• Transactions on Electrical and Electronic Materials
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• 제18권3호
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• pp.129-135
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• 2017

This paper introduces a design and a simulation of a hybrid active power filter (HAPF) for harmonics reduction given an ideal supply source. The synchronous reference frame method has been used here to identify the reference currents. The proposed HAPF uses a new artificial- intelligence technique called Particle Swarm Optimization (PSO) for tuning the parameters of a proportional and integral controller called PI-PSO. The PI-PSO controller is used to archive optimality for the DC-link voltage of the HAPF-inverter. The hysteresis non-linear current control method is used in this approach to compare the extracted reference and the actual currents in order to generate the pulse gate required for the HAPF. Results obtained by simulations with Matlab/Simuling show that the proposed approach is very flexible and effective for eliminating harmonic currents generated by the non-linear load with the HAPF based PSO tuning.

• International Journal of Naval Architecture and Ocean Engineering
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• 제7권6호
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• pp.1034-1043
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• 2015

This is the continuation of a numerical study on vortex shedding from a blunt trailing-edge of a hydrofoil. In our previous work (Lee et al., 2015), numerical schemes for efficient computations were successfully implemented; i.e. multiple domains, the approximation of domain boundary conditions using cubic spline functions, and particle-based domain decomposition for better load balancing. In this study, numerical results through a hybrid particle-mesh method which adopts the Vortex-In-Cell (VIC) method and the Brinkman penalization model are further rigorously validated through comparison to experimental data at the Reynolds number of $2{\times}10^6$. The effects of changes in numerical parameters are also explored herein. We find that the present numerical method enables us to reasonably simulate vortex shedding phenomenon, as well as turbulent wakes of a hydrofoil.

• Geomechanics and Engineering
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• 제16권5호
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• pp.471-482
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• 2018

A flat-jointed bonded-particle model (BPM) has been proved to be an effective tool for simulating mechanical behaviours of intact rocks. However, the tedious and time-consuming calibration procedure imposes restrictions on its widespread application. In this study, a systematic approach is proposed for simplifying the calibration procedure. The initial relationships between the microscopic, constitutive parameters and macro-mechanical rock properties are firstly determined through dimensionless analysis. Then, sensitivity analyses and regression analyses are conducted to quantify the relationships, using results from numerical simulations. Finally, four examples are used to demonstrate the effectiveness and robustness of the proposed systematic approach for the calibration procedure of BPMs.

• 정보처리학회논문지:소프트웨어 및 데이터공학
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• 제6권10호
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• pp.479-486
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• 2017

최근 GPGPU를 이용하여 저하된 컴퓨터 성능 향상폭을 높일 수 있게 되었고, 이로 인하여 높은 연산을 요구로 하는 물리 기반의 실시간 시뮬레이션을 PC에서 구동할 수 있게 되었다. 물리 시뮬레이션에서 적용되는 물리 계산은 병렬 처리로 수행되어질 수 있으며, 최근 OpenGL 4.3 및 Unity4.0에서 지원되는 Compute shader를 통한 병렬 연산을 이용하면 효율적으로 구동할 수 있다. 본 논문에서는 다양한 플랫폼을 지원하는 디지털 콘텐츠 제작 툴인 Unity와 다양한 플랫폼에서 구동되어지는 OpenGL에서의 실시간 물리 시뮬레이션에서의 성능을 측정 및 비교한다. 본 논문에서 particle 시뮬레이션의 실험 결과 Unity를 이용한 particle 시뮬레이션이 OpenGL을 이용한 particle 시뮬레이션에 비해 최대 136.04% 빠른 성능을 보인다. 이를 통하여 추후 멀티 플랫폼을 지원하는 디지털 콘텐츠를 제작함에 있어 더 나은 개발 도구를 선정할 수 있을 것으로 기대된다.

• Geomechanics and Engineering
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• 제10권4호
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• pp.455-470
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• 2016

In ballasted railway tracks, ballast fouling due to finer material intrusion has been identified as a challenging issue in track maintenance works. In this research, deformation characteristics of crushed stone-sand mixtures, simulating fresh and fouled ballasts were studied from laboratory and a 3-D discrete element method (DEM) triaxial compression tests. The DEM simulation was performed using a recently developed DEM approach, named, Yet Another Dynamic Engine (YADE). First, void ratio characteristics of crushed stone-sand mixtures were studied. Then, triaxial compression tests were conducted on specimens with 80 and 50% of relative densities simulating dense and loose states respectively. Initial DEM simulations were conducted using sphere particles. As stress-strain behaviour of crushed stone-sand mixtures evaluated by sphere particles were different from laboratory specimens, in next DEM simulations, the particles were modeled by a clump particle. The clump shape was selected using shape indexes of the actual particles evaluated by an image analysis. It was observed that the packing behaviour of laboratory crushed stone-sand mixtures were matched well with the DEM simulation with clump particles. The results also showed that the strength properties of crushed stone deteriorate when they are mixed by 30% or more of sand, specially under dense state. The results also showed that clump particles give closer stress-strain behaviour to laboratory specimens than sphere particles.

