 2021

The maneuvering performance of a ship on the actual sea is very different from that in calm water due to wave-induced motion. Enhancement of a ship's maneuverability in waves at the design stage is an important way to ensure that the ship navigates safely. This paper focuses on the maneuvering prediction of a Russian trawler in wind and irregular waves. First, a unified seakeeping and maneuvering analysis of a Russian trawler is proposed. The hydrodynamic forces acting on the hull in calm water were estimated using empirical formulas based on a database containing information on several fishing vessels. A simulation of the standard maneuvering of the Russian trawler was conducted in calm water, which was checked using the International Maritime Organization (IMO) standards for ship maneuvering. Second, a unified model of seakeeping and maneuvering that considers the effect of wind and waves is proposed. The wave forces were estimated by a three-dimensional (3D) panel program (ANSYS-AQWA) and used as a database when simulating the ship maneuvering in wind and irregular waves. The wind forces and moments acting on the Russian trawler are estimated using empirical formulas based on a database of wind-tunnel test results. Third, standard maneuvering of a Russian trawler was conducted in various directions under wind and irregular wave conditions. Finally, the influence of wind and wave directions on the drifting distance and drifting angle of the ship as it turns in a circle was found. North wind has a dominant influence on the turning trajectory of the trawler.

• 상하수도학회지
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• 제18권4호
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• pp.500-507
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• 2004

This study was conducted to evaluate the equity of the flow distribution from rapid mixing basin to the flocculation basins. Also, several types of inlet structures of the open channel affecting the flow pattern and distribution trend were studied using Computational Fluid Dynamics (CFD) simulation. For investigating the factual phenomena in distribution channel, we selected a certain domestic water treatment plant with capacity of $361,000m^3/d$. From the measurements of flow discharge, it is investigated that this existing inlet geometry resulted in significant inequitable distribution. The both largest deviations in the basins and rows were over 10%. In order to reduce the these deviation, this study suggested installing a baffle against the influent, and showed the effectiveness which the largest deviation was less than 3%. Also, it was concluded that the existing design method of open channel could be improved by three-dimensional hydrodynamic analysis for optimizing the even flow.

• 대한조선학회논문집
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• 제37권3호
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• pp.99-109
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• 2000

본 연구에서는 대파고 파랑 중을 항해하는 선박의 슬래밍 충격에 대한 선체 전체의 동적 탄성응답 해석법을 개발하였다. 선체구조는 전단효과를 고려하는 박판보 유한요소이론을 활용하였으며, 선체 각 단면에 작용하는 유체력은 통상의 선형 운동체 이론에 덧붙여 물체 경계의 비선형성을 고려하여 추정하였다. 즉 매 순간 선체와 파 입자간의 접수 형상을 고려하는 비선형 유체력 추정법을 모멘텀 슬래밍 이론에 근거하여 정식화하였다. 개발된 해석법의 검증을 위해 V형 단면 선형과 S-175 선형 모델을 대상으로 수치해석을 수행하였다. 시간 영역에 있어 각 단면에서의 파면에 대한 상대 변위 성분과 속도 성분들을 계산하였으며, 선체 중앙 단면에서의 굽힘 모멘트 값의 시간이력을 검토하였다.