• Journal of the Society of Naval Architects of Korea
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• v.51 no.1
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• pp.58-66
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• 2014

Numerical simulations for the flows containing free surface remain difficult problems because the drastic differences of physical properties of water and air, The difference of densities makes the solution instable in particular. For the stabilities of the solutions, the most typical methods to simulate free surface flows, such as Volume Of Fluid(VOF) and Level-Set(LS) methods, impose transient zones where the physical prosperities are continuously distributed. The thickness of the transient zone is the source of the numerical errors. The other side, marker-density method does not use such a transient zone. In the traditional marker-density method, however, the air velocities of free surface cells are extrapolated from the water velocity, and the pressures on the free surface are extrapolated from the air pressures for the stability of the solution. In this study, the marker-density method is modified for the decrease of such numerical errors. That is, the pressure on the free surface is determined to coincide with the pressure gradient terms of the governing equations, and the velocity of free surface cells are calculated with the governing equations. Two-dimensional steady spilling breakers behind of a submersed hydrofoil and three-dimensional spilling breaker near a wedge shaped ship model are simulated using INHAWAVE-II including the modified marker-density(MMD) method. The results are compared with the results of Fluent V6.3 including VOF method and several published research results.

• Wind and Structures
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• v.4 no.3
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• pp.177-196
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• 2001

At present the most popular turbulence models used for engineering solutions to flow problems are the $k-{\varepsilon}$ and Reynolds stress models. The shortcoming of these models based on the isotropic eddy viscosity concept and Reynolds averaging in flow fields of the type found in the field of Wind Engineering are well documented. In view of these shortcomings this paper presents the implementation of a non-linear model and its evaluation for flow around a building. Tests were undertaken using the classical bluff body shape, a surface mounted cube, with orientations both normal and skewed at $45^{\circ}$ to the incident wind. Full-scale investigations have been undertaken at the Silsoe Research Institute with a 6 m surface mounted cube and a fetch of roughness height equal to 0.01 m. All tests were originally undertaken for a number of turbulence models including the standard, RNG and MMK $k-{\varepsilon}$ models and the differential stress model. The sensitivity of the CFD results to a number of solver parameters was tested. The accuracy of the turbulence model used was deduced by comparison to the full-scale predicted roof and wake recirculation zone lengths. Mean values of the predicted pressure coefficients were used to further validate the turbulence models. Preliminary comparisons have also been made with available published experimental and large eddy simulation data. Initial investigations suggested that a suitable turbulence model should be able to model the anisotropy of turbulent flow such as the Reynolds stress model whilst maintaining the ease of use and computational stability of the two equations models. Therefore development work concentrated on non-linear quadratic and cubic expansions of the Boussinesq eddy viscosity assumption. Comparisons of these with models based on an isotropic assumption are presented along with comparisons with measured data.

• Wind and Structures
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• v.15 no.4
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• pp.331-343
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• 2012

Thunderstorm downbursts are responsible for numerous structural failures around the world. The wind characteristics in thunderstorm downbursts containing vortex rings differ with those in 'traditional' boundary layer winds (BLW). This paper initially performs an unsteady-state simulation of the flow structure in a downburst (modelled as a impinging jet with its diameter being $D_{jet}$) using a computational fluid dynamics (CFD) method, and then analyses the pressure distribution on a solar updraft tower (SUT) in the downburst. The pressure field shows agreement with other previous studies. An additional pair of low-pressure region and high-pressure region is observed due to a second vortex ring, besides a foregoing pair caused by a primary vortex ring. The evolutions of pressure coefficients at five orientations of two representative heights of the SUT in the downburst with time are investigated. Results show that pressure distribution changes over a wide range when the vortices are close to the SUT. Furthermore, the fluctuations of external static pressure distribution for the SUT case 1 (i.e., radial distance from a location to jet center x=$D_{jet}$) with height are more intense due to the down striking of the vortex flow compared to those for the SUT case 2 (x=$2D_{jet}$). The static wind loads at heights z/H higher than 0.3 will be negligible when the vortex ring is far away from the SUT. The inverted wind load cases will occur when vortex is passing through the SUT except on the side faces. This can induce complex dynamic response of the SUT.

• Plant Journal
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• v.12 no.3
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• pp.24-31
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• 2016

Wind-induced vibration is a phenomenon that a struture is oscillated due to wind force such as buffeting, vortex shedding wake and etc., which is one of important characteristics to be considered for design in case that stack has significant slenderness ratio or low natural frequency. International design standards of stack define several criteria for evaluating the suitability of stack design, which describe the required design considerations for each range of design parameters and provide the instruction to verify the stack design against wind-induced vibration simply. However, there is a limitation that they cannot provide quantitative information in case code requirement cannot be satisfied due to constraints of plant space or economical design. In order to overcome the limiation of code, integrated numerical analysis of computational fluid dynamics, harmonic analysis and finite element analysis were proposed to investigate wind-induced vibration for multiple stacks in actual plant. Simulated results of mutual wake interference effect between adjacent stacks were evaluated and compared to the criteria in international standards.

