• 한국항공우주학회지
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• 제45권3호
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• pp.259-267
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• 2017

본 연구에서는 카이스트에서 개발된 비정렬격자 기반의 유동 해석자를 이용하여 KARI-11-180 익형, SDM과 천음속 비행체 주변 유동장에 대한 수치해석을 수행하였다. 유동장을 해석하기 위하여 RANS가 수치적으로 풀이되었으며, Roe가 제안한 FDS 방법을 사용하여 비점성 플럭스를 계산하였다. 난류 모델은 SA 모델, SST 모델, ${\gamma}-{\widetilde{Re}}_{{\theta}t}$모델이 사용되었다. KARI-11-180 익형 유동 해석 결과 천이현상을 고려하였을 때 항력 계수가 더 작게 예측되었으며, 계산된 공력 특성은 전반적으로 실험 결과와 잘 일치하였다. SDM의 경우 날개 앞전에서 유동 박리현상이 발생하였으며, 계산된 공력 특성이 EFD 결과와 유사한 경향을 보였다. 천음속 비행체의 경우 자유류 마하수가 0.9일 때 주 날개에서 발생하는 충격파를 성공적으로 포착하였으며, 실험 결과와 해석된 결과 사이의 유사성을 확인하였다.

• 한국수자원학회논문집
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• 제41권2호
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• pp.197-212
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• 2008

최근 하천의 흐름해석 분야에서는 수위 및 하상변동 양상과 오염된 지류유입으로 인한 본류에서의 유속분포 양상 및 혼합과정 등의 실제적인 문제를 해결할 수 있는 1, 2차원적 해석이 이루어지고 있으며, 이는 복잡한 하천을 균일화된 모양과 단순화된 방정식으로 일괄 적용함으로써 많은 한계점을 나타내고 있다. 본 연구에서는 서울시 관내 지방 2급 하천인 우이천 시험유역을 대상으로 하천의 물리적인 특성 변화에 따른 흐름해석을 수행하기 위하여 3차원 RANS (Reynolds Averaged Navier-Stokes Equation)를 지배방정식으로 하는 CFD (Computational Fluid Dynamics)모형인 FLOW-3D를 이용하였고, ${\kappa}-{\varepsilon}$, RNG (Renormalized Group) ${\kappa}-{\varepsilon}$, LES (Large Eddy Simulation) 등의 난류모형을 적용하여 각각의 수치모의 결과를 비교 분석 하였다. 또한, 수치 해석을 통한 교량 설치부와 하류 횡단월류형 구조물에서의 난류영향 및 유속분포, 수위 압력분포, 와류특성 등을 분석하였고, 구조물의 철거에 의한 영향을 비교하여 분석하였다. 이는 향후 도시하천의 기능을 상실한 횡단 구조물 철거에 의한 장기적인 하상변동과 토사의 퇴적, 세굴 및 수질악화 등의 영향을 파악해 볼 수 있는 기초자료로 활용될 수 있으리라 사료된다.

• 한국입자에어로졸학회지
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• 제9권2호
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• pp.103-110
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• 2013

The electrostatic precipitator (ESP) has been used for degrading atmospheric pollutants. These devices induce the electrical forces to facilitate the removal of particulate pollutants. The ions travel from the high voltage electrode to the grounded electrode by Coulomb force induced by the electric field when a high voltage is applied between two electrodes. The ions collide with gas molecules and exchange momentum with each other thus inducing fluid motion, electrohydrodynamic (EHD) flow. In this study, for the simulation of electric field and EHD flow in ESPs, an open source EHD solver, "espFoam", has been developed using open source CFD toolbox, OpenFOAM(R) (Open Field Operation and Manipulation). The electric potential distribution and ionic space charge density distribution were obtained with the developed solver, and validated with experimental results in the literature. The comparison results showed good agreement. Turbulence model is also incorporated to simulate turbulent flow; hence the developed solver can analyze laminar and turbulent flow. In distributions of electric potential and space charge, the distributions become distorted and asymmetric as the flow velocity increases. The effect of electrical drift flow was investigated for different flow velocities and the secondary flow in a flow of low velocity is successfully predicted.

