• 한국습지학회지
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• 제10권2호
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• pp.67-79
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• 2008

하천에서의 부정류 해석을 위해서 1차원 유한차분법(FDM)인 Abbott-Ionescu scheme과 2차원 유한체적법(FVM)인 근사의 Riemann solver(Osher scheme)에 대하여 살펴보았다. 두 모형은 직선 하도, 약간 굽어진 사행하도 및 사행하도에서의 흐름 문제들에 적용되었으며 결과의 비교는 균일한 직사각형 수로에 대하여 이루어졌다. 하천의 복잡한 형상의 표현하기 위해서는 이를 고려할 수 있는 유한체적법을 이용하였다. 유한차분법과 유한체적법 결과는 수위 및 유량 수문곡선에 대하여 매우 만족스러운 것으로 나타났다. 균일한 직선하도에 대해서는 1차원분석으로도 충분하다는 사실을 파악할 수 있었으며, 사행하도의 경우 흐름을 정확하게 모형화하기 위해서는 2차원 또는 3차원 모형을 사용하여야 할 것이다.

• Wind and Structures
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• 제12권1호
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• pp.21-47
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• 2009

The logarithmic form for turbulent flow analysis guarantees the positivity of the turbulence variables as ${\kappa}$ and ${\varepsilon}$ of the ${\kappa}-{\varepsilon}$ model by using the natural logarithm of these variables. In the present study, the logarithmic form is incorporated into the finite element solution procedure for the unsteady turbulent flow analysis. A backward facing step flow using the standard ${\kappa}-{\varepsilon}$ model and a flow around a 2D square cylinder using the modified ${\kappa}-{\varepsilon}$ model (the Kato-Launder model) are simulated. These results show that the logarithmic form effectively keeps adequate balance of turbulence variables and makes the analysis stable during transient or unsteady processes.

• Nuclear Engineering and Technology
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• 제53권5호
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• pp.1446-1453
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• 2021

In the past, several experimental investigations aiming at characterizing the natural circulation (NC) behavior in test facilities were carried out. They showed a variety of flow patterns characterized by an inverted U-shape of the NC flow curve versus primary mass inventory. On the other hand, attempts to reproduce such curves using thermal-hydraulic system codes, showed 10-30% differences between the measured and calculated NC mass flow rate. Actually, the used computer codes are generally based upon nodalization using single U-tube representation. Such model may not allow getting accurate simulation of most of the NC phenomena occurring during such tests (like flow redistribution and flow reversal in some SG U-tubes). Simulations based on multi-U-tubes model, showed better agreement with the overall behavior, but remain unable to predict NC phenomena taking place in the steam generator (SG) during the experiment. In the current study, the CATHARE code is considered in order to assess a NC characterization test performed in the four loops PKL facility. For this purpose, four different SG nodalizations including, single and multi-U-tubes, 1D and 3D SG inlet/outlet zones are considered. In general, it is shown that the 1D and 3D models exhibit similar prediction results up to a certain point of the rising part of the inverted U-shape of the NC flow curve. After that, the results bifurcate with, on the one hand, a tendency of the 1D models to over-predict the measured NC mass flow rate and on the other hand, a tendency of the 3D models to under-predict the NC flow rate.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2019년도 학술발표회
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• pp.158-158
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• 2019

Recent studies show the possibility of more frequent extreme events as a result of the changing climate. These weather extremes, such as excessive rainfall, result to debris flow, river overflow and urban flooding, which post a substantial threat to the community. Therefore, an effective flood model is a crucial tool in flood disaster mitigation. In recent years, a number of flood models has been established; however, the major challenge in developing effective and accurate inundation models is the inconvenience of running multiple models for separate conditions. Among the solutions in recent researches is the development of the combined 1D-2D flood modeling. The coupled 1D-2D river flood modeling allows channel flows to be represented in 1D and the overbank flow to be modeled over two-dimension. To test the efficiency of this approach, this research aims to assess the capability of HEC-RAS model's implementation of the combined 1D-2D hydraulic simulation of river overflow inundation, and compare with the results of GERIS and FLUMENS 2D flood model. Results show similar output to the flood models that had used different methods. This proves the applicability of the HEC-RAS 1D-2D coupling method as a powerful tool in simulating accurate inundation for flood events.