• Journal of the Society of Naval Architects of Korea
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• v.55 no.3
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• pp.215-221
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• 2018

The structural design of offshore installations against explosions has been required to protect vital areas (e.g. control room, worker's area etc.) and minimize the damage from explosion accidents. Because the explosion accident will not only result in significant casualties and economic losses, but also cause serious pollution and damage to surrounding environment and coastal marine ecosystems. Over the past two decades, an incredible efforts was made to develop reliable methods to reduce and manage the explosion risk. Among the methods Quantitative Risk Assessment and Management (QRA&M) is the one of cutting-edge technologies. The explosion risk can be quantitatively assessed by the product of explosion frequency based on probability calculation and consequence analyzed using computer simulations, namely Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). However to obtain reliable consequence analysis results by CFD and FEA, uncertainties associate with modeling and simulation are needed to be identified and validated by comparison with experimental data. Therefore, large-scaled explosion test procedure is developed in this study. And developed test procedure can be helpful to obtain precious test data for the validation of consequence analysis using computer simulations, and subsequently allow better assessment and management of explosion risks.

• Journal of Korea Water Resources Association
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• v.44 no.6
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• pp.439-447
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• 2011

This study aims to test the feasibility of combined use of EFDC (Environmental Fluid Dynamics Code) hydrodynamic model and WASP7.3 (Water Quality Analysis Program) model to improve accuracy of water quality predictions of the Yongdam Lake, Korea. The orthogonal curvilinear grid system was used for EFDC model to represent riverine shape of the study area. Relationship between volume, surface and elevation results were checked to verify if the grid system represents morphology of the lake properly. Monthly average boundary water quality conditions were estimated using the monthly monitored water quality data from Korean Ministry of Environment DB system. Monthly tributary flow rates were back-routed using dam discharge data and allocated in proportion to each basin area as direct measurements were not available. The optimum number of grid system was determined to be 372 horizontal cells and 10 vertical layers of the site for 1 year simulation of hydrodynamics and water quality out of iterative trials. Monthly observed BOD, TN, TP and Chl-a concentrations inside the lake were used for calibration of WASP7.3 model. This study shows that EFDC and WASP can be used in series successfully to improve accuracy in water quality modeling. However, it was observed that the amount of data to develop inflow water quality and flow rate boundary conditions and water quality data inside lake for calibration were not enough for accurate modeling. It is suggested that object-oriented data collection systems would be necessary to ensure accuracy of EFDC-WASP model application and thus for efficient lake water quality management strategy development.

• Proceedings of the Korea Water Resources Association Conference
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• 2010.05a
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• pp.952-956
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• 2010

저수지에 설치된 물순환장치는 저수지 수문상황과 자연조건의 변화에 따라 유동특성이 상이하므로 연구자가 고려하는 다양한 조건에서 현장 측정하기는 불가능하다. 이런 문제를 대체하는 방안으로 전산유체유동(CFD) 모형을 적용한 모사를 실시함으로서 다양한 조건에 따른 효과를 평가할수 있게 된다. 본 연구에서는 전산유체유동을 통한 대류식 물순환장치의 유동영향범위와 수질변화 등을 평가하고, 다양한 조건에서 모사를 실시하여 최적운영방안을 도출하고 실제운영에 활용토록 하고자 한다. 수체거동을 모사하기 위해 실제 저수지를 형상화한 Domain을 3가지로 구성하였다. 첫번째는 반경 20m, 깊이 40m Domain에 물순환장치를 중앙에 설치한 것이며(D20), 두 번째는 반경 40m, 깊이 40m에 두 개의 물순환장치를 양쪽에 설치하였고(D40), 세 번째는 반경 100m, 깊이 40m로 설정(D100)하였고 양쪽에 두 개의 물순환장치를 설치한 것으로 구성하였다. CFD에 의한 개별 대류식 순환장치의 유동모사결과 D20은 시간의 경과에 따라 수온성층이 하강하는 현상이 나타났으며 이러한 결과로 판단할때 40m 간격으로 대류식 순환장치를 설치하여 운영하면 탈성층이 나타날 수 있을것으로 판단되었다. D40에 대하여 CFD에 의한 유동모사를 실시하였으며 시간의 경과에 따라 수온성층의 변화는 나타나지 않았다. 이러한 결과로 판단할때 40m 간격으로 한줄로 대류식 순환장치를 설치하여 운영하면 성층을 깨는 현상은 나타나지 않을 것으로 평가되었다. 반면 Dye테스트시 심층에서 상승한 수체는 수온성층 표면에서 수평방향으로 계속 퍼져 나가면서 옆장치에서 상승된 수체와 혼합이 활발히 이루어 지는 현상을 나타내었다. 장치간 거리가 100m인 대류식 물순환장치에 의한 유동모사시 수온성층의 변화는 전혀 나타나지 않았으며, Dye테스트시 심층에서 상승한 수체는 수온성층 위에서 수평방향으로 퍼져 나가면서 옆에서 가동되는 장치에서 상승된 수체와 혼합 현상을 나타내기는 하나 D40보다 혼합시간이 더 걸리는 것으로 나타났다.

