• Wind and Structures
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• 제26권5호
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• pp.267-277
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• 2018

Numerical simulations are performed of a long flexible cylinder undergoing vortex-induced vibration at a Reynolds number of 500. The cylinder is pinned at both ends, having an aspect ratio of 100 (cylinder length to cylinder diameter) and a mass ratio of 4.2 (structural mass to displaced fluid mass). Temporal and spatial information on the cross-flow (CF) and in-line (IL) vibrations is extracted. High modal vibrations up to the $6^{th}$ in the CF direction and the $11^{th}$ in the IL direction are observed. Both the CF and IL vibrations feature a multi-mode mixed pattern. Mode competition is observed. The $2^{nd}$ mode with a low frequency dominates the IL vibration and its existence is attributed to a wave group propagating back and forth along the span. Distributions of fluid force coefficients are correlated to those of the CF and IL vibrations along the span. Histograms of the x'-y motion phase difference are evaluated from the total simulation time and a complete vibration cycle representing the standing or travelling wave pattern. Correlations between the phase difference and the vibrations are discussed. Vortex structures behind the cylinder show an interwoven near-wake pattern when the standing wave pattern dominates, but an oblique near-wake pattern when the travelling wave pattern prevails.

• Steel and Composite Structures
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• 제10권6호
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• pp.541-561
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• 2010

This paper presents finite element analyses, experimental measurements and finite element model updating of an arch type steel laboratory bridge model using semi-rigid connections. The laboratory bridge model is a single span and fixed base structure with a length of 6.1 m and width of 1.1m. The height of the bridge column is 0.85 m and the maximum arch height is 0.95 m. Firstly, a finite element model of the bridge is created in SAP2000 program and analytical dynamic characteristics such as natural frequencies and mode shapes are determined. Then, experimental measurements using ambient vibration tests are performed and dynamic characteristics (natural frequencies, mode shapes and damping ratios) are obtained. Ambient vibration tests are performed under natural excitations such as wind and small impact effects. The Enhanced Frequency Domain Decomposition method in the frequency domain and the Stochastic Subspace Identification method in the time domain are used to extract the dynamic characteristics. Then the finite element model of the bridge is updated using linear elastic rotational springs in the supports and structural element connections to minimize the differences between analytically and experimentally estimated dynamic characteristics. At the end of the study, maximum differences in the natural frequencies are reduced on average from 47% to 2.6%. It is seen that there is a good agreement between analytical and experimental results after finite element model updating. Also, connection percentages of the all structural elements to joints are determined depending on the rotational spring stiffness.

• Structural Engineering and Mechanics
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• 제62권6호
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• pp.749-758
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• 2017

In structural mechanics, traditional analyses methods usually employ matrix operations for obtaining displacement and internal forces of the structure under the external effects, such as distributed loads, earthquake or wind excitations, and temperature changing inter alia. These matrices are derived from the well-known principle of mechanics called minimum potential energy. According to this principle, a system can be in the equilibrium state only in case when the total potential energy of system is minimum. A close examination of the expression of the well-known equilibrium condition for linear problems, $P=K{\Delta}$, where P is the load vector, K is the stiffness matrix and ${\Delta}$ is the displacement vector, it is seen that, basically this principle searches the displacement set (or deformed shape) for a system that minimizes the total potential energy of it. Instead of using mathematical operations used in the conventional methods, with a different formulation, meta-heuristic algorithms can also be used for solving this minimization problem by defining total potential energy as objective function and displacements as design variables. Based on this idea the technique called Total Potential Optimization using Meta-heuristic Algorithms (TPO/MA) is proposed. The method has been successfully applied for linear and non-linear analyses of trusses and truss-like structures, and the results have shown that the approach is much more successful than conventional methods, especially for analyses of non-linear systems. In this study, the application of TPO/MA, with Harmony Search as the selected meta-heuristic algorithm, to cables net system is presented. The results have shown that the method is robust, powerful and accurate.

