• Wind and Structures
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• v.22 no.4
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• pp.503-524
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• 2016

The suitability of Computational Fluid Dynamics (CFD) simulations on the built environment for the purpose of estimating average roughness characteristics and for studying wind flow patterns within the environment is assessed. Urban models of various levels of complexity are considered including an empty domain, array of obstacles arranged in regular and staggered manners, in-homogeneous roughness with multiple patches, a semi-idealized built environment, and finally a real built environment. For each of the test cases, we conducted CFD simulations using RANS turbulence closure and validated the results against appropriate methods: existing empirical formulas for the homogeneous roughness case, empirical wind speed models for the in-homogeneous roughness case, and wind tunnel tests for the semi-idealized built environment case. In general, results obtained from the CFD simulations show good agreement with the corresponding validation methods, thereby, giving further evidence to the suitability of CFD simulations for built environment studies consisting of wide-ranging roughness. This work also provides a comprehensive overview of roughness modeling in CFD-from the simplest approach of modeling roughness implicitly through wall functions to the most elaborate approach of modeling roughness explicitly for the sake of accurate wind flow simulations within the built environment.

• 한국데이터정보과학회:학술대회논문집
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• 2005.04a
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• pp.145-154
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• 2005

There have been some controversies on the use of the coefficient of determination for linear no-intercept model. One definition of the coefficient of determination, $R^2=\sum\;{y}{^{\hat{2}}/\sum\;{y^2}$, is being widely accepted only for linear no-intercept models though Kvalseth(1985) demonstrated some possible pitfalls in using such $R^2$. Main objective of this article is to provide a cautionary notice for use of the $R^2$ by pointing out its tricky aspects by means of empirical simulations.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.93-93
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• 2010

In this talk, I will introduce a reactive force field (ReaxFF) molecular dynamics (MD) simulation. In contrast to common MD simulations with empirical FFs, we can predict chemical reactions (bond breaking and formation) in large scale systems with the ReaxFF simulation where all of the ReaxFF parameters are from quantum mechanical calculations such as density functional theory to provide high accuracy. Accordingly, the ReaxFF simulation provides both accuracy of quantum mechanical calculations and description of large scale systems of atomistic simulations at the same time. Here, I will first discuss a theory in the ReaxFF including the differences from other empirical FFs, and then show several applications for studying chemical reactions of SiHx radicals on Si surfaces, which is an important issue in Si process.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.231-232
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• 2002

Various methods that utilize erosion rate measurement of standard cannon, 155mm Howitzer M185, as reference, are being used to calculate erosion rate of an interested unknown cannon tubes. We know ten measured erosion values of the standard cannon from 391 rounds to 4.000. An approximate function fitting these value s is derived. The new erosion equation is also suggested and computer simulations arc presented.

• Journal of the Korean Statistical Society
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• v.30 no.4
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• pp.563-571
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• 2001

In this paper, we study selecting a transformation so that the transformed variable is nearly symmetrically distributed. The large sample properties of an M-estimator of transformation parameter that is obtained by minimizing the integrated square of the imaginary part of the empirical characteristic function are investigated when a random sample is selected from some unspecified distribution. According to influence function calculations and Monte Carlo simulations, these estimates are less sensitive, than the normal model maximum likelihood estimates, to a few outliers.

• IEIE Transactions on Smart Processing and Computing
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• v.4 no.2
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• pp.89-96
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• 2015

In this paper, time-frequency analysis algorithms, empirical mode decomposition and local mean decomposition, are reviewed and their applications to nonlinear and nonstationary real-world data are discussed. In addition, their generic extensions to complex domain are addressed for the analysis of multichannel data. Simulations of these algorithms on synthetic data illustrate the fundamental structure of the algorithms and how they are designed for the analysis of nonlinear and nonstationary data. Applications of the complex version of the algorithms to the synthetic data also demonstrate the benefit of the algorithms for the accurate frequency decomposition of multichannel data.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.5
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• pp.1777-1783
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• 2013