• KIEAE Journal
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• v.5 no.3
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• pp.11-16
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• 2005

The recent trends of high-density and high-rise in apartment housing have caused the problems of decrease in ventilation rates and increase of indoor pollutant contaminants. SHS(Sick House Syndrome) has now become a major issue and threats the health of residents. To solve these indoor air problems, increase in ventilation rate is considered as one of the most efficient approach. Thus, the recent housing development is pursuing improvement in the site design and the layout of apartment building blocks to promote natural ventilation is now investigated as one of the fundamental solutions. This study was focused on the air flow characteristics of outdoor environment in an apartment complex to keep the pollutants out of the site. Age of air and pressure difference have been used as indices of the outdoor air quality. Four different types of apartment building layouts have been analyzed by CFD simulation. This study again selected a real apartment housing complex as a case study model. By analyzing the pressure differences between the front and rear of an apartment building block, the ventilation performance in each individual unit was evaluated, and its impact on ventilation performance is investigated by analyzing the stagnant air around the apartment building blocks. During this process, existing patterns of apartment housing layout have been evaluated, and the most appropriate site layout has been chosen to analyze the outdoor airflow patterns. Based on the analysis of airflow patterns of site layout, the possibilities of improving ventilation performance of an individual apartment housing is proposed.

• Journal of the Korean Society for Geothermal and Hydrothermal Energy
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• v.11 no.2
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• pp.12-18
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• 2015

The energy pile, used for both structural foundations and heat exchangers, brings about heat exchange with the ground formation by circulating a working fluid for heating and cooling buildings. As heat exchange occurs in the energy pile, thermal stress and strain is generated in the pile body and surrounding ground formation. In order to investigate the thermo-mechanical behavior of an energy pile, a comprehensive experimental program was conducted, monitoring the thermal stress of a cast-in place energy pile equipped with five pairs of U-type heat exchanger pipes. The heating and cooling simulation both continued for 30 days. The thermal strain in the longitudinal direction of the energy pile was monitored for a 15 operation days and another 15 days monitoring followed, without the application of heat exchange. In addition, a finite element model was developed to simulate the thermo-mechanical behavior of the energy pile. A non-linear contact model was adopted to interpret the interaction at the pile-soil interface, and thermal-induced structure mechanics was considered to handle the thermo-mechanical coupled multi-field problem.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.51 no.9
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• pp.449-454
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• 2002

A rectangular straight microchannel, integrated with the resistance temperature detectors(RTDs) for temperature sensing and a micro-heater for generating the Temperature gradient along the channel, was fabricated. Its dimension is 57${\mu}{\textrm}{m}$(H)$\times$200${\mu}{\textrm}{m}$(W)$\times$48,050${\mu}{\textrm}{m}$(L), and RTDs were placed at the inner-channel wall. Si wafer was used as a substrate. For the fabrication of RTDs, 5300$\AA$ thick Pt/Ti layer was sputtered on a Pyrex glass wafer. Finally, the glass wafer was bonded with Si wafer by anodic bonding, so that the RTDs are located inside the microchannel. Temperature coefficient of resistance(TCR) values of the fabricated Pt-RTDs were 2800~2950ppm$^{\circ}C$ and the variation of TCR value In the range of O~10$0^{\circ}C$ was less than 0.3％. Therefore, it was proved that the fabricated Pt-RTDs without annealing were excellent as temperature sensors. The temperature distribution in the microchannel was investigated as a function of mass flow rate and heating power. The temperature increase rate diminished with decreasing the applied power and increasing the mass flow rate. It was confirmed from the comparison with the simulation results that the temperature measured inside the microchannel is more accurate than measuring the temperature measured at the outer wall. The proposed temperature sensing method and microchannel are expected to be useful in microfluidics researches.