• Journal of Korean Society of Coastal and Ocean Engineers
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• v.33 no.4
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• pp.168-178
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• 2021

Numerical simulations were implemented to look into the modified seabed topography due to the presence of breakwaters of varying reflection characteristics. The numerical model was composed of OlaFlow, an OpenFoam-based tool box, and a physics-based morphology model [Seoul Foam]. In doing so, the interaction between the seabed, which undergoes deformation due to siltation and scouring, and the incoming waves was described using Dynamic Mesh. The rubble-mound, vertical, and curved slit caisson breakwaters with varying reflection characteristics resulted in standing waves that differ from each other, shown to have a significant influence on the seabed topography. These results are in line with Nielsen's study (1993) that sands saltated under the surface nodes of standing waves, where the near-bed velocities are most substantial, convected toward the surface antinodes by boundary-layer drift. Moreover, the crest of sand waves was formed under the surface antinodes of standing waves, and the trough of sand waves was formed under the surface antinodes. In addition, sand wave amplitude reaches its peak in the curved slit caisson with a significant reflection coefficient, and the saltation of many grains of sand would cause this phenomenon due to the increased near-bed velocity under the nodes when the reflection coefficient is getting large.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.81-86
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• 2001

Interfacial pressure jump terms based on the physics of phasic interface and bubble dynamics are introduced into the momentum equations of the two-fluid model for bubbly flow. The pressure discontinuity across the phasic interface due to the surface tension force is expressed as the function of fluid bulk moduli and bubble radius. The consequence is that we obtain from the system of equations the real eigenvalues representing the void-fraction propagation speed and the pressure wave speed in terms of the bubble diameter. Inversely, we obtain an analytic closure relation for the radius of bubbles in the bubbly flow by using the kinematic wave speed given empirically in the literature. It is remarkable to see that the present mechanistic model using this practical bubble radius can indeed represent both the mathematical well-posedness and the physical wave speeds in the bubbly flow.

• Nuclear Engineering and Technology
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• v.55 no.5
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• pp.1802-1813
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• 2023

Conjugate heat transfer between liquid metal and solid is a common phenomenon in a liquid-metal-cooled fast reactor's fuel assembly and heat exchanger, dramatically affecting the reactor's safety and economy. Therefore, comprehensively studying the sophisticated conjugate heat transfer in a liquid-metal-cooled fast reactor is profound. However, it has been evidenced that the traditional Simple Gradient Diffusion Hypothesis (SGDH), assuming a constant turbulent Prandtl number (Pr_t,, usually 0.85 - 1.0), is inappropriate in the Computational Fluid Dynamics (CFD) simulations of liquid metal. In recent decades, numerous studies have been performed on the four-equation model, which is expected to improve the precision of liquid metal's CFD simulations but has not been introduced into the conjugate heat transfer calculation between liquid metal and solid. Consequently, a four-equation model, consisting of the Abe k - ε turbulence model and the Manservisi k_𝜃 - ε_𝜃 heat transfer model, is applied to study the conjugate heat transfer concerning liquid metal in the present work. To verify the numerical validity of the four-equation model used in the conjugate heat transfer simulations, we reproduce Johnson's experiments of the liquid lead-bismuth-cooled turbulent pipe flow using the four-equation model and the traditional SGDH model. The simulation results obtained with different models are compared with the available experimental data, revealing that the relative errors of the local Nusselt number and mean heat transfer coefficient obtained with the four-equation model are considerably reduced compared with the SGDH model. Then, the thermal-hydraulic characteristics of liquid metal turbulent pipe flow obtained with the four-equation model are analyzed. Moreover, the impact of the turbulence model used in the four-equation model on overall simulation performance is investigated. At last, the effectiveness of the four-equation model in the CFD simulations of liquid sodium conjugate heat transfer is assessed. This paper mainly proves that it is feasible to use the four-equation model in the study of liquid metal conjugate heat transfer and provides a reference for the research of conjugate heat transfer in a liquid-metal-cooled fast reactor.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.43.1-43.1
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• 2015