• 한국산학기술학회논문지
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• 제20권4호
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• pp.580-589
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• 2019

인도네시아는 전력 공급이 매우 부족한 현실에 처해 있으며, 이에 대한 해결책으로 화력발전소 건설 계획이 증가하고 있다. 화력발전소는 발전에 수반되는 엔진과 장비의 과열을 식히기 위해 냉각수 계통을 필요로 하며, 냉각수 계통 중 순환수 취수펌프장(circulating water pump chamber)은 일반적으로 ANSI (1998) 기준에 따라 설계된다. 본 연구에서는 인도네시아 K-석탄화력발전소의 순환수취수펌프장이 현장 여건상 ANSI (1998)의 확산각 설계기준 $20^{\circ}$를 만족시킬 수 없어, 수리적으로 안정된 흐름 및 구조물이 되도록 3차원 수치모형실험을 수행하였다. 수치모형으로 Flow-3D 모형을 이용하였다. Flow-3D 모형의 적용성을 검토하기 위해 Rodi (1997)의 사각형 구조물 주변에 형성되는 흐름 연구 결과와 금회 수치해석 결과를 비교하였다. 수치해석에서 도출된 종방향 유속 분포는 잘 일치함을 보여주고 있다. 순환수취수펌프장 내 설계유속을 만족시키기 위해 유속 저감에 유리한 사각형 형태의 배플을 적용하였다. 순환수취수펌프장으로 유입되는 유속이 1.5 m/s ~ 2.5 m/s로 분포되는 경우, 배플에서 분리흐름의 각도는 약 $15^{\circ}{\sim}20^{\circ}$로 발생하였다. 이를 고려하여 분리흐름 각도 이하로 하류에 배플을 배치함으로 Inlet bay 설계유속 0.5 m/s 이하를 만족시켰다.

• 한국지열·수열에너지학회논문집
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• 제15권3호
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• pp.7-13
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• 2019

To improve the flow distribution at channel locations in the welded plate heat exchanger with "L"-type inflow, the flow visualization of Model 1 was carried out. Besides, the characteristics of flow distribution was investigated experimentally according to the header shape. The inlet flow rate for each channel location was increased at the side channels but decreased at the central channels. In the case of Model 2, which has a slant structure added to the basic header of Model 1, the unevenness of inlet flow increased by 23% from 0.019 to 0.023 as compared to Model 1. On the other hand, Model 3, which has a baffle structure added to Model 2, showed 0.064 unevenness in inlet flow, which was a 36% reduction one compared to Model 1. To improve the distribution at each channel in the welded plate heat exchanger with "L"-type flow, it is necessary to improve the header external shape for the guide of flow as well as the baffle structure for reduction of vortex flow.

• Nuclear Engineering and Technology
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• 제31권6호
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• pp.572-585
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• 1999

The characteristics of two-phase flow instability in a vertical boiling channel connecting with an unheated riser are investigated through the linear stability analysis model. Various two-phase flow models, including thermal non-equilibrium effects, are taken into account for establishing a physical model in the time domain. A classical approach to the frequency response method is adopted for the stability analysis by employing the D-partition method. The adequacy of the linear model is verified by evaluating experimental data at high quality conditions. It reveals that the flow-pattern-dependent drift velocity model enhances the prediction accuracy while the homogeneous equilibrium model shows the most conservative predictions. The characteristics of density wave oscillations under low-power and low-quality conditions are investigated by devising a simple model which accounts for the gravitational and frictional pressure losses along the channel. The necessary conditions for the occurrences of type-I instability and flow excursion are deduced from the one-dimensional D-partition analysis. The parametric effects of some design variables on low quality oscillations are also investigated.

