• Journal of Radiation Protection and Research
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• v.27 no.3
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• pp.171-179
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• 2002

The three-dimensional long-range dispersion model has been developed to understand the characteristics of the transport and diffusion of radioactive materials released into atmosphere. The model is designed to compute air concentration and ground deposition at distances up to some thousands of kilometers from the source point in horizontal direction. The vertical turbulent motion is considered separately within the mixing layer and above the mixing layer. The test simulation was performed In the area of Northeast Asia. The release point was assumed in the east part of China. The calculated concentration distributions art mainly advected toward the southeast part of release point by the wind fields. The developed model will be used to estimate the radiological consequences against a nuclear accident. The model will be supplemented by the comparative study using the data of the long-range field experiments.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.45.1-45.1
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• 2016

Spatially rotating magnetic fields have been observed in the solar wind and in the Earth's magnetopause as well as in reversed field pinch (RFP) devices. Such field configurations have a similarity with extended current layers having a spatially varying plasma pressure instead of the spatially varying guide field. It is thus expected that magnetic reconnection may take place in a rotating magnetic field no less than in an extended current layer. We have investigated the spontaneous evolution of a collisionless plasma system embedding a rotating magnetic field with a two-and-a-half-dimensional electromagnetic particle-in-cell (PIC) simulation. In magnetohydrodynamics, magnetic flux can be decreased by diffusion in O-lines. In kinetic physics, however, an asymmetry of the velocity distribution function can generate new magnetic flux near O- and X-lines, hence a dynamo effect. We have found that a magnetic-flux-reducing diffusion phase and a magnetic-flux-increasing dynamo phase are alternating with a certain period. The temperature of the system also varies with the same period, showing a similarity to sawtooth oscillations in tokamaks. We have shown that a modified theory of sawtooth oscillations can explain the periodic behavior observed in the simulation. A strong guide field distorts the current layer as was observed in laboratory experiments. This distortion is smoothed out as magnetic islands fade away by the O-line diffusion, but is soon strengthened by the growth of magnetic islands. These processes are all repeating with a fixed period. Our results suggest that a rotating magnetic field configuration continuously undergoes deformation and relaxation in a short time-scale although it might look rather steady in a long-term view.

• Journal of the Korean Institute of Intelligent Systems
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• v.26 no.4
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• pp.286-294
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• 2016

In the initial design stage, the geometry of systems needs to be optimized regarding its performance. However, performance analysis is very time-consuming. Therefore, optimization becomes difficult/impossible problems because we need to evaluate the system performance for alternative design cases. To overcome this problem, many researchers perform prediction of system performance using the approximation model. The response surface method (RSM) is typically used to predict the system performance in the various research fields, but it presents prediction errors for highly nonlinear systems. The major objective of this paper is to propose a proper prediction method for marine system problems. Case studies of marine systems (the substructure of a floating offshore wind turbine considering hydrodynamic performance and bulk carrier bottom stiffened panels considering structure performance) verify that the proposed method is applicable to performance prediction in marine systems.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.101.2-101.2
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• 2012

Shocks are ubiquitous in astrophysical plasmas: bow shocks are formed by the interaction of solar wind with planetary magnetic fields, and supernova explosions and jets produce shocks in interstellar and intergalactic spaces. The global morphologies of these shocks are usually described by a set of magnetohydrodynamic (MHD) equations which tacitly assumes local thermal equilibrium, and the resulting Rankine-Hugoniot shock jump conditions are applied to obtain the relationship between the upstream and downstream physical quantities. While thermal equilibrium can be achieved easily in collisional fluids, it is generally believed that collisions are infrequent in astrophysical settings. In fact, shock widths are much smaller than collisional mean free paths and a variety of kinetic phenomena are seen at the shock fronts according to in situ observations of planetary shocks. Hence, both the MHD and kinetic equations have been adopted in theoretical and numerical studies to describe different aspects of the physical phenomena associated with astrophysical shocks. In this paper, we present the results of 3D relativistic particle-in-cell (PIC) simulations for ion-electron plasmas, with focus on the shock structures: when a jet propagates into an unmagnetized ambient plasma, a shock forms in the nonlinear stage of the Weibel instability. As the shock shows the structures that resemble those predicted in MHD systems, we compare the results with those predicted in the MHD shocks. We also discuss the thermalization processes of the upstream flows based on the time evolutions of the phase space and the velocity distribution, as well as the wave spectra analyses.

• Journal of Environmental Science International
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• v.25 no.5
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• pp.683-694
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• 2016

Sensitivity analysis of the WRF model according to the impact of nudging (e.g., nudging techniques and application domains) was conducted during high nocturnal ozone episode to improve the prediction of the regional ozone concentration in the southeastern coastal area of the Korean peninsula. The analysis was performed by six simulation experiments: (1) without nudging (e.g., CNTL case), (2) with observation nudging (ONE case) to all domains (domain 1~4), (3) with grid nudging (GNE case) to all domains, (4)~(6) with grid nudging to domain 1, domain 1~2 and domain 1~3, respectively (GNE-1, GNE-2, GNE-3 case). The results for nudging techniques showed that the GNE case was in very good agreement with those observed during all analysis periods (e.g., daytime, nighttime, and total), as compared to the ONE case. In particular, the large effect of grid nudging on the near-surface meteorological factors (temperature, relative humidity, and wind fields) was predicted at the coastline and nearby sea during daytime. The results for application domains showed that the effects of nudging were distinguished between the meteorological factors and between the time periods. When applied grid nudging until subdomain, the improvement effects of temperature and relative humidity had differential tendencies. Temperature was increased for all time, but relative humidity was increased in daytime and was decreased in nighttime. Thus, GNE case showed better result than other cases.

