• Current Optics and Photonics
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• v.3 no.6
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• pp.522-530
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• 2019

We present simulation methods to accurately determine the transmission efficiency and far-field patterns (FFPs) of a shallow-etched waveguide grating antenna (WGA) formed on a silicon-on-insulator wafer based on the finite-difference time-domain (FDTD) approach. The directionality and the FFP of a WGA with >1-mm in length can be obtained reliably by simulating a truncated WGA structure using a three-dimensional FDTD method and a full-scale WGA using a two-dimensional FDTD with the effective index method. The developed FDTD methods are applied to the simulation of an optical phased array (OPA) composed of a uniformly spaced WGA array, and the steering-angle dependent transmission efficiency and FFPs are obtained in OPA structures having up to 128-channel WGAs.

• Communications for Statistical Applications and Methods
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• v.27 no.5
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• pp.547-568
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• 2020

Modeling the statistical autocorrelations in spatial data is often achieved through the estimation of the variograms, where the selection of the appropriate valid variogram model, especially for small samples, is crucial for achieving precise spatial prediction results from kriging interpolations. To estimate such a variogram, we traditionally start by computing the empirical variogram (traditional Matheron or robust Cressie-Hawkins or kernel-based nonparametric approaches). In this article, we conduct numerical studies comparing the performance of these empirical variograms. In most situations, the nonparametric empirical variable nearest-neighbor (VNN) showed better performance than its competitors (Matheron, Cressie-Hawkins, and Nadaraya-Watson). The analysis of the spatial groundwater dataset used in this article suggests that the wave variogram model, with hole effect structure, fitted to the empirical VNN variogram is the most appropriate choice. This selected variogram is used with the ordinary kriging model to produce the predicted pollution map of the nitrate concentrations in groundwater dataset.

• Structural Engineering and Mechanics
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• v.55 no.1
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• pp.93-109
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• 2015

Finite element analysis (FEA) combined with the concepts of linear elastic fracture mechanics (LEFM) provides a practical and convenient means to study the fracture and crack growth of materials. In this paper, a numerical modeling of crack propagation in the cement mantle of the reconstructed acetabulum is presented. This work is based on the implementation of the displacement extrapolation method (DEM) and the strain energy density (SED) theory in a finite element code. At each crack increment length, the kinking angle is evaluated as a function of stress intensity factors (SIFs). In this paper, we analyzed the mechanical behavior of cracks initiated in the cement mantle by evaluating the SIFs. The effect of the defect on the crack propagation path was highlighted.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.311-319
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• 2008

We present an innovative method of multi physics application involving energetic materials. Energetic materials are related to reacting flows in extreme environments such as fires and explosions. They typically involve high pressure, high temperature, strong shock waves and high strain rate deformation of metals. We use an Eulerian methodology to address these problems. Our approach is naturally free from large deformation of materials that make it suitable for high strain rate multi-material interacting problems. Furthermore we eliminate the possible interface smearing by using the level sets. We heave devised a new level set based tracking framework that can elegantly handle large gradients typically found in reacting gases and metals. We show several work-in-progress application of our integrated framework.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.43.2-43.2
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• 2019

Tomography is a method to reconstruct three-dimensional structure of an optically thin object. We can obtain the three-dimensional information by combining a number of projected images at different angles. Solar rotational tomography (SRT) is the tomographic method to estimate the coronal structures using the solar rotation. There are a few practical difficulties in solar coronal observation. One of the most crucial difficulty is handling the blocking area by the occulter or the Sun itself. So we have to use the iterative reconstruction for the SRT which can resolve that problem by using the forward modeling. In this study, we propose an alternative method to reconstruct the solar coronal structure: the filtered backprojection (FBP) algorithm. The FBP algorithm is based on the simple analytic solution. Thus it is easy to understand, and the computing cost is much cheaper than that of the iterative reconstruction. Recently we found a solution for the FBP algorithm to the problem of the blocking area in the solar EUV observations. We introduce how to apply the FBP algorithm to the SRT, and show the initial results of the performance test.

• Proceedings of the KSRS Conference
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• 1999.11a
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• pp.371-374
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• 1999

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.10a
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• pp.543-544
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• 2006

• Nuclear Engineering and Technology
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• v.42 no.3
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• pp.271-278
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• 2010

While the deterministic lattice physics/depletion codes use leakage-corrected critical spectrum (although approximate due to the B1 buckling search employed), Monte Carlo depletion codes currently in use do not have such a feature in spite of their heterogeneity and continuous-energy modeling capability. This paper describes an approach to Monte Carlo depletion with leakage-corrected critical spectrum derived from first principles. This is based on the concept of albedo eigenvalue treated as weight of the reflected neutron in Monte Carlo simulation.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.47.3-48
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• 2020

I will present the Sejong Suite, an extensive collection of state-of-the-art high-resolution cosmological hydrodynamical simulations spanning a variety of cosmological and astrophysical parameters, primarily developed for modeling the Lyman-Alpha forest. Adopting a particle-based implementation, we follow the evolution of gas, dark matter (cold and warm), massive neutrinos, and dark radiation, and consider several combinations of box sizes and number of particles. Noticeably, for the first time, we simulate extended mixed scenarios describing the combined effects of warm dark matter, neutrinos, and dark radiation, modeled consistently by taking into account the neutrino mass splitting. Along the way, I will also highlight some new results focused on the matter and flux statistics.

• Journal of the Korea Computer Graphics Society
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• v.14 no.4
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• pp.7-18
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• 2008

To realistically produce fluids such as smoke for the visual effects in the films or animations, we need two main processes: a physics-based modeling of smoke and a rendering of smoke simulation data, based on light transport theory. In the computer graphics community, the physics-based fluids simulation is generally adopted for smoke modeling. Recently, the interest of the particle-based Lagrangian simulation methods is increasing due to the advantages at simulation time, instead of the grid-based Eulerian simulation methods which was widely used. As a result, because the smoke rendering technique depends heavily on the modeling method, the research for rendering of the particle-based smoke data still remains challenging while the research for rendering of the grid-based smoke data is actively in progress. This paper focuses on realistic rendering technique for the smoke particles produced by Lagrangian simulation method. This paper introduces a technique which is called particle map, that is the expansion and modification of photon mapping technique for the particle data. And then, this paper suggests the novel particle map technique and shows the differences and improvements, compared to previous work. In addition, this paper presents irradiance map technique which is the pre-calculation of the multiple scattering term in the volume rendering equation to enhance efficiency at rendering time.