• 한국수자원학회논문집
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• 제55권1호
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• pp.33-42
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• 2022

본 연구에서는 3차원 상용 수치모형 Flow-3D를 활용하여 상류 수위(유입유량) 변동, 격벽의 간격(pool의 길이) 변화, 잠공 유무 등에 따른 아이스 하버 어도 내 흐름 양상을 분석하였다. 수치모의 결과의 적합성을 입증하기 위해 경안천 대곡교 하류에 설치된 실제 어도에서 3차원 초음파 유속계를 사용하여 어도 출구부 단면 유속과 유량을 관측하였다. Flow-3D 모형에는 난류 모듈로 4가지를 선택할 수 있는데, 관측 자료로 검증 결과 RNG 모형이 아이스하버 어도 내 흐름 특성을 가장 잘 재현하였다. 하천의 유량 전량이 어도로만 유입되어 흐른다는 조건에서 수위 변화에 따른 어도 내 유속구조를 모의해 보았다. 그 결과 최저수위에서는 잠입류와 표면류가 혼재하여 발생하였는데, 최저수위에서 약 10 cm 이상만 수위가 상승하여도 모든 pool에서 잠입류가 사라지고 오직 표면류만 발생하였다. 예상과 달리 수위가 조금 상승해도 흐름이 꽤 단순하게 표면류 위주로 발생하였다. 격벽간 간격을 늘려주면 수위가 상승하더라도 잠입류와 표면류가 혼재되어 나타나는 현상이 지속되었다. 그리고 격벽 하단에 잠공이 있는 경우가 없는 경우에 비해 잠입류가 여러 pool에서 생성되는 경향을 보였다. 보다 적극적인 어도 사후 관리로 잠공 폐색을 막아 어도 내 다양한 흐름이 생성될 수 있도록 유도하는 것이 필요해 보인다.

• Geomechanics and Engineering
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• 제35권1호
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• pp.13-27
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• 2023

The model production for large-scale (lateral length ≥ 2.0 m) capillary barrier (CB) model tests is time and cost-intensive. To address these limitations, the framework of a small-scale CB (SSCB) model test under the lateral no-flow condition has been established. In this study, to validate the experimental methodology of the SSCB model test, a series of seepage analyses on the SSCB model test and engineered slopes in the same and additional test conditions was performed. First, the seepage behavior and diversion length (L_D) of the CB system were investigated under three rainfall conditions. In the seepage analysis for the engineered slopes with different slope angles and sand layer thicknesses, the L_D increased with the increase in the slope angle and sand layer thickness, although the increase rate of the L_D with the sand layer thickness exhibited an upper limit. The L_D values from the seepage analysis agreed well with the results estimated from the laboratory SSCB mode test. Therefore, it can be concluded that the experimental methodology of the SSCB model test is one of the promising alternatives to efficiently evaluate the water-shielding performance of the CB system for an engineered slope.

• 한국자동차공학회논문집
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• 제17권2호
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• pp.10-19
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• 2009

The working of diesel particulate filters(DPF) needs to periodically burn soot that has been accumulated during loading of the DPF. The prediction of the relation between an uniformity of gas velocity and soot regeneration efficiency with simulations helps to make design decisions and to shorten the development process. This work presents a comprehensive combined 'DOC+CDPF' model approach. All relevant behaviors of flow fluid are studied in a 3D model. The obtained flow fields in the front of DPF is used for 1D simulation for the prediction of the thermal behavior and regeneration efficiency of CDPF. Validation of the present simulation are performed for the axial and radial direction temperature profile and shows goods agreement with experimental data. The coupled simulation of 3D and 1D shows their impact on the overall regeneration efficiency. It is found that the flow non-uniformity may cause severe radial temperature gradient, resulting in degrading regeneration efficiency.