• Nuclear Engineering and Technology
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• 제56권1호
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• pp.9-18
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• 2024

In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a severe accident management (SAM) strategy that has been adopted and used in many nuclear reactors such as AP1000, APR1400, and light water reactor etc. Some reactor accidents have raised concerns about nuclear reactors among residents, leading to a decrease in residents' acceptability and many studies on SAM are being conducted. Experiments on IVR-ERVC are almost impossible due to its specificity, so fluid characteristics are analyzed through BALI experiments with similar condition. In this study, computational fluid dynamics (CFD) via Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) for BALI experiments were performed. Steady-state CFD analysis was performed on three turbulence models, and SST k-ω model was in good agreement with the experimental measurement temperature within the maximum error range of 1.9%. LES CFD analysis was performed based on the RANS analysis results and it was confirmed that the temperature and wall heat flux for depth was consistent within an error range of 1.0% with BALI experiment. The LES CFD analysis results were compared with those of the Lagrangian-based solver. LES matched the temperature distribution better than SOPHIA, but SOPHIA calculated the position of boundary between stratified layer and convective layer more accurately. On the other hand, Lagrangian-based solver predicted several small eddy behaviors of the convective layer and LES predicted large vortex behavior. The vibration characteristics near the cooling part of the BALI experimental device were confirmed through Fast Fourier Transform (FFT) investigation. It was found that the power spectral density for pressure at least 10 times higher near the side cooling than near the top cooling.

• 한국안전학회지
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• 제35권3호
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• pp.96-104
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• 2020

The numerical simulations were conducted to investigate the thermal-fluid phenomena occurred inside the experimental apparatus during a PCCS, used to remove heat released in accidents from a containment of light water nuclear power plant, operation. Numerical simulations of the flow and heat transfer caused by wall condensation inside the containment simulation vessel (CSV), which equipped with 18 vertical heat exchanger tubes, were conducted using the commercial computational fluid dynamics (CFD) software ANSYS-CFX. Shear stress transport (SST) and the wall condensation model were used for turbulence closure and wall condensation, respectively. The simulation using the actual size of the apparatus. However, rather than simulating the whole experimental apparatus in consideration of the experimental cases, calculation resources, and calculation time, the simulation model was prepared only in CSV. Selective simulation was conducted to verify the effects of non-condensable gas(NC gas) concentration, CSV internal pressure, and wall sub-cooling conditions. First, as a result of the internal flow of CSV, it was observed that downward flow due to condensation occurred surface of the vertical tube and upward flow occurred in the distant place. Natural convection occurred actively around the heat exchanger tube. Due to this rising and falling internal flow, natural circulation occurred actively around the heat exchanger tubes. Next, in order to check the performance of built-in condensation model using according to the non-condensable gas concentration, CSV internal flow and wall sub-cooling, the heat flux values were compared with the experimental results. On average, the results were underestimated with and error of about 25%. In addition, the influence of CSV internal pressure and wall sub-cooling was small, but when the condensate was highly generated due to the low non-condensable gas concentration, the error was large compared to the experimental values. This is considered to be due to the nature of the condensation model of the CFX code. However, in spite of the limitations of CFD, it is valid to use the built-in condensation model of CFD for PCCS performance prediction from a conservative perspective.

• Korean Chemical Engineering Research
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• 제55권2호
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• pp.201-213
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• 2017

물이 포함된 원유는 oil separator 를 거쳐 물이 제거된다. 본 연구의 목적은 공기-물-기름 3상 혼합물에 대한 3차원 oil separator 의 분리성능을 예측하기 위하여 Eulerian 전산유체역학(CFD: computational fluid dynamics) 모델을 개발하는 것이다. 비압축성, 등온, 비정상상태 CFD 모델식은 공기상을 연속상으로, 물과 기름상을 분산상으로 정의하며, 운동량 보존식은 항력(drag force), 양력(lift force), 다공성매체 저항력을 포함한다. 또한, 난류현상으로 standard k-${\varepsilon}$ 모델이 이용된다. 물과 기름 출구압은 oil separator 의 액위를 결정하는 중요한 인자이며, 정상운전상태 액위 25 cm를 맞추기 위하여 측정압은 각각 6.3 kPa, 5.1 kPa으로 결정되었다. 시간에 따른 공기, 물, 기름의 부피분율의 변화를 조사하였고, 정상상태에 도달하였을 때, 물과 기름상의 침강속도를 oil separator의 종축 길이에 따라 분석하였다. 본 연구에서 제시된 CFD 모델로부터 얻은 oil separator의 기름분리성능은 99.85%이며, 실험값과 거의 일치하였다. 비교적 단순한이 CFD 모델은향후 oil separator의구조를 변경하거나, 최적운전조건을 찾기위하여 유용하게사용될수있을 것이다.