• Journal of the Korean Society of Safety
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• 제31권2호
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• pp.119-126
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• 2016

Across the world, the industrialization has increased the frequency of climate anomaly. The size of damage due to recent natural disasters is growing large and fast, and the human damage and economic loss due to disasters are consistently increasing. Urbanization has a structure vulnerable to natural disasters. Therefore, in order to reduce damage from natural disasters, both hardware and software approaches should be utilized. Currently, however, the development of a statistical access process for 'analysis of disaster occurrence factor' and 'prediction of damage costs' for disaster prevention and overall disaster management is inadequate. In case of local governments, overall disaster management system is not established, or even if it is established, unscientific classification system and management lead to low utility of natural statistics of disaster year book. Therefore, in order to minimize disaster damage and for rational disaster management, the disaster damage survey process should be improved. This study selected gale as the focused analysis target among natural disasters recorded in disaster year book such as storm, torrential rain, gale, high seas, and heavy snow, and analyzed disaster survey process. Based on disaster year book, the gale damage size was analyzed and the issues occurring from the correlation of gale and damage amount were examined, so as to suggest an improvement plan for reliable natural disaster information collection and systematic natural disaster damage survey.

• KIEAE Journal
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• 제10권3호
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• pp.89-96
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• 2010

In comparison with ordinary or heavy-weight concrete, light-weight air void concrete has the good aspects in optimizing super tall structure systems for the process of design considering wind load and seismic load by lightening total dead load of buildings and reducing natural resources used. Light-weight air void concrete has excellent properties of heat and sound insulating due to its high amount porosity of air voids. So, it has been used as partition walls and the floor of Ondol which is the traditional Korean floor heating system. Under the condition of which the supply of light-weight aggregates are limited, the development of light-weight concrete using air voids is highly required in the aspects of reduced manufacturing prices and mass production. In this study, we investigated the physical properties and thermal behaviors of specimens that applied different mixing ratios of foaming agent to evaluate the possibility of use in the structural elements. We proposed the estimating equation for compressive strength of each mix having different ratio of foaming agent. We also confirmed that the density of cement matrix is decreased as the mixing amount of foaming agent increase up to 0.6% of foaming agent mixing ratio which was observed by SEM. Based on porosity and compressive strength of control mortar without foaming agent, we built the estimating equations of compressive strength for mortars with foaming agent. The upper limit of use in foaming agent is about 0.6% of the binder amount. Each air void is independent, and size of voids range from 50 to $100{\mu}m$.

• 한국태양에너지학회:학술대회논문집
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• 한국태양에너지학회 2009년도 춘계학술발표대회 논문집
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• pp.176-179
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• 2009

In this study, Urban Climate Simulation was performed using 3-Dimensional Urban Canopy Model. The characteristics of urban thermal environment was analyzed by classifying land coverage and increasing natural land coverage ratio. The results are as follows. The characteristics of the land coverage on urban thermal environment formation can be summarized by the effects like higher temperature on the artificial coverage, and the contrary effects on the natural coverage. When the water coverage 100% was made up, maximum temperature was declined by $5.5^{\circ}C$, humidity by the 6.5g/kg, wind velocity by 0.6m/s, convective sensible heat by $400W/m^2$ and the evaporative latent heat was increased by $370W/m^2$ compared to when artificial coverage 100% was formed. These simulation results need to be constructed as DB which shows urban quantitative thermal characters by the urban physical structure. These can be quantitative base for suggesting combinations of the building and urban planning features at the point of the desirable urban thermal environment as well as analysing urban climate phenomenon.

• Publications of The Korean Astronomical Society
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• 제15권spc1호
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• pp.103-112
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• 2000