• Journal of Astronomy and Space Sciences
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• 제37권2호
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• pp.85-94
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• 2020

Equatorial noise, also known magnetosonic waves (MSWs), are one of the frequently observed plasma waves in Earth's inner magnetosphere. Observations have shown that wave amplitudes maximize at the magnetic equator with a narrow extent in their latitudinal distribution. It has been understood that waves are generated from an equatorial source region and confined within a few degrees magnetic latitude. The present study investigates whether the MSW instability and saturation amplitudes maximize at the equator, given an energetic proton ring-like distribution derived from an observed wave event, and using linear instability analysis and particle-in-cell simulations with the plasma conditions at different latitudes along the dipole magnetic field line. The results show that waves initially grow fastest (i.e., with the largest growth rate) at high latitude (20°-25°), but consistent with observations, their saturation amplitudes maximize within ±10° latitude. On the other hand, the slope of the saturation amplitudes versus latitude revealed in the present study is not as steep as what the previous statistical observation results suggest. This may be indicative of some other factors not considered in the present analyses at play, such as background magnetic field and plasma inhomogeneities and the propagation effect.

• Bulletin of the Korean Chemical Society
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• 제33권3호
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• pp.779-789
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• 2012

In the kinetic theory of dense fluids the many-particle collision bracket integral is given in terms of a classical collision operator defined in the phase space. To find an algorithm to compute the collision bracket integrals, we revisit the eigenvalue problem of the Liouville operator and re-examine the method previously reported [Chem. Phys. 1977, 20, 93]. Then we apply the notion and concept of the eigenfunctions of the Liouville operator and knowledge acquired in the study of the eigenfunctions to cast collision bracket integrals into more convenient and suitable forms for numerical simulations. One of the alternative forms is given in the form of time correlation function. This form, on a further manipulation, assumes a form reminiscent of the Chapman- Enskog collision bracket integrals, but for dense gases and liquids as well as solids. In the dilute gas limit it would give rise precisely to the Chapman-Enskog collision bracket integrals for two-particle collision. The alternative forms obtained are more readily amenable to numerical simulation methods than the collision bracket integrals expressed in terms of a classical collision operator, which requires solution of classical Lippmann-Schwinger integral equations. This way, the aforementioned kinetic theory of dense fluids is made fully accessible by numerical computation/simulation methods, and the transport coefficients thereof are made computationally as accessible as those in the linear response theory.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.259-264
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• 2001

High-resolution simulations using vortex methods have been performed for simulating unsteady viscous flows around an impulsively started square cylinder. In order to investigate the phenomenon from laminar to transition flow, simulations are performed for Reynolds numbers 25, 50, 150 and 250. At extremely low Reynolds number, flow around a square cylinder is known to separate at the trailing edges rather than the leading edges. With an increase of Reynolds number, the flow separation at the leading edges will be developed. The main flow characteristics of developing recirculation region and separations from leading and trailing edges are studied with the unsteady behavior of the wake after the cylinder starts impulsively. A notable change in the flow evolution is found at Re=150, that is, it is shown that the flow separations begin at both leading and trailing edges of the square cylinder. On the other hand, when Re=250, the strong secondary vorticity from the rear surfaces of the square cylinder increases the drag coefficient as the primary vortex layer is pushed outwards. The comparisons between results of the present study and experimental data show a good consistency.

• 한국항공우주학회지
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• 제44권7호
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• pp.593-602
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• 2016

본 연구에서는 근접연계방식의 초음속기체 금속분말 미립화장치에 대한 수치해석을 수행하였다. 액체금속의 미립화 과정에서 발생하는 1, 2차 액적분열을 모사하기 위해서 난류 모델을 선정하고 VOF(Volume of Fluid), DPM(Discrete Phase Model) 해석을 차례대로 수행하였다. 해석결과, Level-Set function 분포도를 통해 1차 분열액적의 직경을 계산할 수 있었으며 이 데이터를 DPM 해석에 반영해 도메인 출구에서 수집된 입자들의 최종직경분포를 확인할 수 있었다.

• Geomechanics and Engineering
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• 제9권6호
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• pp.815-827
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• 2015

The mechanical behavior of soil and soil-rock mixture is investigated via the discrete element method. A non-overlapping combination method of spheres is used to model convex polyhedron rock blocks of soil-rock mixture in the DEM simulations. The meso-mechanical parameters of soil and soil-rock interface in DEM simulations are obtained from the in-situ tests. Based on the Voronoi cell, a method representing volumtric strain of the sample at the particle scale is proposed. The numerical results indicate that the particle rotation, occlusion, dilatation and self-organizing force chains are a remarkable phenomena of the localization band for the soil and soil-rock mixture samples. The localization band in a soil-rock mixture is wider than that in the soil sample. The current research shows that the 3D discrete element method can effectively simulate the mechanical behavior of soil and soil-rock mixture.