• Nuclear Engineering and Technology
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• 제39권4호
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• pp.327-336
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• 2007

The influence of density differences on the mixing of the primary loop inventory and the Emergency Core Cooling (ECC) water in the downcomer of a Pressurised Water Reactor (PWR) was analyzed at the ROssendorf COolant Mixing (ROCOM) test facility. ROCOM is a 1:5 scaled model of a German PWR, and has been designed for coolant mixing studies. It is equipped with advanced instrumentation, which delivers high-resolution information for temperature or boron concentration fields. This paper presents a ROCOM experiment in which water with higher density was injected into a cold leg of the reactor model. Wire-mesh sensors measuring the tracer concentration were installed in the cold leg and upper and lower part of the downcomer. The experiment was run with 5% of the design flow rate in one loop and 10% density difference between the ECC and loop water especially for the validation of the Computational Fluid Dynamics (CFD) software ANSYS CFX. A mesh with two million control volumes was used for the calculations. The effects of turbulence on the mean flow were modelled with a Reynolds stress turbulence model. The results of the experiment and of the numerical calculations show that mixing is dominated by buoyancy effects: At higher mass flow rates (close to nominal conditions) the injected slug propagates in the circumferential direction around the core barrel. Buoyancy effects reduce this circumferential propagation. Therefore, density effects play an important role during natural convection with ECC injection in PWRs. ANSYS CFX was able to predict the observed flow patterns and mixing phenomena quite well.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2011년 춘계학술대회논문집
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• pp.210-217
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• 2011

Recently, research on evaluating thermal comfort by using CFD has been vigorously active. This research evaluates not only distribution of temperature and air flow analysing but also thermal comfort in indoor space by applying human model. But research of human model's shape, Grid characteristic and turbulence model has not yet been studied. In this paper, human model's shape, physical characteristic of variable Grid, and change of turbulence model has been studies by CFD. In this study. FLUENT is used for analysis and PMV(predicted Mean Vote), PPD(Predicted Percentage Dissatisfied) and EHT(Equivalent Homogeneous Temperature} are used for evaluation and comparison of thermal comfort. As a result, it shows that shape of CSP and lattice features does not affect on global flow field or evaluation on PMV, PPD. However, it demonstrates more precise result from evaluation of thermal comfort by equivalent temperature when it used detailed human model considering prism grid.

• Wind and Structures
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• 제24권5호
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• pp.465-480
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• 2017

Modelling an equilibrium atmospheric boundary layer (ABL) in computational wind engineering (CWE) and relevant areas requires the boundary conditions, the turbulence model and associated constants to be consistent with each other. Among them, the inflow boundary conditions play an important role and determine whether the equations of the turbulence model are satisfied in the whole domain. In this paper, the idea of modeling an equilibrium ABL through specifying proper inflow boundary conditions is extended to the SST $k-{\omega}$ model, which is regarded as a better RANS model for simulating the blunt body flow than the standard $k-{\varepsilon}$ model. Two new sets of inflow boundary conditions corresponding to different descriptions of the inflow velocity profiles, the logarithmic law and the power law respectively, are then theoretically proposed and numerically verified. A method of determining the undetermined constants and a set of parameter system are then given, which are suitable for the standard wind terrains defined in the wind load code. Finally, the full inflow boundary condition equations considering the scale effect are presented for the purpose of general use.

• Wind and Structures
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• 제31권4호
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• pp.351-362
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• 2020

In this study, the impact of roof slope on the flow characteristics over low-sloped gable roofs was investigated using steady computational fluid dynamics (CFD) simulations based on a k-ω SST turbulence model. A measurement database of the flow field over a scaled model of 15° was created using particle image velocimetry (PIV). Sensitivity analyses for the grid resolutions and turbulence models were performed. Among the three common Reynolds-averaged Navier-Stokes equations (RANS) models, the k-ω SST model exhibited a better performance, followed by the RNG model and then the realizable k-ε model. Next, the flow properties over the differently sloped (0° to 25°) building models were determined. It was found that the effect of roof slope on the flow characteristics was identified by changing the position and size of the separation bubbles, 15° was found to be approximately the sensitive slope at which the distribution of the separation bubbles changed significantly. Additionally, it is suggested additional attention focused on the distributions of the negative pressure on the windward surfaces (especially 5° and 10° roofs) and the possible snow redistribution on the leeward surfaces.