• KIEAE Journal
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• v.10 no.1
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• pp.33-44
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• 2010

These days, the use of alternative energy is recognized as one of countermeasures to rising oil prices and climate-change agreements. Especially, it is regarded as a main element to realize 'Low-Carbon Green Growth', one of the key policies of MB government. However, though alternative energy, renewable energy, is a main policy issue of the present administration, there are many problems to solve: the lack of recognition of its energy, the low level of its technology, its expert's shortage, and its limited application fields. Therefore, to stimulate the use of alternative energy in Korea and to change the recognition of this energy, it is needed to utilize it to urban street furniture that many people visit and use. In particular, if alternative energy is first applied to street furniture that needs energy for its management, the effect of publicity and energy saving will be so great. In this study, for the basic research to introduce and promote various alternative energy sources such as solar power, wind power, small hydro power, and human power, we analyzed the present condition of development of Korean and foreign street furniture items. Furthermore, we evaluated the level of development of these street furniture products and the design types of them and from these evaluations suggested the design direction that can solve problems in development and improve the possibility of application of these energy sources to these street furniture items. Through these analyses, this study aims to enhance the image of future environment-friendly city and make new index for the application of alternative energy.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.2
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• pp.205-210
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• 2014

Recently, pattern recognition methods have been widely used by researchers for fault diagnoses of mechanical systems. A pattern recognition method determines the soundness of a mechanical system by detecting variations in the system's vibration characteristics. Hidden Markov models (HMMs) and artificial neural networks (ANNs) have recently been used as pattern recognition methods in various fields. In this study, a HMM-ANN hybrid method for the fault diagnosis of a mechanical system is introduced, and a rotating wind turbine blade with a crack is selected for fault diagnosis. The existence, location, and depth of said crack are identified in this research. For improving the diagnostic accuracy of the method in spite of the presence of noise, a moment with a few specific frequencies is applied to the structure.

• Journal of Ocean Engineering and Technology
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• v.30 no.5
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• pp.349-355
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• 2016

Divinycell, which functions as both insulation and a supporting structure, is generally applied in the NO96-type liquefied natural gas (LNG) insulation system. Polymer-material-based Divinycell, which has a high strength and low weight, has been widely used in the offshore, transportation, wind power generation, and civil engineering fields. In particular, this type of material receives attention as an insulation material because its thermal conductivity can be lowered depending on the ambient temperature. However, it is difficult to obtain research results for Divinycell, even though the component materials of the NO96-type LNG cargo containment system, such as 36% nickel steel (invar steel), plywood, perlite, and glass wool, have been extensively studied and reported. In the present study, temperature and strain-rate dependent compressive tests on Divinycell were performed. Both the quantitative experimental data and elastic recovery are discussed. Finally, the mechanical characteristics of Divinycell were compared to the results of polyurethane foam insulation material.

• Korean Journal of Agricultural Science
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• v.46 no.1
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• pp.137-149
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• 2019

Accurate estimation of reference evapotranspiration (RET) is important to quantify crop evapotranspiration for sustainable water resource management in hydrological, agricultural, and environmental fields. It is estimated by different methods from direct measurements with lysimeters, or by many empirical equations suggested by numerous modeling using local climatic variables. The potential to use some such equations depends on the availability of the necessary meteorological parameters for calculating the RET in specific climatic conditions. The objective of this study was to determine the proper RET equations using limited climatic data and to analyze the temporal and spatial trends of the RET in South Korea. We evaluated the FAO-56 Penman-Monteith equation (FAO-56 PM) by comparing several simple RET equations and observed small fan evaporation. In this study, the modified Penman equation, Hargreaves equation, and FAO Penman-Monteith equation with missing solar radiation (PM-Rs) data were tested to estimate the RET. Nine weather stations were considered with limited climatic data across South Korea from 1973 - 2017, and the RET equations were calculated for each weather station as well as the analysis of the mean error (ME), mean absolute error (MAE), and root mean square error (RMSE). The FAO-56 PM recommended by the Food Agriculture Organization (FAO) showed good performance even though missing solar radiation, relative humidity, and wind speed data and could still be adapted to the limited data conditions. As a result, the RET was increased, and the evapotranspiration rate was increased more in coastal areas than inland.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.6
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• pp.414-421
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• 2019

In this paper, thermomechanical coupled analysis of carbon/phenolic composites structures in reentry environment was performed. The interface of thermomechanical coupled analysis was constructed using commercial software. The governing equations of temperature and displacement fields were considered to simulate change of physical behavior due to pyrolysis and ablation effects. The results of thermomechanical coupled analysis were compared with the results of ablation test using arc-heated wind tunnel. Also, the structural stability of reentry capsule was analyzed using the analysis interface. The excellent ablation characteristics and thermal protection effects of the carbon/phenolic composites were confirmed and the constructed analysis interface can be effectively used to perform thermal protection system design.