We have discovered an unexplored population of galaxies featuring weak broad-line regions (BLRs) at z < 0.2 from detailed analysis of galaxy spectra in the Sloan Digital Sky Survey Data Release 7. These objects predominantly show a stellar continuum but also a broad $H{\alpha}$ emission line, indicating the presence of a low-luminosity active galactic nucleus (AGN) oriented so that we are viewing the central engine directly without significant obscuration. These accreting black holes have previously eluded detection due to their weak nature. The new BLR AGNs we found increased the number of known type 1 AGNs by 49%. Some of these new BLR AGNs were detected at the Chandra X-ray Observatory, and their X-ray properties confirm that they are indeed type 1 AGN. Based on our new and more complete catalogue of type 1 AGNs, we derived the type 1 fraction of AGNs as a function of [OIII] ${\lambda}5007$ emission luminosity and explored the possible dilution effect on the obscured AGN due to star-formation. The new type 1 AGN fraction shows much more complex behavior with respect to black hole mass and bolometric luminosity than suggested by the existing receding torus model. The type 1 AGN fraction is sensitive to both of these factors, and there seems to be a sweet spot (ridge) in the diagram of black hole mass and bolometric luminosity. Furthermore, we present a hint that the Eddington ratio plays a role in determining the opening angles. This work is submitted to ApJS.

• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.168-168
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• 1999

Recently, Fe/Si multilayered films (MLF) have been a focus of interest due to the strong antiferromagnetic (AF) coupling observed in such kind of MLF originates from the same nature as in the metal/metal MLF. In particular, a question of whether the spacer layer in the Fe/Si MLF is metallic or semiconducting is of interest. In spite of various experimental techniques envolved in the study, the chemical composition and the properties of the interfacial regions in the MLF exhibiting the AF coupling is still questionable. The nature of the AF coupling and the interfacial properties of Fe/Si MLF are investigated in this study. A series of Fe/Si MLF with a fixed nominal thickness of Fe(3nm) and a variable thickness of Sk(1.0-2.2nm) were deposited by RF-sputtering onto glass substrates at room temperature. The atomic structures and the actual sublayer thicknesses of the Fe/Si MLF are investigated by using x-ray diffraction. The magnetic-field dependence of the equatorial Kerr effect clearly shows an appearance of the AF coupling between Fe sublayers at tsi = 1.5 - 1.8 nm. the drastic discrepancies between the experimental magnetooptical (MO) and optical properties, and based on the assumption of sharp interfaces between Fe and Si sublayers leads to a conclusion that pure si is absent in the AF-coupled Fe/Si MLF. Introducing in the model nonmagnetic semiconducting FeSi alloy layers between Fe and Si sublayers or as spacer between pure Fe sublayers only slightly improves the agreement between model and experiment. A reasonable agreement between experimental and simulated MO spectra was reached with using the fitted optical properties for the spacer with a typical metallic type of behavior. The results of the magnetic properties measured by vibrating sample magnetometer and magnetic circular dichroism are also analyzed in connection with the MO and optical properties.

• KIPS Transactions on Software and Data Engineering
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• v.10 no.1
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• pp.19-28
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• 2021

Recently, in the field of education, various attempts have been made to apply virtual reality technology to an educational field and use it as an educational medium. Accordingly, in the science subject area, it is necessary to simulate science experiments that can make various and active experiments out of various limitations such as space and situation by using virtual reality environment construction technology. In this study, after selecting a physics course from a science subject, an experimental simulation using a parabolic motion formula, one of physical phenomena, is implemented in a virtual space, and then used in actual physics education based on the learning standards of the STEAM theory. Prove this is possible. Through this, it was confirmed that a specific educational model using virtual reality space can be designed, and it shows that education can be conducted with more effective educational methods in various subjects of education through the combination of traditional educational model and modern technology. Regarding the results of the research, it suggests the possibility of future research plans and practical use in the educational field.