Symbiotic stars are known as binary systems of a giant with heavy mass loss and a white dwarf accompanied by an emission nebula. They often show bipolar nebulae, and are believed to form an accretion disk around the white dwarf component by attracting the slow but heavy stellar wind around the giant companion. However, the existence and physical properties of the accretion disk in these systems still remain controversial. Unique to the spectra of symbiotic stars is the existence of the symbiotic bands around $6830{\AA}$ and $7088{\AA}$, which have been identified by Schmid (1989) as the Raman scattered features of the O VI $1032{\AA}$ and $1038{\AA}$ doublet by atomic hydrogen. Due to the incoherency of the Raman scattering, these features have very broad profiles and they are also strongly polarized. In the accretion disk emission model, it is expected that the Raman features are polarized perpendicular to the binary axis and show multiple peak structures in the profile, because the neutral scatterers located near the giant component views the accretion disk in the edge-on direction. Assuming the presence of scattering regions outflowing in the polar directions, we may explain the additional red wing or red peak structure, which is polarized parallel to the binary axis. We argue that in the accretion disk emission model it is predicted that the profile of the Raman feature around $6830{\AA}$ is different from the profile of the $7088{\AA}$ because the O VI line optical depth varies locally around the white dwarf component. We conclude that the Raman scattered features are an important tool to investigate the physical conditions and geometrical configuration of the accretion disk in a symbiotic star.

• Journal of Ocean Engineering and Technology
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• 제27권1호
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• pp.85-92
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• 2013

This paper deals with the resonant type of a WEC (wave energy converter) and the determination method of its geometric parameters which were obtained to construct the robust and optimal structure, respectively. In detail, the optimization problem is formulated with the constraints composed of the response surfaces which stand for the resonance period(heave, pitch) and the meta center height of the buoy. Use of a signal-to-noise ratio calculated from normalized multi-objective results with the weight factor can help to select the robust design level. In order to get the sample data set, the motion responses of the power buoy were analyzed using the BEM (boundary element method)-based commercial code. Also, the optimization result is compared with a robust design for a feasibility study. Finally, the power efficiency of the WEC with the optimum design variables is estimated as the captured wave ratio resulting from absorbed power which mainly related to PTO (power take off) damping. It could be said that the resultant of the WEC design is the economical optimal design which satisfy the given constraints.

• Journal of Ocean Engineering and Technology
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• 제32권1호
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• pp.21-27
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• 2018

The design and analysis of a quayside mooring system for safe mooring of Prelude FLNG under extreme environmental conditions were carried out. The design of the mooring system considered the yard operation conditions and maximum wind speed during a typhoon. In order to secure the mooring safety of Prelude FLNG under an extreme environment, a special steel structure was designed between the quay and Prelude FLNG to maintain the distance from the quay to a certain extent to avoid a collision with the inclined base. The mooring safety was also ensured by installing additional new parts on the quay. A mooring analysis and mooring safety review were performed with more rigorous modeling considering the nonlinearity of the mooring rope and fender. In order to secure additional safety of the mooring system under extreme environmental conditions, a safety assessment was conducted on the failures of the mooring components proposed in the marine mooring guidelines. Based on the results of the mooring analysis, it was confirmed that the Prelude FLNG can be safely moored even under the extreme conditions of typhoons, and a worst case scenario analysis verified that the mooring system design was robust enough. The proposed mooring analysis and design method will provide a basis for the safe mooring of ultra-large floating offshore structures of similar size in the future.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 한국전기전자재료학회 2010년도 하계학술대회 논문집
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• pp.36-36
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• 2010

Currently, there are several newly developed energy resources for the future to replace petroleum resources such as hydrogen fuel cell, solar cell, wind power, and etc. Among them, solar cell has attracted a worldwide concern, because it has an enormous amount of resources. In general, a study of solar cells can be classified in to an area of bulk type and thin-film type. Inorganic solar cells based on silicon have been tremendously developed in technology and efficiency. However, since there are many lithographic steps, high processing temperature approximately $1000^{\circ}C$, and expensive raw materials, a manufacturing cost of device are nearly reaching a limit. Contrary to those disadvantages, organic solar cells can be manufactured at room temperature. Also, it has many advantages such as a low cost, easy fabrication of thin film, and possible manufacture to a large size. Because it can be made to be flexible, research and development on solar cells are actively in progress for the next generation. ever though an efficiency of the organic solar cell is low compared to that of inorganic one, a continuous study is needed. In this paper, we report optimal device structure obtained by a program simulation for design and development of highly efficient organic photovoltaic cells. we have also compared simulated results to experimental ones.