Parshall flume is more practical one of hydraulic structures for measuring flowrate in open channels and also has more advantages when the magnitude of flow velocity is relatively lower or much more sediments are brought from upstream. International Organization for Standardization (ISO) has suggested the empirical formulas standardized by the sizes and dimensions of Parshall flume. However, the related studies using the numerical simulations and experiments are relatively rare. Therefore, in this study, it was examined whether the numerical simulation was adequacy for reproducing the hydraulic characteristics of Parshall flume as much as laboratory experiments by comparing the results from numerical simulations and empirical equation. And for arbitrary Parshall flume, that is unlisted in the ISO standards due to environmental conditions, constructional difficulties etc, thus, the hydraulic experiments should be conducted to obtain the empirical formulas for it, the results from numerical simulations were compared with those of laboratory experiments. Consequently, it was convinced that the numerical simulation about Parshall flume was simulated appropriately instead of experimental approach. And the dimensionless discharge equation of arbitrary ones was suggested using the results of numerical simulations, and the equation was validated by comparing with laboratory experimental results showing the maximum relative error of 2.3%. If the actual topography, the shape of inlet and submerged flow, which is excluded in this study, were carefully considered, it would be possible to supply a simple empirical discharge equation based on numerical results. Also, it can replete hard works for hydraulic experiment being error-prone with complex procedures to a minimum of economic effort.

• Wind and Structures
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• v.21 no.1
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• pp.85-103
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• 2015

Aerodynamic forces of vortex-induced vibration and galloping are going to be coupled when their onset velocities are close to each other, which will induce the cross-wind amplitudes of the structures increased continuously with ever-increasing wind velocities. The main purpose of the present work is going to propose an empirical formula to predict the response amplitude of VIV-galloping interaction. Firstly, two typical mathematical models for the coupled oscillations, i.e., Tamura & Shimada model and Parkinson & Corless model are comparatively summarized. Then, the key parameter affecting response amplitude is determined through comparative numerical simulations with Tamura & Shimada model. For rectangular cylinders with the side ratio from 0.5 to 2.5, which are actually prone to develop the VIV and galloping induced interaction responses, an empirical amplitude prediction formula is proposed after regression analysis on comprehensively collected experimental data with the predetermined key parameter.

• KIEAE Journal
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• v.13 no.1
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• pp.109-123
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• 2013

The study is to probe a technical alternative to enhance the reliability and accuracy of the results of various landscape simulations. This study to present technical criteria that are necessary in each stage of target site analysis, picture taking, and computer synthesis and, through these to present supplementary plans to enhance the reliability and accuracy of landscape simulations. In order to derive more practical and empirical results in terms of the reliability of the results of landscape simulations, examples that actually passed landscape review were selected. With regard to study process, an analysis was made first to analyze the landscape report data of designs that passed the review to analyze their characteristics, to be followed by an integrated analysis of problems that were revealed in various landscape simulations. Important factors that affect directly the work of landscape simulations such as the specification of camera lenses that were used in picture taking, distance, and angle. Design the work was carried out using Auto CAD, 3DS Max, and Photoshop program in the same way as in actual design. For verification of their accuracy and reliability, the results were entrusted to experts who have implemented similar jobs. To seduce differences from those too landscape simulations that conduct trial experiment of the virtual space that are to be created in the future with accurate numerical values.

• Structural Engineering and Mechanics
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• v.68 no.2
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• pp.237-245
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• 2018

This study proposes an empirical formulation to predict the maximum deformation of offshore blast wall structure that is subjected to impact loading caused by hydrocarbon explosion. The blast wall model is assumed to be supported by a simply-supported boundary condition and corrugated panel is modelled. In total, 1,620 cases of LS-DYNA simulations were conducted to predict the maximum deformation of blast wall, and they were then used as input data for the development of the empirical formulation by regression analysis. Stainless steel was employed as materials and the strain rate effect was also taken into account. For the development of empirical formulation, a wide range of parametric studies were conducted by considering the main design parameters for corrugated panel, such as geometric properties (corrugation angle, breadth, height and thickness) and load profiles (peak pressure and time). In the case of the blast profile, idealised triangular shape is assumed. It is expected that the obtained empirical formulation will be useful for structural designers to predict maximum deformation of blast wall installed in offshore topside structures in the early design stage.