• Nuclear Engineering and Technology
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• v.55 no.1
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• pp.310-323
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• 2023

The k-eigenvalue neutronics/thermal-hydraulics coupling calculation is a key issue for reactor design and analysis. Jacobian-free Newton-Krylov (JFNK) method, featured with super-linear convergence rate and high efficiency, has been attracting more and more attention to solve the multi-physics coupling problem. However, it may converge to the high-order eigenmode because of the multiple solutions nature of the k-eigenvalue form of multi-physics coupling issue. Based on our previous work, a modified JFNK with a line search method is proposed in this work, which can find the fundamental eigenmode together with thermal-hydraulics feedback in a wide range of initial values. In detail, the existing modified JFNK method is combined with the line search strategy, so that the intermediate iterative solution can avoid a sudden divergence and be adjusted into a convergence basin smoothly. Two simplified 2-D homogeneous reactor models, a PWR model, and an HTR model, are utilized to evaluate the performance of the newly proposed JFNK method. The results show that the performance of this proposed JFNK is more robust than the existing JFNK-based methods.

• International Union of Geodesy and Geophysics Korean Journal of Geophysical Research
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• v.23 no.1
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• pp.1-11
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• 1995

Thermospheric neutral winds and temperatures have been collected from the ground-based Fabry-Perot interferometer (FPI) at Thule Air Base ($76.5^{\circ}N{\;}69.0^{\circ}W$), Greenland since 1985. The thermospheric observations are obtained by determining the Doppler characteristics f the [OI] 6300 ${\AA}$ emissions of atomic oxygen. The FPI operates routinely during the winter season, with a limitation in the observation by the existence of clouds. For this study, data acquired from 1985 to 1991 were analyzed. The neutral wind measurements from these long-term measurements are used to investigate the influence of solar cycle variation on the high-latitude thermospheric dynamics. These data provide experimental results of the geomagnetic polar cap are also compared with the predictions of two semiempirical models : the vector spherical harmonics (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The experimental results show a good positive correlation between solar activity and thermospheric wind speed over the geomagnetic polar cap. The calculated correlation coefficient indicates that an increase of 100 in F10.7 index corresponds to an increase in wind speed of about 100 m/s. The model predictions reveal similar trends of wind speed variation as a function of solar activity, with the VSH and HWM models tending to overestimate and underestimate the wind speed, respectively.

• Journal of The Korean Society of Agricultural Engineers
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• v.57 no.5
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• pp.43-49
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• 2015

Most of agricultural structures located in seashore could not avoid rapid deterioration of concrete because chloride-ion and $CO_2$ gradually penetrate into concrete. However, since most of models can be able to describe the phenomenon of penetration by using one or two dimensional models based on finite difference method (FDM), those modes can not simulate the real geometry and it takes a lot of computational time to complete even the calculation. To overcome those weaknesses, three dimensional numerical model considering time dependent variables such as surface concentration of chloride and diffusion coefficient of domain based on finite element method (FEM) was suggested. This model also included the neutralization occurred by the penetration of $CO_2$. Because the model used various sizes of tetrahedral mesh instead of equivalent rectangular mesh, it reduced the computational time to compare with FDM. As this model is based on FEM, it will be easily extended to execute multi-physics simulation including water evaporation and temperature change of concrete.

• Journal of the Korean Data and Information Science Society
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• v.20 no.1
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• pp.225-235
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• 2009

Lattice process models are used to explain phase transitions in statistical mechanics, a branch of physics. The Ising model, a specific form of lattice process model, was proposed by Ising in 1925. Since then, variants of the Ising model such as the Potts model and the unitary cell model have been proposed. Like the Ising model, it is believed that the more general models exhibit phase transitions on the critical surface, which is based on the mathematical equation. In statistical sense, phase transitions can be simulated through Markov Chain Monte Carlo (MCMC). We applied Swendsen-Wang algorithm, a block Gibbs algorithm, to a general lattice process models and we simulate phase transition phenomena of the unitary cell model.